MIC2169B 500kHz PWM Synchronous Buck Control IC General Description Features The MIC2169B is a high-efficiency, simple to use 500kHz PWM synchronous buck control IC housed in small MSOP-10 and MSOP-10 ePad packages. The MIC2169B allows compact DC/DC solutions with a minimal external component count and cost. The device features highoutput driver capability to drive loads up to 30A. The MIC2169B operates from a 3V to 14.5V input, without the need of any additional bias voltage. The output voltage can be precisely regulated down to 0.8V. The adaptive all N-Channel MOSFET drive scheme allows efficiencies over 95% across a wide load range within the smallest possible printed circuit board space area. The MIC2169B senses current across the high-side NChannel MOSFET, eliminating the need for an expensive and lossy current-sense resistor. Current-limit accuracy is maintained by a positive temperature coefficient that tracks the increasing RDS(ON) of the external MOSFET. Further cost and space are saved by the internal in-rush currentlimiting digital soft-start. The MIC2169B is identical to the MIC2169A with the exception that the MIC2169B supports pre-bias loads and has a lower impedance gate-drive circuit. Internal pre-bias circuit prevents output voltage drooping and excessive reverse inductor current when powering up with a pre-bias voltage at the output. The MIC2169B is available in a 10-pin MSOP and a thermally-capable 10-pin ePad MSOP package, with a wide junction operating range of -40C to +125C. All support documentation can be found on Micrel's web site at www.micrel.com. * * * * * * * * * * * * * * * * 3V to 14.5V input voltage range Adjustable output voltage down to 0.8V 500kHz PWM operation Up to 95% efficiency Output Pre-biased Protection Build-in 2.2 drivers to drive two n-channel MOSFETs Adaptive gate drive increases efficiency Simple, externally-compensated voltage-mode PWM control Short minimum ON time of 30ns allowing very-low duty cycle Fast transient response Adjustable current limit senses high-side N-Channel MOSFET current Hiccup mode short-circuit protection No external current-sense resistor Internal soft-start current source Dual function COMP and EN pin allows low-power shutdown Available in small-size 10-pin MSOP and 10-pin MSOP ePad packages Applications * * * * * * * Point-of-load DC/DC conversion High-Current Power Supplies Telecom/Datacom and Networking Power Supplies Servers and Workstations Graphic cards and other PC Peripherals Set-top boxes LCD power supplies ___________________________________________________________________________________________________________ Micrel Inc. * 2180 Fortune Drive * San Jose, CA 95131 * USA * tel +1 (408) 944-0800 * fax + 1 (408) 474-1000 * http://www.micrel.com April 2010 M9999-041210-B Micrel, Inc. MIC2169B Typical Application VIN = 5V to 12V SD103BWS 100F 10F 0.1F 10F VIN BST CS COMP/EN 150pF 100nF IRF7821 HSD MIC2169B VSW LSD 1F GND EP 1.0H 3.3V IRF7821 1000pF 330F x 2 FB EFFICIENCY (%) VDD 0.1F MIC2169B Efficienc 100 95 90 85 80 75 70 65 60 55 50 VIN = 5V VOUT = 3.3V 0 2 4 6 8 10 12 14 16 ILOAD (A) MIC2169B Adjustable Output 500kHz Converter April 2010 2 M9999-041210-B Micrel, Inc. MIC2169B Ordering Information Part Number Frequency Junction Temperature Range(1) Package Lead Finish MIC2169BYMME 500kHz -40 to +125C 10-Lead ePad MSOP Pb-Free MIC2169BYMM 500kHz -40 to +125C 10-Lead MSOP Pb-Free Pin Configuration 10-Pin ePad MSOP (MME) 10-Pin MSOP Pin Description Pin Number April 2010 Pin Name Pin Function 1 VIN Supply Voltage (Input): +3V to +14.5V. 2 VDD 5V Internal Linear Regulator (Output): VDD is the external MOSFET gate-drive supply voltage and an internal supply bus for the IC. When VIN is <5V, short VDD to the input supply through a 10 resistor. 3 CS Current Sense (Input): Current-limit comparator noninverting input. The current limit is sensed across the MOSFET during the ON time. The current can be set by the resistor in series with the CS pin. 4 COMP/EN Compensation / Enable (Input): Dual function pin. Pin for external compensation. If this pin is pulled below 0.25V, with the reference fully up the device shuts down (50A typical current draw). 5 FB 6 GND Ground (Return). 7 LSD Low-Side Drive (Output): High-current driver output for external synchronous MOSFET. 8 VSW Switch (Return): High-side MOSFET driver return. 9 HSD High-Side Drive (Output): High-current output-driver for the high-side MOSFET. When VIN is between 3.0V to 5V, 2.5V threshold MOSFETs should be used. At VIN > 5V, 4.5V threshold MOSFETs should be used. 10 BST Boost (Input): Provides the drive voltage for the high-side MOSFET driver. The gate-drive voltage is higher than the source voltage by VDD minus a diode drop. ePad EP Feedback (Input): Input to error amplifier. Regulates error amplifier to 0.8V. Connect to Ground. 3 M9999-041210-B Micrel, Inc. MIC2169B Absolute Maximum Ratings(1) Operating Ratings(2) Supply Voltage (VIN) ...................................... -0.3V to 15.5V Booststrapped Voltage (VBST) .................... -0.3V to VIN +6V VSW .............................................................. -0.3V to 15.5V CS ............................................................................15.25V LSD,FB............................................................... -0.3V to 6V Storage Temperature (TS)..........................