L6985F 38 V, 500 mA synchronous step-down switching regulator with 30 A quiescent current Datasheet - production data Description HTSSOP16 (RTH = 40 C/W) Features 0.5 A DC output current 4 V to 38 V operating input voltage Low consumption mode or low noise mode 30 A IQ at light-load (LCM VOUT = 3.3 V) 8 A IQ-SHTDWN Output voltage adjustable from 0.85 V to VIN Adjustable fSW (250 kHz - 2 MHz) Embedded output voltage supervisor Synchronization Adjustable soft-start time Internal current limiting Overvoltage protection The L6985F is a step-down monolithic switching regulator able to deliver up to 0.5 A DC. The output voltage adjustability ranges from 0.85V to VIN. Thanks to the P-channel MOSFET high-side power element, the device features 100% of the duty cycle operation. The wide input voltage range meets the specification for the 5 V, 12 V and 24 V power supplies. The "Low Consumption Mode" (LCM) is designed for applications active during the idle mode, so it maximizes the efficiency at the light-load with the controlled output voltage ripple. The "Low Noise Mode" (LNM) makes the switching frequency constant overload current range, meeting the low noise application specification. The output voltage supervisor manages the reset phase for any digital load (C, FPGA). The RST open collector output can also implement output voltage sequencing during the power-up phase. The synchronous rectification, designed for high efficiency at the medium - heavy load, and the high switching frequency capability make the size of the application compact. Pulse-by-pulse current sensing on both power elements implements effective constant current protection. Output voltage sequencing Peak current mode architecture RDSON HS = 360 m, RDSON LS = 150 m Thermal shutdown Applications Designed for 12 V and 24 V buses Programmable logic controllers (PLCs) Decentralized intelligent nodes Sensors and low noise applications (LNM) February 2016 This is information on a product in full production. DocID027836 Rev 2 1/65 www.st.com Contents L6985F Contents 1 Application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2 Pin settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1 Pin connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.2 Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.3 Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.4 Thermal data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.5 ESD protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3 Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 4 Functional description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 4.1 Power supply and voltage reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Switchover feature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 4.2 Voltages monitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 4.3 Soft-start and inhibit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 4.3.1 Ratiometric startup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 4.3.2 Output voltage sequencing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 4.4 Error amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 4.5 Light-load operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 4.6 4.7 4.5.1 Low noise mode (LNM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 4.5.2 Low consumption mode (LCM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Switchover feature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 4.6.1 LCM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 4.6.2 LNM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Overcurrent protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 OCP and switchover feature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 2/65 4.8 Overvoltage protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 4.9 Thermal shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 DocID027836 Rev 2 L6985F 5 6 Contents Closing the loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 5.1 GCO(s) control to output transfer function . . . . . . . . . . . . . . . . . . . . . . . . 36 5.2 Error amplifier compensation network . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 5.3 Voltage divider . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 5.4 Total loop gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 5.5 Compensation network design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 Application notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 6.1 Output voltage adjustment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 6.2 Switching frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 6.3 MLF pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 6.4 Voltage supervisor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 6.5 Synchronization (LNM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 6.6 Design of the power components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 6.6.1 Input capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 6.6.2 Inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 6.6.3 Output capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 7 Application board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 8 Efficiency curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 9 Package information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 9.1 HTSSOP16 package information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 10 Order codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 11 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 DocID027836 Rev 2 3/65 65 Application schematic 1 L6985F Application schematic Figure 1. L6985F application schematic 4/65 DocID027836 Rev 2 L6985F Pin settings 2 Pin settings 2.1 Pin connection Figure 2. Pin connection (top view) 2.2 Pin description Table 1. Pin description No. Pin Description 1 RST The RST open collector output is driven low when the output voltage is out of regulation. The RST is released after an adjustable time DELAY once the output voltage is over the active delay threshold. 2 VCC Connect a ceramic capacitor ( 470 nF) to filter internal voltage reference. This pin supplies the embedded analog circuitry. 3 SS/INH An open collector stage can disable the device clamping this pin to GND (INH mode). An internal current generator (4 A typ.) charges the external capacitor to implement the soft-start. 4 SYNCH/ ISKP The pin features Master / Slave synchronization in LNM (see Section 6.5 on page 45) and skip current level selection in LCM (see Section 4.5.2 on page 24). 5 FSW A pull-up resistor (E24 series only) to VCC or pull-down to GND selects the switching frequency. Pinstrapping is active only before the soft-start phase to minimize the IC consumption. 6 MLF A pull-up resistor (E24 series only) to VCC or pull-down to GND selects the low noise mode/low consumption mode and the active RST threshold. Pinstrapping is active only before the soft-start phase to minimize the IC consumption. 