-65C to +150C Peak Reflow Temperature (10 to 20 sec) ................ +260C ESD (HBM) (3) ................................................................. 2kV ESD (MM).....................................................................200V Supply Voltage (VIN)...................................... +3V to +14.5V Ambient Temperature (TA) ...........................-40C to +85C Junction Temperature (TJ) ..........................-40C to+125C Junction Thermal Resistance ePad MSOP (JA)............................................76.7C/W ePad MSOP (JC) .............................................9.6C/W MSOP (JA) ......................................................130C/W MSOP (JC).....................................................42.6C/W Output Voltage Range............................. 0.8V to VIN x DMAX Electrical Characteristics(4) TJ = 25C, VIN = 5V; Bold values indicate -40C TJ +125C; unless otherwise specified. Min Typ Max Units (1%) 0.792 0.8 0.808 V (2% over temp) 0.784 0.8 0.816 V Feedback Bias Current 150 350 nA Output Voltage Line Regulation 0.03 %/V Output Voltage Load Regulation 0.5 % Parameter Condition Feedback Voltage Reference Feedback Voltage Reference Output Voltage Total Regulation 3V VIN 14.5V; 1A IOUT 10A; (VOUT = 2.5V)(4) 0.6 1.5 % 500 550 kHz Oscillator Section Oscillator Frequency 450 Maximum Duty Cycle 92 Minimum On-Time (5) % 30 60 ns Input and VDD Supply PWM Mode Supply Current VCS = VIN -0.25V; VFB = 0.7V (output switching but excluding external MOSFET gate current.) 1.5 3 mA Shutdown Quiescent Current VCOMP/EN = 0V 50 150 A 0.25 0.35 V 0.1 VCOMP Shutdown Threshold VCOMP Shutdown Blanking Period CCOMP = 100nF Digital Supply Voltage (VDD) VIN 6V April 2010 675 4.7 4 5 s 5.3 V M9999-041210-B Micrel, Inc. MIC2169B Electrical Characteristics(4) (continued) TJ = 25C, VIN = 5V; Bold values indicate -40C TJ +125C; unless otherwise specified. Parameter Min Condition Typ Max Units Error Amplifier DC Gain(5) 70 dB Transconductance 1.1 m-1 Soft-Start Soft-Start Current After time out of internal timer. VCOMP = 0.8V 4 8.5 13 A 160 200 240 A Current Sense CS Over Current Trip Point VCS = VIN -0.25V Temperature Coefficient 1800 ppm/C ns Gate Drivers Rise/Fall Time Into 3000pF at VIN > 5V 15 Output Driver Impedance Source, VIN = 4.5V 2.2 3 Sink, VIN = 4.5V 1.3 3 Source, VIN = 3V 2.7 4 Sink, VIN = 3V 1.7 4 Driver Non-Overlap Time (5) 50 ns Notes: 1. Absolute maximum ratings indicate limits beyond which damage to the component may occur. Electrical specifications do not apply when operating the device outside of its operating ratings. The maximum allowable power dissipation is a function of the maximum junction temperature, TJ(max), the junction-to-ambient thermal resistance, JA, and the ambient temperature, TA. The maximum allowable power dissipation will result in excessive die temperature. 2. The device is not guaranteed to function outside its operating rating. 3. Devices are ESD sensitive, handling precautions required. 4. Specification for packaged product only. 5. Guaranteed by design. April 2010 5 M9999-041210-B Micrel, Inc. MIC2169B Typical Characteristics 0.8005 0.5 5 10 SUPPLY VOLTAGE (V) 15 VDD Line Regulation 3 2 1 0.794 0 0.792 -60 -30 0 30 60 90 120 150 TEMPERATURE (C) 0 0.0 -60 -30 0 30 60 90 120 150 TEMPERATURE (C) 15 Oscillator Frequency vs. Temperature 540 530 520 510 500 490 480 470 460 450 -60 -30 0 30 60 90 TEMPERATURE (C) 120 150 10 15 5.00 4.98 4.96 4.94 4.92 4.90 0 5 10 15 20 25 LOAD CURRENT (mA) 30 Oscillator Frequency vs. Suppl Voltage 1.5 550 FREQUENCY (kHz) 1.5 1.0 0.5 10 5 VDD Load Regulation 5.02 VIN (V) VDD Line Regulation vs. Temperature 3.5 3.0 2.5 2.0 5 0 VIN (V) VDD REGULATOR VOLTAGE (V) VDD (V) VFB (V) 0 4 0.796 VDD LINE REGULATION (%) 0.7980 6 0.802 April 2010 VFB (V) 0.7985 5 0.798 0.7995 0.7990 1.0 0.804 5.0 4.5 4.0 0.8000 1.5 VFB vs. Temperature 0.800 VFB Line Regulation 0.8010 FREQUENCY VARIATION (%) 0.806 PWM Mode Supply Current vs. Suppl Voltage 2.0 QUIESCENT CURRENT (mA) IDD (mA) PWM Mode Supply Current vs. Temperature 2.9 2.7 2.5 2.3 2.1 1.9 1.7 1.5 1.3 1.1 0.9 0.7 0.5 -40 -20 0 20 40 60 80 100120140 TEMPERATURE (C) 1.0 0.5 0 -0.5 -1.0 -1.5 0 5 10 15 VIN (V) 6 M9999-041210-B Micrel, Inc. MIC2169B Typical Characteristics (continued) 260 Overcurrent Trip Point vs. Temperature 240 ICS ( A) 220 200 180 160 140 120 100 -60 -30 0 30 60 90 120 150 TEMPERATURE (C) Functional Diagram MIC2169B Block Diagram April 2010 7 M9999-041210-B Micrel, Inc. MIC2169B power up, and the chip's internal oscillator starts switching. At this point in time, the COMP pin current source increases to 40A and an internal 12-bit counter starts counting which takes approximately 2ms to complete. During counting, the COMP voltage is clamped at 0.65V. After this counting cycle the COMP current source is reduced to 8.5A and the COMP pin voltage rises from 0.65V to 0.95V, the bottom edge of the saw-tooth oscillator. This is the beginning of 0% duty cycle and it increases slowly causing the output voltage to rise slowly. The MIC2169B has one hysteretic comparator whose output is asserted high when VOUT is within -3% of steady state. When the output voltage reaches 97% of programmed output voltage then the gm error amplifier is enabled along with the hysteretic comparator output is asserted high. This point onwards, the voltage control loop (gm error amplifier) is fully in control and will regulate the output voltage. Soft-start time can be calculated approximately by adding the following four time frames: Functional Description The MIC2169B is a voltage-mode, synchronous stepdown switching regulator controller designed for high power. Current limit is implemented without the use of an external sense resistor. It includes an internal soft-start function which reduces the power supply input surge current at start-up by controlling the output voltage rise time, a PWM generator, a reference voltage, two MOSFET drivers, and short-circuit current limiting circuitry to form a complete 500kHz switching regulator. MIC2169B is identical to the MIC2169A except it supports pre-bias loads and has a lower impedance gate-drive circuit. Theory of Operation The MIC2169B is a voltage mode step-down regulator. The figure above illustrates the block diagram for the voltage control loop. The output voltage variation due to load or line changes will be sensed by the inverting input of the transconductance error amplifier via the feedback resistors R3, and R2 and compared to a reference voltage at the non-inverting input. This will cause a small change in the DC voltage level at the output of the error amplifier which is the input to the PWM comparator. The other input to the comparator is a 0.95V to 1.45V triangular waveform. The comparator generates a rectangular waveform whose width tON is equal to the time from the start of the clock cycle t0 until t1, the time the triangle crosses the output waveform of the error amplifier. To illustrate the control loop, let us assume the output voltage drops due to sudden load turn-on, this would cause the inverting input of the error amplifier, which is divided down version of VOUT, to be slightly less than the reference voltage, causing the output voltage of the error amplifier to go high. This will cause the PWM comparator to increase tON time of the top side MOSFET, causing the output voltage to go up and bringing VOUT back in regulation. t1 = Cap_COMP x 0.25V/8.5A t2 = 12 bit counter, approx 2ms t3 = Cap_COMP x 0.3V/8.5A V t 4 = OUT VIN Soft-Start Time(Cap_COMP=100nF) = t1 + t2 + t3 + t4 = 2.9ms + 2ms + 3.5ms + 1.6ms = 10ms Current Limit The MIC2169B uses the RDS(ON) of the top power MOSFET to measure output current. Since it uses the drain to source resistance of the power MOSFET, it is not very accurate. This scheme is adequate to protect the power supply and external components during a fault condition by cutting back the time the top MOSFET is on if the feedback voltage is greater than 0.67V. In case of a hard short when feedback voltage is less than 0.67V, the MIC2169B discharges the COMP capacitor to 0.65V, resets the digital counter and automatically shuts off the top gate drive, the gm error amplifier is completely disabled, the -3% hysteretic comparators is asserted low, and the soft-start cycles restart from t2 to t4. This mode of operation is called the "hiccup mode" and its purpose is to protect the down stream load in case of a hard short. The circuit in Figure 1 illustrates the MIC2169B current limiting circuit. Soft-Start The COMP/EN pin on the MIC2169B is used for the following three functions: 1. Disables the part by grounding this pin 2. External compensation to stabilize the voltage control loop 3. Soft-start For better understanding of the soft-start feature, assume VIN = 12V, and the MIC2169B is allowed to power-up by un-grounding the COMP/EN pin. The COMP pin has an internal 8.5A current source that charges the external compensation capacitor. As soon as this voltage rises to 250mV (t = Cap_COMP x 0.25V/8.5A) and VIN crosses the 2.6V UVLO threshold, the MIC2169B allows the internal VDD linear regulator to April 2010 Cap _ COMP x 0.5 x 8.5A 8 M9999-041210-B Micrel, Inc. MIC2169B VIN C2 CIN HSD MOSFET Gate Drive The MIC2169B high-side drive circuit is designed to switch an N-Channel MOSFET. The Functional Block Diagram shows a bootstrap circuit, consisting of D1 and CBST, supplies energy to the high-side drive circuit. Capacitor CBST is charged while the low-side MOSFET is on and the voltage on the VSW pin is approximately 0V. When the high-side MOSFET driver is turned on, energy from CBST is used to turn the MOSFET on. As the MOSFET turns on, the voltage on the VSW pin increases to approximately VIN. Diode D1 is reversed biased and CBST floats high while continuing to keep the high-side MOSFET on. When the low-side switch is turned back on, CBST is recharged through D1. The drive voltage is derived from the internal 5V VDD bias supply. The nominal low-side gate drive voltage is 5V and the nominal high-side gate drive voltage is approximately 4.5V due the voltage drop across D1. An approximate 50ns delay between the high-side and low-side driver transitions is used to prevent current from simultaneously flowing unimpeded through both MOSFETs (shoot-through). Adaptive gate drive is implemented on the high-side (off) to low-side (on) driver transition to reduce losses in the flywheel diode and to prevent shoot-through. This is operated by detecting the VSW pin; once this pin is detected to reach 1.5V, the high-side MOSFET can be assumed to be off and the low side driver is enabled. Q1 MOSFET N 0.1F L1 Inductor