7 COMP Output of the error amplifier. The designed compensation network is connected at this pin. 8 DELAY An external capacitor connected at this pin sets the time DELAY to assert the rising edge of the RST o. c. after the output voltage is over the reset threshold. If this pin is left floating, RST is like a Power Good. 9 FB 10 SGND Signal GND 11 PGND Power GND Inverting input of the error amplifier DocID027836 Rev 2 5/65 65 Pin settings L6985F Table 1. Pin description (continued) No. Pin 12 PGND 13 LX Switching node 14 LX Switching node 15 VIN DC input voltage 16 VBIAS - E. p. 2.3 Description Power GND Typically connected to the regulated output voltage. An external voltage reference can be used to supply a part of the analog circuitry to increase the efficiency at the light-load. Connect to GND if not used. Exposed pad must be connected to SGND, PGND. Maximum ratings Stressing the device above the rating listed in Table 2: Absolute maximum ratings may cause permanent damage to the device. These are stress ratings only and operation of the device at these or any other conditions above those indicated in the operating sections of this specification is not implied. Exposure to absolute maximum rating conditions may affect device reliability. Table 2. Absolute maximum ratings Symbol Description Min. Max. Unit VIN -0.3 40 V DELAY -0.3 VCC + 0.3 V PGND SGND - 0.3 SGND + 0.3 V SGND V VCC -0.3 (VIN + 0.3) or (max. 4) V SS / INH -0.3 VIN + 0.3 V -0.3 VCC + 0.3 V -0.3 VCC + 0.3 V VOUT -0.3 10 V FSW -0.3 VCC + 0.3 V SYNCH -0.3 VIN + 0.3 V VBIAS -0.3 (VIN + 0.3) or (max. 6) V RST -0.3 VIN + 0.3 V LX -0.3 VIN + 0.3 V -40 150 C MLF COMP See Table 1 TJ Operating temperature range TSTG Storage temperature range -65 to 150 C TLEAD Lead temperature (soldering 10 sec.) 260 C IHS, ILS High-side / low-side switch current 2 A 6/65 DocID027836 Rev 2 L6985F 2.4 Pin settings Thermal data Table 3. Thermal data Symbol 2.5 Parameter Value Unit Rth JA Thermal resistance junction ambient (device soldered on the STMicroelectronics(R) demonstration board) 40 C/W Rth JC Thermal resistance junction to exposed pad for board design (not suggested to estimate TJ from power losses). 5 C/W Value Unit HBM 2 kV MM 200 V CDM 500 V ESD protection Table 4. ESD protection Symbol ESD Test condition DocID027836 Rev 2 7/65 65 Electrical characteristics 3 L6985F Electrical characteristics TJ = 25 C, VIN = 12 V unless otherwise specified. Table 5. Electrical characteristics Symbol Parameter VIN Operating input voltage range VINH VINL IPK IVY ISKIP IVY_SNK Test condition Note Min. Typ. Max. 4 38 VCC UVLO rising threshold 2.7 3.5 VCC UVLO falling threshold 2.4 3.5 Peak current limit Duty cycle < 20% 0.8 Duty cycle = 100% closed loop operation 0.65 Valley current limit Skip current limit Reverse current limit 0.9 LCM, VSYNCH = GND (1) LCM, VSYNCH = VCC (2) LNM or VOUT overvoltage 0.15 0.35 0.5 0.1 0.5 1 2 ISW = 0.5 A 0.36 0.72 RDSON LS Low-side RDSON ISW = 0.5 A 0.15 0.30 fSW Selected switching frequency FSW pinstrapping before SS IFSW FSW biasing current SS ended IMLF D TON MIN MLF biasing current V A RDSON HS High-side RDSON Low noise mode / LCM/LNM Low consumption mode selection Unit See Table 6: fSW selection 0 MLF pinstrapping before SS 500 nA See Table 7 on page 11 SS ended 0 (2) Duty cycle 0 Minimum On time 500 nA 100 % 80 ns VCC regulator VCC LDO output voltage SWO VBIAS threshold (3 V< VBIAS < 5.5 V) 8/65 VBIAS = GND (no switchover) 2.9 3.3 3.6 VBIAS = 5 V (switchover) 2.9 3.3 3.6 Switch internal supply from VIN to VBIAS 2.85 3.2 Switch internal supply from VBIAS to VIN 2.78 3.15 DocID027836 Rev 2 V L6985F Electrical characteristics Table 5. Electrical characteristics (continued) Symbol Parameter Test condition Note Min. Typ. Max. Unit 4 8 15 A 4 10 15 Power consumption ISHTDWN Shutdown current from VIN VSS/INH = GND LCM - SWO VREF < VFB < VOVP (SLEEP) VBIAS = 3.3 V IQ OPVIN IQ OPVBIAS Quiescent current from VIN Quiescent current from VBIAS LCM - NO SWO VREF < VFB < VOVP (SLEEP) VBIAS = GND (3) A (3) 35 70 120 LNM - SWO VFB = GND (NO SLEEP) VBIAS = 3.3 V 0.5 1.5 5 LNM - NO SWO VFB = GND (NO SLEEP) VBIAS = GND 2 2.8 6 25 50 115 A LNM - SWO VFB = GND (NO SLEEP) VBIAS = 3.3 V 0.5 1.2 5 mA SS rising 200 460 700 100 140 LCM - SWO VREF < VFB < VOVP (SLEEP) VBIAS = 3.3 V mA (3) Soft-start VINH VSS threshold VINH HYST VSS hysteresis ISS CH CSS charging current VSS < VINH OR t < TSS SETUP OR VEA+ > VFB t > TSS SETUP AND VEA+ < VFB VSS START SSGAIN (2) mV 1 A (2) Start of internal error amplifier ramp 4 0.995 SS/INH to internal error amplifier gain 1.1 1.150 V 0.85 0.859 V 50 500 nA 3 Error amplifier VOUT Voltage feedback IVOUT VOUT biasing current AV ICOMP 0.841 (2) Error amplifier gain EA output current capability 100 6 DocID027836 Rev 2 12 dB 25 A 9/65 65 Electrical characteristics L6985F Table 5. Electrical characteristics (continued) Symbol Parameter Test condition Note Min. Typ. Max. Unit Inner current loop gCS V PP g CS Current sense transconductance (VCOMP to IPK = 0.5 A inductor current gain) (2) Slope compensation (4) 1.67 A/V 0.2 0.3 0.4 Overvoltage trip (VOVP/VREF) 1.15 1.2 1.25 Overvoltage hysteresis 0.5 2 5 A Overvoltage protection VOVP VOVP HYST % Synchronization (fan out: 6 slave devices typ.) fSYN MIN VSYN TH ISYN VSYN OUT Synchronization frequency LNM; fSW = VCC 266.5 SYNCH input threshold LNM, SYNCH rising 0.70 SYNCH pulldown current LNM, VSYN = 1.2 V High level output LNM, 5 mA sinking load Low level output LNM, 0.7 mA sourcing load Selected RST threshold MLF pinstrapping before SS kHz 1.2 0.7 V mA 1.40 0.6 V Reset VTHR VTHR HYST RST hysteresis VRST RST open collector output See Table 7 on page 11 (2) 2 % VIN > VINH AND VFB < VTH 4 mA sinking load 0.4 2 < VIN < VINH 4 mA sinking load 0.8 V Delay VTHD RST open collector released VFB > VTHR as soon as VDELAY > VTHD ID CH CDELAY charging current 1.19 1.234 1.258 VFB > VTHR 1 2 3 V A Thermal shutdown TSHDWN Thermal shutdown temperature (2) 165 THYS Thermal shutdown hysteresis (2) 30 C 1. Parameter tested in static condition during the testing phase. Parameter value may change over dynamic application condition. 2. Not tested in production. 3. LCM enables SLEEP mode at the light-load. 4. Measured at fsw = 250 kHz. 10/65 DocID027836 Rev 2 L6985F Electrical characteristics TJ = 25 C, VIN = 12 V unless otherwise specified. Table 6. fSW selection Symbol fSW RVCC (E24 series) RGND (E24 series) Tj fSW min. fSW typ. fSW max. 0 NC 225 250 275 1.8 k NC 3.3 k NC 5.6 k NC 380 10 k NC 435 NC 0 18 k NC 33 k NC 56 k NC 755 NC 1.8 k 870 NC 3.3 k NC 5.6 k NC 10 k NC 18 k NC 33 k 285 330 (1) 450 500 550 575 660 (1) 900 kHz 1000 1100 1150 (1) 1310 1500(2) 56 k NC Unit 1575 1750(2) 1925 1800 2000(2) 2200 1. Not tested in production. 2. No synchronization as slave in LNM. TJ = 25 C, VIN = 12 V unless otherwise specified. Table 7. LNM / LCM selection table Symbol VRST RVCC RGND (E24 1%) (E24 1%) 0 NC 8.2 k NC 18 k NC 39 k Operating VRST/VOUT mode (tgt. value) VRST VRST VRST min. typ. max. 93% 0.779 0.791 0.802 80% 0.670 0.680 0.690 87% 0.728 0.740 0.751 NC 96% 0.804 0.816 0.828 NC 0 93% 0.779 0.791 0.802 NC 8.2 k 80% 0.670 0.680 0.690 NC 18 k 87% 0.728 0.740 0.751 NC 39 k 96% 0.804 0.816 0.828 LCM LNM DocID027836 Rev 2 Unit V 11/65 65 Functional description 4 L6985F Functional description The L6985F device is based on a "peak current mode", constant frequency control. As a consequence, the intersection between the error amplifier output and the sensed inductor current generates the PWM control signal to drive the power switch. The device features LNM (low noise mode) that is forced PWM control, or LCM (low consumption mode) to increase the efficiency at the light-load. The main internal blocks shown in the block diagram in Figure 3 are: 12/65 Embedded power elements. Thanks to the P-channel MOSFET as a high-side switch the device features a low dropout operation A fully integrated sawtooth oscillator with adjustable frequency A transconductance error amplifier The high-side current sense amplifier to sense the inductor current A "Pulse Width Modulator" (PWM) comparator and the driving circuitry of the embedded power elements The soft-start blocks to ramp the error amplifier reference voltage and so decreases the inrush current at power-up. The SS/INH pin inhibits the device when driven low. The switchover capability of the internal regulator to supply a portion of the quiescent current when the VBIAS pin is connected to an external output voltage The synchronization circuitry to manage a master / slave operation and the synchronization to an external clock The current limitation circuit to implement the constant current protection, sensing a pulse-by-pulse high-side / low-side switch current. In case of heavy