VOUT RCS VSW LSD CS Q2 MOSFET N 1000pF C1 COUT 200 A Figure 1. The MIC2169B Current Limiting Circuit The current limiting resistor RCS is calculated by the following equation: RCS = RDS(ON)Q1 x IL 200A where: IL = ILOAD + Inductor Ripple Current 2 Inductor Ripple Current = VOUT x (VIN - VOUT ) VIN x FS x L FS = 500kHz 200A is the internal sink current to program the MIC2169B current limit. The MOSFET RDS(ON) varies 30% to 40% with temperature; therefore, it is recommended to add a 50% margin to the load current (ILOAD) in the above equation to avoid false current limiting due to increased MOSFET junction temperature rise. It is also recommended to connect RCS resistor directly to the drain of the top MOSFET Q1, and the RSW resistor to the source of Q1 to accurately sense the MOSFETs RDS(ON). To make the MIC2169B insensitive to board layout and noise generated by the switch node, a 1.4 resistor and a 1000pF capacitor is recommended between the switch node and GND. Total Power Dissipation and Thermal Considerations Total power dissipation in the MIC2169B equals the power dissipation caused by driving the external MOSFETs plus the quiescent supply current: PdissTOTAL = PdissSUPPLY + PdissDRIVE where: PdissSUPPLY = VDD x IDD IDD is shown in the "PWM Mode Supply Current" graph in the Typical Characteristics section of the specification. PdissDRIVE calculations are shown in the Applications section of the specification. The die temperature may be calculated once the total power dissipation is known: Internal VDD Supply The MIC2169B controller internally generates VDD for self biasing and to provide power to the gate drives. This VDD supply is generated through a low-dropout regulator and generates 5V from VIN supply greater than 5V. For supply voltage less than 5V, the VDD linear regulator is approximately 200mV in dropout. Therefore, it is recommended to short the VDD supply to the input supply through a 10 resistor for input supplies between 3.0V to 5V. April 2010 TJ = TA + PdissTOTAL x JA where: TA is the maximum ambient temperature (C) TJ is the junction temperature (C) PdissTOTAL is the power dissipation of the MIC2169B (W) JC is the thermal resistance from junction-toambient air (C/W) 9 M9999-041210-B Micrel, Inc. MIC2169B The following graphs are used to determine the maximum gate charge that can be driven with respect to supply voltage and ambient temperature. Figure 2 shows the power dissipation in the driver for different values of gate charge. CASE TEMPERATURE RISE (C) Figures 4 and 5 show the increase in junction and case temperature for a given power dissipation. POWER DISSIPATION (W) 0.70 0.60 0.50 12Vin 0.40 0.30 50 40 MSOP 30 20 ePAD MSOP 10 0 0.0 0.20 5Vin 0.2 0.4 0.6 0.8 1.0 1.2 POWER DISSIPATION (W) 0.10 0.00 0 20 40 60 80 GATE CHARGE (nC) Figure 4. Case Temperature Rise vs. Power Dissipation 100 JUNCTION TEMPERATURE RISE (C) Figure 2. Power Dissipation vs. Total Gate Charge Figure 3 shows the maximum allowable power dissipation vs ambient temperature. For a given total gate charge, the maximum operating ambient temperature can be found by using the two graphs. MAXIMUM AMBIENT TEMPERATURE (C) 60 120 100 MSOP 80 ePAD MSOP 60 40 20 0 140 0.0 120 0.2 0.4 0.6 0.8 1.0 1.2 POWER DISSIPATION (W) 100 80 Figure 5. Junction Temperature Rise vs. Power Dissipation ePAD MSOP 60 MSOP 40 20 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 POWER DISSIPATION (W) Figure 3. Maximum Ambient Temperature vs. Power Dissipation April 2010 10 M9999-041210-B Micrel, Inc. MIC2169B A convenient figure of merit for switching MOSFETs is the on resistance times the total gate charge RDS(ON)xQG. Lower numbers translate into higher efficiency. Low gate-charge logic-level MOSFETs are a good choice for use with the MIC2169B. Parameters that are important to MOSFET switch selection are: Application Information MOSFET Selection The MIC2169B controller works from input voltages of 3V to 14.5V and has an internal 5V regulator to provide power to turn the external N-Channel power MOSFETs for high- and low-side switches. For applications where VIN < 5V, the internal VDD regulator operates in dropout mode, and it is necessary that the power MOSFETs used are sub-logic level and are in full conduction mode for VGS of 2.5V. For applications when VIN > 5V; logiclevel MOSFETs, whose operation is specified at VGS = 4.5V must be used. For the lower (<5V) applications, the VDD supply can be connected directly to VIN to help increase the driver voltage to the MOSFET. It is important to note the on-resistance of a MOSFET increases with increasing temperature. A 75C rise in junction temperature will increase the channel resistance of the MOSFET by 50% to 75% of the resistance specified at 25C. This change in resistance must be accounted for when calculating MOSFET power dissipation and in calculating the value of current-sense (CS) resistor. Total gate charge is the charge required to turn the MOSFET on and off under specified operating conditions (VDS and VGS). The gate charge is supplied by the MIC2169B gate-drive circuit. At 500kHz switching frequency and above, the gate charge can be a significant source of power dissipation in the MIC2169B. At low output load, this power dissipation is noticeable as a reduction in efficiency. The average current required to drive the high-side MOSFET is: * Voltage rating * On-resistance * Total gate charge The voltage ratings for the top and bottom MOSFET are essentially equal to the input voltage. A safety factor of 20% should be added to the VDS(max) of the MOSFETs to account for voltage spikes due to circuit parasitics. The power dissipated in the switching transistor is the sum of the conduction losses during the on-time (PCONDUCTION) and the switching losses that occur during the period of time when the MOSFETs turn on and off (PAC). PSW = PCONDUCTION + PAC where: PCONDUCTION = ISW (rms )2 x RSW PAC = PAC(off ) + PAC(on) RSW = on-resistance of the MOSFET switch V D = duty cycle = O VIN IG[high -side](avg ) = Q G x f S Making the assumption the turn-on and turn-off transition times are equal; the transition times can be approximated by: where: IG[high-side](avg) = average high-side MOSFET gate current. QG = total gate charge for the high-side MOSFET taken from manufacturer's data sheet for VGS = 5V. The low-side MOSFET is turned on and off at VDS = 0 because the freewheeling diode is conducting during this time. The switching loss for the low-side MOSFET is usually negligible. Also, the gate-drive current for the low-side MOSFET is more accurately calculated using CISS at VDS = 0 instead of gate charge. For the low-side MOSFET: tT = where: CISS and COSS are measured at VDS = 0 IG = gate-drive current (1.4A for the MIC2169B) The total high-side MOSFET switching loss is: PAC = (VIN + VD ) x IPK x t T x fS where: tT = switching transition time (typically 20ns to 50ns) VD = freewheeling diode drop, typically 0.5V fS it the switching frequency, nominally 500kHz The low-side MOSFET switching losses are negligible and can be ignored for these calculations. IG[low - side](avg ) = CISS x VGS x fS Since the current from the gate drive comes from the input voltage, the power dissipated in the MIC2169B due to gate drive is: ( PGATEDRIVE = VIN x IG[high -side](avg) + IG[low -side](avg) April 2010 CISS x VGS + COSS x VIN IG ) 11 M9999-041210-B Micrel, Inc. MIC2169B currents, the core losses can be a significant contributor. Core loss information is usually available from the magnetics vendor. Copper loss in the inductor is calculated by the equation below: Inductor Selection Values for inductance, peak, and RMS currents are required to select the output inductor. The input and output voltages and the inductance value determine the peak-to-peak inductor ripple current. Generally, higher inductance values are used with higher input voltages. Larger peak-to-peak ripple currents will increase the power dissipation in the inductor and MOSFETs. Larger output ripple currents will also require more output capacitance to smooth out the larger ripple current. Smaller peak-to-peak ripple currents require a larger inductance value and therefore a larger and more expensive inductor. A good compromise between size, loss and cost is to set the inductor ripple current to be equal to 20% of the maximum output current. The inductance value is calculated by the equation below. L= ( VOUT x VIN(max) - VOUT PINDUCTORCU The resistance of the copper wire, RWINDING, increases with temperature. The value of the winding resistance used should be at the operating temperature. R WINDING (hot ) = R WINDING ( 20C) x (1 + 0.0042 x (THOT - T20C ) where: THOT = temperature of the wire under operating load T20C = ambient temperature RWINDING(20C) = room temperature winding resistance (usually specified by the manufacturer) ) VIN(max) x f S x 0.2 x IOUT(max) Output Capacitor Selection The output capacitor values are usually determined by the capacitors ESR (equivalent series resistance). Voltage and RMS current capability are two other important factors selecting the output capacitor. Recommended capacitors are tantalum, low-ESR aluminum electrolytics, and POSCAPS. The output capacitor's ESR is usually the main cause of output ripple. The output capacitor ESR also affects the overall voltage feedback loop from stability point of view. See "Feedback Loop Compensation" section for more information. The maximum value of ESR is calculated: where: fS = switching frequency, 500kHz 0.2 = ratio of AC ripple current to DC output current VIN(max) = maximum input voltage The peak-to-peak inductor current (AC ripple current) is: IPP = ( VOUT x VIN(max) - VOUT VIN(max) x fS x L ) The peak inductor current is equal to the average output current plus one half of the peak-to-peak inductor ripple current. RESR IPK = IOUT(max) + 0.5 x IPP VOUT IPP where: VOUT = peak-to-peak output voltage ripple IPP = peak-to-peak inductor ripple current The total output ripple is a combination of the ripple due to the output capacitors' ESR and the ripple due to the output capacitor. The total ripple is calculated below: The RMS inductor current is used to calculate the I2 x R losses in the inductor. I 2 IINDUCTOR = (IOUT _ MAX ) 2 + PP 12 Maximizing efficiency requires the proper selection of core material and minimizing the winding resistance. The high frequency operation of the MIC2169B requires the use of ferrite materials for all but the most cost sensitive applications. Lower cost iron powder cores may be used but the increase in core loss will reduce the efficiency of the power supply. This is especially noticeable at low output power. The winding resistance