short-circuit the current protection is fold back to decrease the stress of the external components A circuit to implement the thermal protection function The OVP circuitry to discharge the output capacitor in case of an overvoltage event The MLF pin strapping sets the LNM/LCM mode and the thresholds of the RST comparator FSW pinstrapping sets the switching frequency The RST open collector output DocID027836 Rev 2 L6985F Functional description Figure 3. Internal block diagram 4.1 Power supply and voltage reference The internal regulator block consists of a start-up circuit, the voltage pre-regulator that provides the current to all the blocks and the bandgap voltage reference. The starter supplies the startup current when the input voltage goes high and the device is enabled (SS/INH pin over the inhibits threshold). The pre-regulator block supplies the bandgap cell and the rest of the circuitry with a regulated voltage that has a very low supply voltage noise sensitivity. Switchover feature The switchover scheme of the pre-regulator block features to derive the main contribution of the supply current for the internal circuitry from an external voltage (3 V < VBIAS < 5.5 V is typically connected to the regulated output voltage). This helps to decrease the equivalent quiescent current seen at VIN. (Please refer to Section 4.6: Switchover feature on page 28). 4.2 Voltages monitor An internal block continuously senses the VCC, VBIAS and VBG. If the monitored voltages are good, the regulator starts operating. There is also a hysteresis on the VCC (UVLO). DocID027836 Rev 2 13/65 65 Functional description L6985F Figure 4. Internal circuit 9&& 67$57(5 35(5(*8/$725 95(* %$1'*$3 ,&%,$6 95() ',1 4.3 Soft-start and inhibit The soft-start and inhibit features are multiplexed on the same pin. An internal current source charges the external soft-start capacitor to implement a voltage ramp on the SS/INH pin. The device is inhibited as long as the SS/INH pin voltage is lower than the VINH threshold and the soft-start takes place when the SS/INH pin crosses VSS START. (See Figure 5). The internal current generator sources a 1 A typ. current when the voltage of the VCC pin crosses the UVLO threshold. The current increases to 4 A typ. as soon as the SS/INH voltage is higher than the VINH threshold. This feature helps to decrease the current consumption in inhibit mode. An external open collector can be used to set the inhibit operation clamping the SS/INH voltage below the VINH threshold. The startup feature minimizes the inrush current and decreases the stress of the power components during the power-up phase. The ramp implemented on the reference of the error amplifier has a gain three times higher (SSGAIN) than the external ramp present at the SS/INH pin. 14/65 DocID027836 Rev 2 L6985F Functional description Figure 5. Soft-start phase The CSS is dimensioned accordingly with Equation 1: Equation 1 I SSCH T SS 4A T SS C SS = SS GAIN -------------------------------- = 3 --------------------------V FB 0.85V where TSS is the soft-start time, ISS CH the charging current and VFB the reference of the error amplifier. The soft-start block supports the precharged output capacitor. DocID027836 Rev 2 15/65 65 Functional description L6985F Figure 6. Soft-start phase with precharged COUT During the normal operation a new soft-start cycle takes place in case of: Thermal shutdown event UVLO event The device is driven in INH mode The soft-start capacitor is discharged with a 0.6 mA typ. current capability for 1 msec time max. For complete and proper capacitor discharge in case of fault condition, a maximum CSS = 67 nF value is suggested. The application example in Figure 7 shows how to enable the L6985F and perform the softstart phase driven by an external voltage step. Figure 7. Enable the device with external voltage step 16/65 DocID027836 Rev 2 L6985F Functional description The maximum capacitor value has to be limited to guarantee the device can discharge it in case of a thermal shutdown and UVLO events (see Section 4.3.1), so restart the switching activity ramping the error amplifier reference voltage. Equation 2 - 1 msec C SS ------------------------------------------------------------------------------------------V SS_FINAL - 0.9 V R SS_EQ ln 1 - ---------------------------------------------- 600 A - R SS_EQ where: Equation 3 R UP R DWN R SS_EQ = --------------------------------R UP + R DWN R DWN V SS_FINAL = V STEP - V DIODE ---------------------------------R UP + R DWN The optional diode prevents to disable the device if the external source drops to ground. RUP value is selected in order to make the capacitor charge at first approximation independent from the internal current generator (4 A typ. current capability, see Table 5 on page 8), so: Equation 4 V STEP - V DIODE - V SS END ----------------------------------------------------------------------- I SS CHARGE 4 A R UP where: Equation 5 V FB V SS END = V SS START + --------------------SS GAIN represents the SS/INH voltage correspondent to the end of the ramp on the error amplifier (see Figure 5); refer to Table 5 for VSS START, VFB and SSGAIN parameters. As a consequence the voltage across the soft-start capacitor can be written as: Equation 6 1 v SS t = V SS_FINAL ----------------------------------------t 1-e - --------------------------------C SS R SS_EQ RSS_DOWN is selected to guarantee the device stays in inhibit mode when the internal generator sources 1 A typ. out of the SS/INH pin and VSTEP is not present: Equation 7 R DWN I SS INHIBIT R DWN 1 A V INH 200 mV so: Equation 8 R DWN 100 k DocID027836 Rev 2 17/65 65 Functional description L6985F RUP and RDWN are selected to guarantee: Equation 9 V SS_FINAL 2 V V SS_END The time to ramp the internal voltage reference can be calculated from Equation 10: Equation 10 V SS_FINAL - V SS START T SS = C SS R SS_EQ ln ----------------------------------------------------------- V SS_FINAL - V SS END that is the equivalent soft-start time to ramp the output voltage. Figure 8 shows the soft-start phase with the following component selection: RUP = 180 k, RDWN = 33 k, CSS = 200 nF, the 1N4148 is a small signal diode and VSTEP = 13 V. Figure 8. External soft-start network VSTEP driven The circuit in Figure 7 introduces a time delay between VSTEP and the switching activity that can be calculated as: Equation 11 V SS_FINAL T SS DELAY = C SS R SS_EQ ln ----------------------------------------------------------- V SS_FINAL - V SS START 18/65 DocID027836 Rev 2 L6985F Functional description Figure 9 shows how the device discharges the soft-start capacitor after an UVLO or thermal shutdown event in order to restart the switching activity ramping the error amplifier reference voltage. Figure 9. External soft-start after UVLO or thermal shutdown DocID027836 Rev 2 19/65 65 Functional description 4.3.1 L6985F Ratiometric startup The ratiometric startup is implemented sharing the same soft-start capacitor for a set of the L6985F devices. Figure 10. Ratiometric startup 9 9287 9287 9287 W $0 As a consequence all the internal current generators charge in parallel the external capacitor. The capacitor value is dimensioned accordingly with Equation 12: Equation 12 I SSCH T SS 4A T SS C SS = n L6985F SS GAIN -------------------------------- = n L6985F 3 --------------------------0.85V V FB where nL6985F represents the number of devices connected in parallel. For better tracking of the different output voltages the synchronization of the set of regulators is suggested. 20/65 DocID027836 Rev 2 L6985F Functional description Figure 11. Ratiometric startup operation DocID027836 Rev 2 21/65 65 Functional description 4.3.2 L6985F Output voltage sequencing The L6985F device implements sequencing connecting the RST pin of the master device to the SS/INH of the slave. The slave is inhibited as long as the master output voltage is outside of the regulation so implementing the sequencing (see Figure 12). Figure 12. Output voltage sequencing 9 9287 9287 9287 W W'(/$< W'(/$< W'(/$< $0 High flexibility is achieved thanks to the programmable RST thresholds (see Table 7 on page 11) and programmable delay time. To minimize the component count the DELAY pin capacitor can be also omitted so the pin works as a normal Power Good. 4.4 Error amplifier The voltage error amplifier is the core of the loop regulation. It is a transconductance operational amplifier whose non inverting input is connected to the internal voltage reference (0.85 V), while the inverting input (FB) is connected to the external divider or directly to the output voltage. Table 8. Uncompensated error amplifier characteristics Description Values Transconductance 155 S Low frequency gain 100 dB The error amplifier output is compared with the inductor current sense information to perform PWM control. The error amplifier also determines the burst operation at the lightload when the LCM is active. 22/65 DocID027836 Rev 2 L6985F 4.5 Functional description Light-load operation The MLF pinstrapping during the power-up phase determines the light-load operation (refer to Table 7 on page 11). 4.5.1 Low noise mode (LNM) The low noise mode implements a forced PWM operation over the different loading conditions. The LNM features a constant switching frequency to minimize the noise in the final application and a constant voltage ripple at fixed VIN. The regulator in steady loading condition never skip pulses and it operates in the continuous conduction mode (CCM) over the different loading conditions thus making this operation mode ideal for noise sensitive applications. Figure 13. Low noise mode operation DocID027836 Rev 2 23/65 65 Functional description 4.5.2 L6985F Low consumption mode (LCM) The low consumption mode maximizes the efficiency at the light-load. The regulator prevents the switching activity whenever the switch peak current request is lower than the ISKIP threshold (350 mA typical). As a consequence the L6985F device works in bursts and it minimizes the quiescent current request in the meantime between the switching operation. In the LCM operation, the pin SYNCH/ISKIP level dynamically defines the ISKIP current threshold (see Table 9). Table 9. ISKIP current level SYNCH / ISKIP (pin 4) ISKIP current threshold LOW ISKIPH = 0.35 A typical HIGH ISKIPL = 0.1 A typical The ISKIP programmability helps to optimize the performance in terms of the output voltage ripple or efficiency at the light-load, that are parameters which disagree each other by definition. A lower skip current level minimizes the voltage ripple but increases the switching activity (time between bursts gets closer) since less energy per burst is transfered to the output voltage at the given load. On the other side, a higher skip level reduces the switching activity and improves the efficiency at the light-load but worsen the voltage ripple. No difference in terms of the voltage ripple and conversion efficiency for the medium and high load current level, that is when the device operates in the discontinuous or continuous mode (DCM vs. CCM). Figure 14 and Figure 15 report the efficiency measurements with VOUT = 3.3 V to highlight the ISKIPH and ISKIPL efficiency gap at the light-load also in comparison with the LNM operation. The same efficiency at the medium / high load is confirmed at different ISKIP levels. Figure 14. Light-load efficiency comparison at different ISKIP - linear scale 24/65 DocID027836 Rev 2 L6985F Functional description Figure 15. Light-load efficiency comparison at different ISKIP - log scale Figure 16 and Figure 17 show the LCM operation at the different ISKIP level. Figure 16 shows the ISKIPH = 350 mA typ. and so 34 mV output voltage ripple. Figure 17 shows the ISKIPL = 100 mA typ. and so 13 mV output voltage ripple. Figure 16. LCM operation with ISKIPH = 350 mA typ. at zero load DocID027836 Rev 2 25/65 65 Functional description L6985F Figure 17. LCM operation with ISKIPH = 100 mA typ. at zero load Given the energy stored in the inductor during a burst, the voltage ripple depends on the capacitor value: Equation 13 T V OUT RIPPLE 26/65 BURST Q IL 0 iL t dt = -------------- = -------------------------------------------C OUT C OUT DocID027836 Rev 2 L6985F Functional description Figure 18. LCM operation over loading condition (part 1) Figure 19. LCM operation over loading condition (part 2) DocID027836 Rev 2 27/65 65 Functional description L6985F Figure 20. The regulator works in CCM 4.6 Switchover feature The switchover maximizes the efficiency at the light-load that is crucial for LCM applications. 4.6.1 LCM The LCM operation satisfies the high efficiency requirements of the battery powered applications. In order to minimize the regulator quiescent current request from the input voltage, the VBIAS pin can be connected to an external voltage source in the range 3 V < VBIAS < 5.5 V (see Section 4.1: Power supply and voltage reference on page 13). In case the VBIAS pin is connected to the regulated output voltage (VOUT), the total current drawn from the input voltage can be calculated as: Equation 14 V BIAS 1 I QVIN = I QOPVIN + -------------------- --------------- I QOPVBIAS V IN L6985F where IQ OP VIN, IQ OP VBIAS are defined in Table 5: Electrical characteristics on page 8 and L6985F is the efficiency of the conversion in the working point. 4.6.2 LNM Equation 14 is also valid when the device works in LNM and it can increase the efficiency at the medium load since the regulator always operates in the continuous conduction mode. 28/65 DocID027836 Rev 2 L6985F 4.7 Functional description Overcurrent protection The current protection circuitry features constant current protection, so the device limits the maximum peak current (see Table 5: Electrical characteristics on page 8) in overcurrent condition. The L6985F device implements a pulse-by-pulse current sensing on both power elements (high-side and low-side switches) for effective current protection over the duty cycle range. The high-side current sensing is called "peak", the low-side sensing "valley". The internal noise generated during the switching activity makes the current sensing circuitry ineffective for a minimum conduction time of the power element. This time is called "masking time" because the information from the analog circuitry is masked by the logic to prevent erroneous detection of the overcurrent event. As a consequence, the peak current protection is disabled for a masking time after the high-side switch is turned on, the valley for a masking time after the low-side switch is turned on. In other words, the peak current protection can be ineffective at extremely low duty cycles, the valley current protection at extremely high duty cycles. The L6985F device assures an effective overcurrent protection sensing the current flowing in both power elements. In case one of the two current sensing circuitry is ineffective because of the masking time, the device is protected sensing the current on the opposite switch. Thus, the combination of the "peak" and "valley" current limits assure the effectiveness of the overcurrent protection even in extreme duty cycle conditions. The valley current threshold is designed higher than the peak to guarantee a proper operation. In case the current diverges because of the high-side masking time, the low-side power element is turned on until the switch current level drops below the valley current sense threshold. The low-side operation is able to prevent the high-side turn on, so the device can skip pulses decreasing the switching frequency. DocID027836 Rev 2 29/65 65 Functional description L6985F Figure 21. Valley current sense operation in overcurrent condition Figure 21 shows the switching frequency reduction