decreases efficiency at the higher output current levels. The winding resistance must be minimized although this usually comes at the expense of a larger inductor. The power dissipated in the inductor is equal to the sum of the core and copper losses. At higher output loads, the core losses are usually insignificant and can be ignored. At lower output April 2010 2 = IINDUCTOR(rms) x R WINDING 2 VOUT I x (1 - D) + (IPP x RESR )2 = PP COUT x fS where: D = duty cycle COUT = output capacitance value fS = switching frequency 12 M9999-041210-B Micrel, Inc. MIC2169B The output voltage is determined by the equation: The voltage rating of capacitor should be twice the voltage for a tantalum and 20% greater for aluminum electrolytic. The output capacitor RMS current is calculated below: IC OUT ( rms ) = R1 VO = VREF x 1 + R 2 where VREF for the MIC2169B is typically 0.8V IPP 12 A typical value of R1 can be between 3k and 10k. If R1 is too large, it may allow noise to be introduced into the voltage feedback loop. If R1 is too small, in value, it will decrease the efficiency of the power supply, especially at light loads. Once R1 is selected, R2 can be calculated using: The power dissipated in the output capacitor is: ( PDISS(C OUT ) = IC OUT ( rms ) )2 x RESR(C OUT ) Input Capacitor Selection The input capacitor should be selected for ripple current rating and voltage rating. Tantalum input capacitors may fail when subjected to high inrush currents, caused by turning the input supply on. A tantalum input capacitor's voltage rating should be at least 2 times the maximum input voltage to maximize reliability. Aluminum electrolytic, OS-CON, and multilayer polymer film capacitors can handle the higher inrush currents without voltage derating. The input voltage ripple will primarily depend on the input capacitor's ESR. The peak input current is equal to the peak inductor current, so: R2 = External Schottky Diode An external freewheeling diode is used to keep the inductor current flow continuous while both MOSFETs are turned off. This dead time prevents current from flowing unimpeded through both MOSFETs and is typically 50ns. The diode conducts twice during each switching cycle. Although the average current through this diode is small, the diode must be able to handle the peak current. VIN = IINDUCTOR(peak ) x RESR(CIN ) The input capacitor must be rated for the input current ripple. The RMS value of input capacitor current is determined at the maximum output current. Assuming the peak-to-peak inductor ripple current is low: ID(avg) = IOUT x 2 x 50ns x fS The reverse voltage requirement of the diode is: VDIODE(rrm) = VIN The power dissipated by the Schottky diode is: PDIODE = ID(avg) x VF where: ICIN( rms ) IOUT(max) x D x (1 - D) The power dissipated in the input capacitor is: ( PDISS(CIN ) = ICIN(rms ) )2 x RESR(C IN VF = forward voltage at the peak diode current The external Schottky diode, D1, is not necessary for circuit operation since the low-side MOSFET contains a parasitic body diode. The external diode will improve efficiency and decrease high frequency noise. If the MOSFET body diode is used, it must be rated to handle the peak and average current. The body diode has a relatively slow reverse recovery time and a relatively high forward voltage drop. The power lost in the diode is proportional to the forward voltage drop of the diode. As the high-side MOSFET starts to turn on, the body diode becomes a short circuit for the reverse recovery period, dissipating additional power. The diode recovery and the circuit inductance will cause ringing during the high-side MOSFET turn-on. An external Schottky diode conducts at a lower forward voltage preventing the body diode in the MOSFET from turning on. The lower forward voltage drop dissipates less power than the body diode. The lack of a reverse recovery mechanism in a Schottky diode causes less ringing and less power loss. ) Voltage Setting Components The MIC2169B requires two resistors to set the output voltage as shown in Figure 6. R1 Error Amp FB 5 R2 VREF 0.8V MIC2169B Figure 6. Voltage-Divider Configuration April 2010 VREF x R1 VO - VREF 13 M9999-041210-B Micrel, Inc. MIC2169B Depending on the circuit components and operating conditions, an external Schottky diode will give a 1/2% to 1% improvement in efficiency. Feedback Loop Compensation The MIC2169B controller comes with an internal transconductance error amplifier used for compensating the voltage feedback loop by placing a capacitor (C1) in series with a resistor (R1) and another capacitor C2 in parallel from the COMP pin to ground. See "Functional Block Diagram." Power Stage The power stage of a voltage mode controller has an inductor, L1, with its winding resistance (DCR) connected to the output capacitor, COUT, with its electrical series resistance (ESR) as shown in Figure 7. The transfer function G(s), for such a system is: L Figure 9. Phase Curve for G(s) It can be seen from the transfer function G(s) and the gain curve that the output inductor and capacitor create a two pole system with a break frequency at: DCR VO fLC = ESR 1 2 x L x COUT Therefore, fLC = 6.2kHz By looking at the phase curve, it can be seen that the output capacitor ESR (0.025) cancels one of the two poles (LCOUT) system by introducing a zero at: COUT Figure 7. The Output LC Filter