during the valley current sense operation in case of an extremely low duty cycle (VIN = 38 V, fSW = 2 MHz short-circuit condition). In a worst case scenario (like in Figure 21) of the overcurrent protection the switch current is limited to: Equation 15 V IN - V OUT I MAX = I VALLEYTH + ------------------------------ T MASKHS L where IVALLEY_TH is the current threshold of the valley sensing circuitry (see Table 5: Electrical characteristics on page 8) and TMASK_HS is the masking time of the high-side switch 100 nsec. typ.). In most of the overcurrent conditions the conduction time of the high-side switch is higher than the masking time and so the peak current protection limits the switch current. Equation 16 IMAX = IPEAK_TH 30/65 DocID027836 Rev 2 L6985F Functional description Figure 22. Peak current sense operation in overcurrent condition The DC current flowing in the load in overcurrent condition is: Equation 17 I RIPPLE V OUT V IN - V OUT I DCOC V OUT = I MAX - ---------------------------------------- = I MAX - ------------------------------ T ON 2 2L DocID027836 Rev 2 31/65 65 Functional description L6985F OCP and switchover feature Output capacitor discharging the current flowing to ground during heavy short-circuit events is only limited by parasitic elements like the output capacitor ESR and short-circuit impedance. Due to parasitic inductance of the short-circuit impedance, negative output voltage oscillations can be generated with huge discharging current levels (see Figure 23). Figure 23. Output voltage oscillations during heavy short-circuit inductor current short-circuit current switching node regulated output voltage 32/65 DocID027836 Rev 2 L6985F Functional description Figure 24. Zoomed waveform inductor current short-circuit current switching node regulated output voltage The VBIAS pin absolute maximum ratings (see Table 2: Absolute maximum ratings on page 6) must be satisfied over the different dynamic conditions. If the VBIAS is connected to GND there are no issues (see Figure 23 and Figure 24). A small resistor value (few ) in series with the VBIAS can help to limit the pin negative voltage (see Figure 25) during heavy short-circuit events if it is connected to the regulated output voltage. DocID027836 Rev 2 33/65 65 Functional description L6985F Figure 25. VBIAS in heavy short-circuit event inductor current switching node VBIAS pin voltage (cyan) regulated output voltage (purple) 4.8 Overvoltage protection The overvoltage protection monitors the FB pin and enables the low-side MOSFET to discharge the output capacitor if the output voltage is 20% over the nominal value. This is a second level protection and should never be triggered in normal operating conditions if the system is properly dimensioned. In other words, the selection of the external power components and the dynamic performance determined by the compensation network should guarantee an output voltage regulation within the overvoltage threshold even during the worst case scenario in term of load transitions. The protection is reliable and also able to operate even during normal load transitions for a system whose dynamic performance is not in line with the load dynamic request. As a consequence the output voltage regulation would be affected. Figure 26 shows the overvoltage operation during a negative steep load transient for a system designed with huge inductor value and a small output capacitor. The inductor value limits the switch current slew rate and the extra charge flowing into the small capacitor value generates an overvoltage event. This can be considered as an example for a system with a dynamic performance not in line with the load request. The L6985F device implements a 1 A typ. negative current limitation to limit the maximum reversed switch current during the overvoltage operation. 34/65 DocID027836 Rev 2 L6985F Functional description Figure 26. Overvoltage operation inductor current switching node VBIAS pin voltage (cyan) regulated output voltage (purple) 4.9 Thermal shutdown The shutdown block disables the switching activity if the junction temperature is higher than a fixed internal threshold (165 C typical). The thermal sensing element is close to the power elements, ensuring fast and accurate temperature detection. A hysteresis of approximately 30 C prevents the device from turning ON and OFF continuously. When the thermal protection runs away a new soft-start cycle will take place. DocID027836 Rev 2 35/65 65 Closing the loop 5 L6985F Closing the loop Figure 27. Block diagram of the loop 9,1 3:0FRQWURO &XUUHQWVHQVH +6 VZ LWFK /&ILOWHU 5HVLVWRUGLYLGHU / ,+6 J &6 /6 &287 VZ LWFK 5 &RPSHQVDWLRQ QHWZRUN 3:0FRPSDUDWRU &3 5& )% 5/2$' 95() 5 (UURUDPSOLILHU && $0 5.1 GCO(s) control to output transfer function The accurate control to output transfer function for a buck peak current mode converter can be written as: Equation 18 G CO s s 1 + ---- z 1 = R LOAD g cs -------------------------------------------------------------------------------------------------------- ---------------------- F H s R LOAD T SW s ------ 1 + ----------------------------------- m C 1 - D - 0.5 1 + L p where RLOAD represents the load resistance, gCS the equivalent sensing conductance of the current sense circuitry, p the single pole introduced by the power stage and z the zero given by the ESR of the output capacitor. FH(s) accounts the sampling effect performed by the PWM comparator on the output of the error amplifier that introduces a double pole at one half of the switching frequency. 36/65 DocID027836 Rev 2 L6985F Closing the loop Equation 19 1 z = --------------------------------ESR C OUT Equation 20 m c 1 - D - 0.5 1 p = --------------------------------------- + ---------------------------------------------L C OUT f SW R LOAD C OUT where: Equation 21 Se m C = 1 + -----Sn S = V g f PP CS SW e V - V IN OUT S = --------------------------- n L Sn represents the on time slope of the sensed inductor current, Se the on time slope of the external ramp (VPP peak-to-peak amplitude) that implements the slope compensation to avoid sub-harmonic oscillations at the duty cycle over 50%. Se can be calculated from the parameter VPP x gCS given in Table 5 on page 8. The sampling effect contribution FH(s) is: Equation 22 1 F H s = --------------------------------------------2 s s 1 + -------------------- + --------2n Qp n where: Equation 23 1 Q p = ----------------------------------------------------------- m c 1 - D - 0.5 DocID027836 Rev 2 37/65 65 Closing the loop 5.2 L6985F Error amplifier compensation network The typical compensation network required to stabilize the system is shown in Figure 28. Figure 28. Transconductance embedded error amplifier 95() )% ($ &203 5& &3 && 9 5 G9 *P G9 & 5& &3 && 9 $0 RC and CC introduce a pole and a zero in the open loop gain. CP does not significantly affect system stability but it is useful to reduce the noise at the output of the error amplifier. The transfer function of the error amplifier and its compensation network is: Equation 24 A V0 1 + s R c C c A 0 s = --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------2 s R0 C0 + Cp Rc Cc + s R0 Cc + R0 C0 + Cp + Rc Cc + 1 where Avo = Gm * Ro The poles of this transfer function are (if Cc >> C0 + CP): Equation 25 1 f PLF = ------------------------------------2 R0 Cc 38/65 DocID027836 Rev 2 L6985F Closing the loop Equation 26 whereas the zero is defined as: 1 f PHF = -------------------------------------------------------2 R0 C0 + Cp Equation 27 1 f Z = ------------------------------------2 Rc Cc 5.3 Voltage divider The simple voltage divider contribution is: Equation 28 R2 G DIV s = -------------------R1 + R2 A small signal capacitor in parallel to the upper resistor (see Figure 29) of the voltage divider implements a leading network (fzero < fpole), sometimes necessary to improve the system phase