in a Voltage-Mode Buck Converter fZERO = (1 + ESR x s x C) G(s) = 2 DCR x s x C + s x L x C + 1 + ESR x s x C 1 2 x x ESR x COUT Therefore, FZERO = 9.6kHz. From the point of view of compensating the voltage loop, it is recommended to use higher ESR output capacitors since they provide a 90 phase gain in the power path. For comparison purposes, Figure 10, shows the same phase curve with an ESR value of 0.002. Plotting this transfer function with the following assumed values (L=1H, DCR=0.009, COUT=660F, ESR=0.025) gives lot of insight as to why one needs to compensate the loop by adding resistor and capacitors on the COMP pin. Figures 8 and 9 show the gain curve and phase curve for the above transfer function. Figure 10. The Phase Curve with ESR = 0.002 Figure 8. The Gain Curve for G(s) April 2010 14 M9999-041210-B Micrel, Inc. MIC2169B It can be seen from Figure 9 that at 50kHz, the phase is approximately -90 versus Figure 10 where the number is -150. This means that the transconductance error amplifier has to provide a phase boost of about 45 to achieve a closed loop phase margin of 45 at a crossover frequency of 50kHz for Figure 9, versus 105 for Figure 10. The simple RC and C2 compensation scheme allows a maximum error amplifier phase boost of about 90. Therefore, it is easier to stabilize the MIC2169B voltage control loop by using high-ESR value output capacitors. gm Error Amplifier It is undesirable to have high error amplifier gain at high frequencies because high frequency noise spikes would be picked up and transmitted at large amplitude to the output, thus, gain should be permitted to fall off at high frequencies. At low frequency, it is desired to have high open-loop gain to attenuate the power line ripple. Thus, the error amplifier gain should be allowed to increase rapidly at low frequencies. The transfer function with R1, C1, and C2 for the internal gm error amplifier can be approximated by the following equation: Figure 11. Error Amplifier Gain Curve 1 + s x R1 x C1 Error Amplifier (z) = gm x s x (C1 + C2 ) x 1 + s x R1 x C1 x C2 C1 + C2 The above equation can be simplified by assuming C2<<C1, Figure 12. Error Amplifier Phase Curve 1 + s x R1 x C1 Error Amplifier (z) = gm x s x C1 x (1 + s x R1 x C2) Total Open-Loop Response The open-loop response for the MIC2169B controller is easily obtained by adding the power path and the error amplifier gains together, since they already are in Log scale. It is desirable to have the gain curve intersect zero dB at tens of kilohertz, this is commonly called crossover frequency; the phase margin at crossover frequency should be at least 45. Phase margins of 30 or less cause the power supply to have substantial ringing when subjected to transients, and have little tolerance for component or environmental variations. Figures 13 and 14 show the open-loop gain and phase margin for the 5V input and 1.8V output application, and it can be seen from Figure 13 that the gain curve intersects the 0dB at approximately 50kHz, and from Figure 14 that at 50kHz, the phase shows approximately 74 of margin. From the above transfer function, one can see that R1 and C1 introduce a zero and R1 and C2 a pole at the following frequencies: FZERO= 1/2 x R1 x C1 FPOLE = 1/2 x C2 x R1 FPOLE@origin = 1/2 x C1 Figures 11 and 12 show the gain and phase curves for the above transfer function with R1 = 4.02k, C1 = 100nF, C2 = 150pF, and gm = 1.1m-1. April 2010 15 M9999-041210-B Micrel, Inc. MIC2169B Figure 13. Open-Loop Gain Margin Figure 15. MIC2169A Startup without Pre-Bias Support Figure 15 shows MIC2169B startup with a pre-bias of 1V on the output, in which the pre-existing output voltage discharges during soft start. Figure 14. Open-Loop Phase Margin Pre-Biased Loads The MIC2169B supports pre-biased loads. Some applications have a pre-existing voltage on the output. This pre-existing or pre-biased load is generated by an external supply (other than the MIC2169B). During startup without pre-bias support, MIC2169A will pull the output voltage to ground through the inductor and low side FET (see Figure 15). The MIC2169B prevents the current sinking of any preexisting voltage source at the output (see Figure 16). It does this by keeping the low-side FET off during the soft start period. In some applications this pre-bias current sink is not a problem, and the MIC2169A may be used. In some applications the pre-bias current sink may cause a problem, and the MIC2169B should be used. The MIC2169B can support up to 90% of a pre-bias condition (up to 90% of the final regulated output voltage) see Figure 17. April 2010 Figure 16. MIC2169B Startup with Pre-Bias Support Figure 16 shows MIC2169B startup with a pre-bias of 1V on the output, in which the pre-existing output voltage has no discharge. 