margin. Figure 29. Leading network example Laplace transformer of the leading network: Equation 29 1 + s R 1 C R1 R2 G DIV s = -------------------- ------------------------------------------------------------R1 + R2 R1 R2 1 + s -------------------- C R1 R1 + R2 DocID027836 Rev 2 39/65 65 Closing the loop L6985F where: Equation 30 1 f Z = ----------------------------------------2 R 1 C R1 1 f p = ------------------------------------------------------R1 R2 2 -------------------- C R1 R1 + R2 fZ fp 5.4 Total loop gain In summary, the open loop gain can be expressed as: Equation 31 G s = G DIV s G CO s A 0 s Example 1 VIN = 12 V VOUT = 3.3 V and ROUT = 6.6 Selecting L = 4.7H, COUT = 10 F and ESR = 1 m, RC= 110 k, CC= 68 pF, CP = 1.2 pF (please refer to Example 2), the gain and phase bode diagrams are plotted respectively in Figure 30 and Figure 31. Equation 32 BW = 108kHz phase margin = 64 40/65 DocID027836 Rev 2 0 L6985F Closing the loop Figure 30. Module plot Figure 31. Phase plot The solid trace represents the transfer function including the sampling effect term (see Equation 22 on page 37), the dotted trace neglects the contribution. DocID027836 Rev 2 41/65 65 Closing the loop 5.5 L6985F Compensation network design The maximum bandwidth of the system can be designed up to fSW/6 or up to 150 kHz to guarantee a valid small signal model. Equation 33 f SW BW = min --------- 150kHz 6 Equation 34 2 BW C OUT V OUT R C = ---------------------------------------------------------------0.85V g CS g m TYP where gCS represents the current sense transconductance (see Table 5: Electrical characteristics on page 8) and gm TYP the error amplifier transconductance. Equation 35 5 C C = -------------------------------------2 R C BW Example 2 Considering L6985F, VIN = 12 V, VOUT = 3.3 V, L = 4.7 H, COUT = 10 F, fSW = 2 MHz. Assuming to design the compensation network to achieve system bandwidth of 110 kHz: Equation 36 ** f POLE = 2. 4kHz Equation 37 V OUT R LOAD = -------------- = 6.6 I OUT so accordingly with Equation 34 and Equation 35: Equation 38 R C = 107k 110k Equation 39 C C = 66pF 68pF 42/65 DocID027836 Rev 2 L6985F Application notes 6 Application notes 6.1 Output voltage adjustment The internal reference is equal to 0.85 V typical, and the output voltage is adjusted accordingly to Equation 40 (see Figure 32): Equation 40 R1 R1 V OUT = V REF 1 + ------- = 0.85 1 + ------- R2 R2 where the Cr1 capacitor is sometimes useful to increase the small signal phase margin (please refer to Section 5.5: Compensation network design). Figure 32. L6985F application circuit 6.2 Switching frequency A resistor connected to the FSW pin features the selection of the switching frequency. The pinstrapping is performed at power-up, before the soft-start takes place. The FSW pin is pinstrapped and then driven floating in order to minimize the quiescent current from VIN. Please refer to Table 6: fSW selection on page 11 to identify the pull-up / pull-down resistor value. fSW = 250 kHz / fSW = 500 kHz preferred codifications don't require any external resistor. DocID027836 Rev 2 43/65 65 Application notes 6.3 L6985F MLF pin A resistor connected to the MLF pin features the selection of the between low noise mode / low consumption mode and the different RST thresholds. The pinstrapping is performed at power-up, before the soft-start takes place. The FSW pin is pinstrapped and then driven floating in order to minimize the quiescent current from VIN. Please refer to Table 7 on page 11 to identify the pull-up / pull-down resistor value. (LNM, RST threshold 93%) / (LCM, RST threshold 93%) preferred codifications don't require any external resistor. 6.4 Voltage supervisor The embedded voltage supervisor (composed of the RST and the DELAY pins) monitors the regulated output voltage and keeps the RST open collector output in low impedance as long as the VOUT is out of regulation. In order to ensure a proper reset of digital devices with a valid power supply, the device can delay the RST assertion with a programmable time. Figure 33. Voltage supervisor operation The comparator monitoring the FB voltage has four different programmable thresholds (80%, 87%, 93%, 96% nominal output voltage) for high flexibility (see Section 6.3: MLF pin and Table 7 on page 11). When the RST comparator detects the output, voltage is in regulation, a 2 A internal current source starts to charge an external capacitor to implement a voltage ramp on the DELAY pin. The RST open collector is then released as soon as VDELAY = 1.234 V (see Figure 33). 44/65 DocID027836 Rev 2 L6985F Application notes The CDELAY is dimensioned accordingly with Equation 41: Equation 41 I SSCH T DELAY 2A T DELAY C DELAY = ------------------------------------------ = ------------------------------------V DELAY 1.234V The maximum suggested capacitor value is 270 nF. 6.5 Synchronization (LNM) Beating frequency noise is an issue when multiple switching regulators populate the same application board. The L6985F synchronization circuitry features the same switching frequency for a set of regulators simply connecting their SYNCH pin together, so preventing beating noise. The master device provides the synchronization signal to the others since the SYNCH pin is I/O able to deliver or recognize a frequency signal. For proper synchronization of multiple regulators, all of them have to be configured with the same switching frequency (see Table 6 on page 11), so the same resistor connected at the FSW pin. In order to minimize the RMS current flowing through the input filter, the L6985F device provides a phase shift of 180 between the master and the SLAVES. If more than two devices are synchronized, all slaves will have a common 180 phase shift with respect to the master. Considering two synchronized L6985F devices which regulates the same output voltage (i.e.: operating with the same duty cycle), the input filter RMS current is optimized and is calculated as: Equation 42 I RMS I OUT ----------- 2D 1 - 2D 2 = I OUT - 2D - 1 2 - 2D ---------- 2 if D < 0.5 if D > 0.5 The graphical representation of the input RMS current of the input filter in the case of two devices with a 0 phase shift (synchronized to an external signal) or a 180 phase shift (synchronized connecting their SYNCH pins) regulating the same output voltage is provided in Figure 34. To dimension the proper input capacitor please refer to Section 6.6.1: Input capacitor selection on page 50. DocID027836 Rev 2 45/65 65 Application notes L6985F Figure 34. Input RMS current 506FXUUHQWQRUPDOL]HG ,UPV,287 WZR UHJXODWRUV RSHUDWLQJ LQ SKDVH WZR UHJXODWRUV RSHUDWLQJ RXW RI SKDVH 'XW\F\FOH $09 Figure 35 shows two regulators not synchronized. Figure 35. Two regulators not synchronized 46/65 DocID027836 Rev 2 L6985F Application notes Figure 36 shows the same regulators working synchronized. The MASTER regulator (LX_5V trace) delivers the synchronization signal (SYNCH_3V3, SYNCH_5V pins are connected together) to the SLAVE device (LX_3V3). The SLAVE regulator works in phase with the synchronization signal which is out of phase with the MASTER switching operation. Figure 36. Two regulators synchronized Multiple L6985F devices can be synchronized to an external frequency signal fed to the SYNCH pin. In this case the regulator set is phased to the reference and all the devices will work with a 0 phase shift. The frequency range of the synchronization signal is 275 kHz - 1.4 MHz and the minimum pulse width is 100 nsec (see Figure 37). Figure 37. Synchronization pulse definition L)[ G4:/$)30 .)[ G4:/$)30 OTFDNJO G4:/$)30 OTFDNJO ".7 Since the slope compensation contribution that is required to prevent subharmonic oscillations in peak current mode architecture depends on the switching frequency, it is important to select the same oscillator frequency for all regulators (all of them operate as SLAVE) as close as possible to the frequency of the reference signal (please refer to Table 6: fSW selection on page 11). As a consequence all the regulators have the same DocID027836 Rev 2 47/65 65 Application notes L6985F resistor value connected to the FSW pin, so the slope compensation is optimized accordingly with the frequency of the synchronization signal. The slope compensation contribution is latched at power-up and so fixed during the device operation. The L6985F normally operates in MASTER mode, driving the SYNCH line at the selected oscillator frequency as shown in Figure 35 and Figure 36. In SLAVE mode the L6985F sets the internal oscillator at 250 kHz typ. (see Table 6 on page 11 - first row) and drives the line accordingly. Figure 38. L6985F synchronization driving capability In order to safely guarantee that each regulator recognizes itself in SLAVE mode during the normal operation, the external master must drive the SYNCH pin with a clock signal frequency higher than the maximum oscillator spread (refer to Table 6) for at least 10 internal clock cycles. Example 3 Selecting RFSW = 0 to GND. Table 10. Example of oscillator frequency selection from Table 6 Symbol RVCC (E24 series) RGND (E24 series) fSW min. fSW typ. fSW max. fSW NC 0 450 500 550 The device enters into the slave mode after 10 pulses at frequency higher than 550 kHz and so it is able to synchronize to a clock signal in the range 275 kHz - 1.4 MHz (see Figure 37). Anyway it is suggested to limit the frequency range within 20% fSW resistor nominal frequency (see details in text below). If a not spread spectrum is required, all the regulators synchronize to a frequency higher to the maximum oscillator spread (550 kHz in Example 3). The device keeps operating in slave mode as far as the master is able to drive the SYNCH pin faster than 275 kHz (maximum oscillator spread for 250 kHz oscillator), otherwise it goes back into MASTER mode at the nominal oscillator frequency after successfully driving one pulse at 250 kHz (see Figure 39) in the SYNCH line. 48/65 DocID027836 Rev 2 L6985F Application notes Figure 39. Slave to master mode transition switching node SLAVE mode 250kHz typ. standalone operation at nominal fsw SYNCH signal The external master can force a latched SLAVE mode driving the SYNCH pin low at powerup, before the soft-start starts the switching activity. So the oscillator frequency is 250 kHz typ. fixed until a new UVLO event is triggered regardless FSW resistor value, that otherwise counts to design the slope compensation. The same considerations above are also valid. The master driving capability must be able to provide the proper signal levels at the SYNCH pin (see Table 5 on page 8 - Synchronization section): Low level < VSYN THL= 0.7 V sinking 5 mA High level > VSYN THH = 1.2 V sourcing 0.7 mA Figure 40. Master driving capability to synchronize the L6985F 7$$. N" 74:/@5)@) N" 74:/@5)@- 3) 3- ". DocID027836 Rev 2 49/65 65 Application notes L6985F As anticipated above, in the SLAVE mode the internal oscillator operates at 250 kHz typ. but the slope compensation is dimensioned accordingly with FSW resistors so, even if the L6985F supports synchronization over the 275 kHz - 1.4 MHz frequency range, it is important to limit the switching operation around a working point close to the selected frequency (FSW resistor). As a consequence, to guarantee the full output current capability and to prevent the subharmonic oscillations the master must limit the driving frequency range within 20% of the selected frequency. A wider frequency range may generate subharmonic oscillation for duty > 50% or limit the peak current capability (see IPK parameter in Table 5 on page 8) since the internal slope compensation signal may be saturated. In order to guarantee the synchronization as a slave over distribution, temperature and the output load, the external clock frequency must be lower than 1.4 MHz. 6.6 Design of the power components 6.6.1 Input capacitor selection The input capacitor voltage rating must be higher than the maximum input operating voltage of the application. During the switching activity a pulsed current flows into the input capacitor and so its RMS current capability must be selected accordingly with the application conditions. Internal losses of the input filter depends on the ESR value so usually low ESR capacitors (like multilayer ceramic capacitors) have a higher RMS current capability. On the other hand, given the RMS current value, lower ESR input filter has lower losses and so contributes to higher conversion efficiency. The maximum RMS input current flowing through the capacitor can be calculated as: Equation 43 D D I RMS = I OUT 1 - ---- --- Where IOUT is the maximum DC output current, D is the duty cycles, is the efficiency. This function has a maximum at D = 0.5 and, considering = 1, it is equal to IOUT/2. In a specific application the range of possible duty cycles has to be considered in order to find out the maximum RMS input current. The maximum and minimum duty cycles can be calculated as: Equation 44 V OUT + V LOWSIDE D MAX = -----------------------------------------------------------------------------------------------V INMIN + V LOWSIDE - V HIGHSIDE Equation 45 V OUT + V LOWSIDE D MIN = -------------------------------------------------------------------------------------------------V INMAX + V LOWSIDE - V HIGHSIDE Where VHIGH_SIDE and VLOW_SIDE are the voltage drops across the embedded switches. 50/65 DocID027836 Rev 2 L6985F Application notes The peak-to-peak voltage across the input filter can be calculated as: Equation 46 I OUT D D V PP = ------------------------- 1 - ---- ---- + ESR I OUT + I L C IN f SW In case of negligible ESR (MLCC capacitor) the equation of CIN as a function of the target VPP can be written as follows: Equation 47 I OUT D D C IN = -------------------------- 1 - ---- ---V PP f SW Considering = 1 this function has its maximum in D = 0.5: Equation 48 I OUT C INMIN = ---------------------------------------------4 V PPMAX f SW Typically CIN is dimensioned to keep the maximum peak-to-peak voltage across the input filter in the order of 5% VIN_MAX. Table 11. Input capacitors Manufacturer TDK Taiyo Yuden 6.6.2 Series Size Cap value (F) Rated voltage (V) C3225X7S1H106M 1210 10 50 C3216X5R1H106M 1206 UMK325BJ106MM-T 1210 Inductor selection The inductor current ripple flowing into the output capacitor determines the output voltage ripple (please refer to Section 6.6.3). Usually the inductor value is selected in order to keep the current ripple lower than 20% - 40% of the output current over the input voltage range. The inductance value can be calculated by Equation 49: Equation 49 V OUT V IN - V OUT I L = ------------------------------ T ON = -------------- T OFF L L Where TON and TOFF are the on and off time of the internal power switch. The maximum current ripple, at fixed VOUT, is obtained at maximum TOFF that is at a minimum duty cycle (see Section 6.6.1: Input capacitor selection to calculate minimum duty). DocID027836 Rev 2 51/65 65 Application notes L6985F So fixing IL = 20% to 40% of the maximum output current, the minimum inductance value can be calculated: Equation 50 V OUT 1 - D MIN L MIN = ------------------- ----------------------F SW I LMAX where fSW is the switching frequency 1/(TON + TOFF). For example for VOUT = 3.3 V, VIN = 12 V, IOUT = 0.5 A and FSW = 2 MHz the minimum inductance value to have IL = 30% of IOUT is about 8.2 H. The peak current through the inductor is given by: Equation 51 I L I L PK = I OUT + -------2 So if the inductor value decreases, the peak current (that has to be lower than the current limit of the device) increases. The higher is the inductor value, the higher is the average output current that can be delivered, without reaching the current limit. In Table 12 some inductor part numbers are listed. Table 12. Inductors 6.6.3 Manufacturer Series Inductor value (H) Saturation current (A) Coilcraft XAL40xx 2.2 to 15 5.6 to 2.8 XAL50xx 2.2 to 22 9.2 to 3.6 XFL40xx 2.2 to 4.7 3.7 to 2.7 Output capacitor selection The triangular shape current ripple (with zero average value) flowing into the output capacitor gives the output voltage ripple, that depends on the capacitor value and the equivalent resistive component (ESR). As a consequence the output capacitor has to be selected in order to have a voltage ripple compliant with the application requirements. The voltage ripple equation can be calculated as: Equation 52 I LMAX V OUT = ESR I LMAX + --------------------------------------8 C OUT f SW Usually the resistive component of the ripple can be neglected if the selected output capacitor is a multi layer ceramic capacitor (MLCC). The output capacitor is important also for loop stability: it determines the main pole and the zero due to its ESR. (See Section 5: Closing the loop on page 36 to consider its effect in the system stability). 52/65 DocID027836 Rev 2 L6985F Application notes For example with VOUT = 3.3 V, VIN = 12 V, IL = 0.25 A, fSW = 2 MHz (resulting by the inductor value) and COUT = 10 F MLCC: Equation 53 0 * 25 V OUT I LMAX 1 1 * 6mV 1 ------------------ -------------- ------------------------------ = -------- ---------------------------------------------- = ------------------- = 0.05% V OUT V OUT C OUT f SW 8 10F 2MHz 3.3 3.3 The output capacitor value has a key role to sustain the output voltage during a steep load transient. When the load transient slew rate exceeds the system bandwidth, the output capacitor provides the current to the load. In case the final application specifies a high slew rate load transient, the system bandwidth must be maximized and the output capacitor has to sustain the output voltage for time response shorter than the loop response time. In Table 13 some capacitor series are listed. Table 13. Output capacitors Manufacturer Series Cap value (F) Rated voltage (V) ESR (m) GRM32 22 to 100 6.3 to 25 <5 GRM31 10 to 47 6.3 to 25 <5 ECJ 10 to 22 6.3 <5 EEFCD 10 to 68 6.3 15 to 55 SANYO TPA/B/C 100 to 470 4 to 16 40 to 80 TDK C3225 22 to 100 6.3 <5 MURATA PANASONIC DocID027836 Rev 2 53/65 65 Application board 7 L6985F Application board The reference evaluation board schematic is shown in Figure 41. Figure 41. Evaluation board schematic (VOUT= 3.3 V) 51 3 3 /. 3 /. 51 44*/) 6 /. /. $ O' 3 3 L 7#*"4 7$$ + 7065 %&-": O' O' - V) -" /. $0.1 4(/% 1(/% $ 44*/) $ 51 3 L $ 1(/% $ Q 3 L Q TJHOBM(/% 3 Q L 51 QPXFS(/% 1(/% + - 51 - 7*/ V) 7*/@'-5 .1;4" 51 V' V' $ $ V' 7*/@&.* $ 51 1(/% (/% The additional input filter (C16, L3, C15, L2, C14) limits the conducted emission on the power supply (refer to Figure 52 on page 61). Table 14. Bill of material Reference Part number C1, C10, C11 C2 Description Manufacturer Not mounted C2012X7S2A105K 1 F - 0805 - 50 V - X7S - 10% C3 470 nF - 10 V - 0603 C4 1.2 pF - 10 V - 0603 C5 100 nF - 10 V - 0603 C6 10 nF - 10 V - 0603 C7 2 pF - 10 V - 0603 C8 68 pF - 10 V - 0603 TDK C9 CGA5L3X5R1H106K 10 F - 1206 - 50 V - X5R - 10% TDK C14, C15, C16 CGA5L3X5R1H475k 4.7 F - 1206 - 50 V - X7R - 10% TDK C13 UWD1H101MCQ1GS 100 F - 50 V - 20% Nichicon 54/65 7065 + .-' $ . + '48 &1 7 -9 -9 3 /. 7 /. $ $ $ /. V' V'7 /. 3 $ $" 3 345 7*/ 345 $ + 4:/$) V' 7 $ 51 4:/$) 7*/@'-5 -' R2 0 - 0603 R6 1 M - 1%- 0603 R4 56 k - 1% - 0603 DocID027836 Rev 2 (/% L6985F Application board Table 14. Bill of material (continued) Reference Part number Description R7 240 k - 1% - 0603 R8 110 k - 1% - 0603 R9 82 k - 1% - 0603 R11 10 - 1% - 0603 R1, R3, R5, R10 Not mounted Manufacturer L1 XAL4030-472MEC 4.7 H Coilcraft L2 XAL4030-472MEC 4.7 H Coilcraft L3 MPZ2012S221A EMC bead TDK J1 Open J2 Open J3 Closed J4 Open To adjust the ISKIP current level in LCM operation. Leave open in LNM. J5 U1 Switchover L6985F STMicroelectronics DocID027836 Rev 2 55/65 65 Application board L6985F The layout of the board is shown in Figure 42 and in Figure 43. Figure 42. Top layer Figure 43. Bottom layer 56/65 DocID027836 Rev 2 L6985F Application board Figure 44 and Figure 45 show the magnitude and phase margin Bode's plots related to the evaluation board presented in Figure 41 with the BOM of Table 14. The small signal dynamic performance of the demonstration board is: Equation 54 BW = 110kHz phase margin = 64 0 Figure 44. Magnitude Bode's plot of L6985F evaluation board Figure 45. Phase margin Bode's plot of L6985F evaluation board DocID027836 Rev 2 57/65 65 Efficiency curves 8 L6985F Efficiency curves Figure 46. L6985F efficiency curves: VIN = 13.5 V - VOUT = 3.3 V - fsw = 2 MHz Figure 47. L6985F efficiency curves: VIN = 13.5 V - VOUT = 3.3 V - fsw = 2 MHz (log scale) 58/65 DocID027836 Rev 2 L6985F Efficiency curves Figure 48. L6985F efficiency curves: VIN = 24V - VOUT = 3.3 V - fsw = 2 MHz Figure 49. L6985F efficiency curves: VIN = 24 V - VOUT = 3.3 V - fsw = 2 MHz (log scale) DocID027836 Rev 2 59/65 65 Efficiency curves L6985F Figure 50. L6985F efficiency curves: VIN = 13.5 V - VOUT = 5 V - fsw = 2 MHz Figure 51. L6985F efficiency curves: VIN = 13.5 V - VOUT = 5 V - fsw = 2 MHz (log scale) 60/65 DocID027836 Rev 2 L6985F Efficiency curves Figure 52. L6985F efficiency curves: VIN = 24 V - VOUT = 5 V - fsw = 2 MHz Figure 53. L6985F efficiency curves: VIN = 24 V - VOUT = 5 V - fsw = 2 MHz (log scale) DocID027836 Rev 2 61/65 65 Package information 9 L6985F Package information In order to meet environmental requirements, ST offers these devices in different grades of ECOPACK(R) packages, depending on their level of environmental compliance. ECOPACK specifications, grade definitions and product status are available at: www.st.com. ECOPACK is an ST trademark. 9.1 HTSSOP16 package information Figure 54. HTSSOP16 package outline 62/65 DocID027836 Rev 2 L6985F Package information . Table 15. HTSSOP16 package mechanical data Dimensions (mm) Symbol Min. Typ. Max. A 1.20 A1 0.15 A2 0.80 b 0.19 0.30 c 0.09 0.20 D 4.90 5.00 5.10 D1 2.8 3 3.2 E 6.20 6.40 6.60 E1 4.30 4.40 4.50 E2 2.8 3 3.2 e L 1.05 0.65 0.45 L1 k 1.00 0.60 0.75 1.00 0.00 aaa 8.00 0.10 DocID027836 Rev 2 63/65 65 Order codes 10 L6985F Order codes Table 16. Order codes Part numbers Package L6985F Tube HTSSOP16 L6985FTR 11 Packaging Tape and reel Revision history Table 17. Document revision history Date Revision 05-May-2015 1 Initial release. 2 Updated Table 3: Thermal data on page 7 (added Rth JC). Updated Table 6: fSW selection on page 11 (added note 2. below table). Updated Section 6.5: Synchronization (LNM) on page 45 (replaced value of "range" "2 MHz" by "1.4 MHz", added text). 17-Feb-2016 64/65 Changes DocID027836 Rev 2 L6985F IMPORTANT NOTICE - PLEASE READ CAREFULLY STMicroelectronics NV and its subsidiaries ("ST") reserve the right to make changes, corrections, enhancements, modifications, and improvements to ST products and/or to this document at any time without notice. Purchasers should obtain the latest relevant information on ST products before placing orders. ST products are sold pursuant to ST's terms and conditions of sale in place at the time of order acknowledgement. Purchasers are solely responsible for the choice, selection, and use of ST products and ST assumes no liability for application assistance or the design of Purchasers' products. No license, express or implied, to any intellectual property right is granted by ST herein. Resale of ST products with provisions different from the information set forth herein shall void any warranty granted by ST for such product. ST and the ST logo are trademarks of ST. All other product or service names are the property of their respective owners. Information in this document supersedes and replaces information previously supplied in any prior versions of this document. (c) 2016 STMicroelectronics - All rights reserved DocID027836 Rev 2 65/65 65