16 M9999-041210-B Micrel, Inc. MIC2169B Figure 17. MIC2169B Startup with Pre-Bias Support, Pre-Bias At 90% of VOUT_FINAL * Keep both the VIN and power GND connections short. * Place several vias to the ground plane close to the VIN input capacitor ground terminal. * Use either X7R or X5R dielectric input capacitors. Do not use Y5V or Z5U type capacitors. * Do not replace the ceramic input capacitor with any other type of capacitor. Any type of capacitor can be placed in parallel with the input capacitor. * If a Tantalum input capacitor is placed in parallel with the input capacitor, it must be recommended for switching regulator applications and the operating voltage must be derated by 50%. * In "Hot-Plug" applications, a Tantalum or Electrolytic bypass capacitor must be used to limit the overvoltage spike seen on the input supply with power is suddenly applied. * An additional Tantalum or Electrolytic bypass input capacitor of 22uF or higher is required at the input power connection. * Use a 5 resistor from the input supply to the VDD pin on the MIC2169B. Also, place a 1F ceramic capacitor from this pin to GND, preferably not through a via. The capacitor must be located right at the IC. The Vdd terminal is very noise sensitive and placement of the capacitor is very critical. Connections must be made with wide trace. Figure 17 shows MIC2169B startup with a pre-bias of 2.2V on the output (90% of VOUT) without the pre-existing output voltage discharge. Design and PCB Layout Guideline WARNING!!! TO MINIMIZE EMI AND OUTPUT NOISE, FOLLOW THESE LAYOUT RECOMMENDATIONS: Inductor PCB Layout is critical to achieve reliable, stable and efficient performance. A ground plane is required to control EMI and minimize the inductance in power, signal and return paths. The following guidelines should be followed to insure proper operation of the MIC2169B converter. * Keep the inductor connection to the switch node (SW) short. * Do not route any digital lines underneath or close to the inductor. * Keep the switch node (SW) away from the feedback (FB) pin. * To minimize noise, place a ground plane underneath the inductor. IC * Place the IC and MOSFETs close to the point of load (POL). * Use fat traces to route the input and output power lines. * Signal and power grounds should be kept separate and connected at only one location. Output Capacitor * Use a wide trace to connect the output capacitor ground terminal to the input capacitor ground terminal. * Phase margin will change as the output capacitor value and ESR changes. Contact the factory if the output capacitor is different from what is shown in the BOM. * The feedback trace should be separate from the power trace and connected as close as possible to the output capacitor. Sensing a long high-current load trace can degrade the DC load regulation. Input Capacitor * Place the VIN input capacitor next. * Place the VIN input capacitors on the same side of the board and as close to the MOSFETs as possible. April 2010 17 M9999-041210-B Micrel, Inc. MIC2169B MOSFETs Others * * Connect the current limiting (R2) resistor directly to the drain of top MOSFET Q3. * The feedback resistors R3 and R4/R5/R6 should be placed close to the FB pin. The top side of R3 should connect directly to the output node. Run this trace away from the switch node (junction of Q3, Q2, and L1). The bottom side of R3 should connect to the GND pin on the MIC2169B. * The compensation resistor and capacitors should be placed right next to the COMP pin and the other side should connect directly to the GND pin on the MIC2169B rather than going to the plane. * Add a place holder for a gate resistor on the top MOSFET gate drive. Do not use a resistor in series with the low-side MOSFET gate. Low gate charge MOSFETs should be used to maximize efficiency, such as Si4800, Si4804BDY, IRF7821, IRF8910, FDS6680A and FDS6912A, etc. RC Snubber * Add a RC snubber of 1.4 resistor and a 1000pF capacitor from the switch node to ground pin. Place the snubber on the same side of the board and as close to the MOSFETs as possible. See page 8, Current Limiting section for more detail. Schottky Diode (Optional) * Place the Schottky diode on the same side of the board as the MOSFETs and VIN input capacitor. * The connection from the Schottky diode's Anode to the input capacitors ground terminal must be as short as possible. * The diode's Cathode connection to the switch node (SW) must be keep as short as possible. Evaluation Board Schematics MIC2169B Evaluation Board Schematic April 2010 18 M9999-041210-B Micrel, Inc. MIC2169B Bill of Materials Item Part Number Manufacturer Description U1 MIC2169B-YMME Micrel, Inc. Buck Controller 1 Q1, Q2 IRF7821-TR SI4174DY IR Vishay 30V, N-Channel HEXFET, Power MOSFET 2 0 Q3 2N7002E On Semiconductor 60V, N-Channel MOSFET 0 D1 SD103BWS Vishay 30V, Schottky Diode 1 D2 1N5819HW SL04 CMMSH1-40 Diodes, Inc. Vishay Central Semi 40V, Schottky Diode 1 0 0 L1 CDRH127LDNP-1R0NC HC5-1R0 SER1360-1R0 Sumida Cooper Electronic Coilcraft 1.0H, 10A Inductor 1 0 0 C1 C3225X7R1C106M TDK 10F/16V, X7R Ceramic Capacitor 1 C2, C3 TPSD686M020R0070 594D686X0020D2T AVX Vishay/Sprague 68F, 20V Tantalum 2 0 C4 C2012X5R0J106M TDK 10F/6.3V, 0805 Ceramic Capacitor 1 C5, C10, C12, C16 VJ1206Y104KXXAT Vishay Victramon 0.1F/25V Ceramic Capacitor 4 C6, C7 TPSD337M006R0045 AVX 330F/6.3V, Tantalum 2 Open 0 C8, C11, C17 Qty. C13 C2012X7R1C105K GRM21BR71C105KA01B VJ1206S105KXJAT TDK muRata Vishay Victramon 1F/16V, 0805 Ceramic Capacitor 1 0 0 C15 VJ0603A102KXXAT Vishay Victramon 1000pF/25V, 0603, NPO 1 R2 CRCW06034700JRT1 Vishay 470, 0603, 1/16W, 5% 1 R3 CRCW08051002FRT1 Vishay 10k, 0805, 1/10W, 1% 1 R4 CRCW08053161FRT1 Vishay 3.16k, 0805, 1/10W, 1% 1 R5 CRCW08054641FRT1 Vishay 4.64k, 0805, 1/10W, 1% 1 R6 CRCW08051132FRT1 Vishay 11.3k, 0805, 1/10W, 1% 1 R7 CRCW08051003FRT1 Vishay 100k, 0805, 1/10W, 1% 1 C9 VJ0603A151KXAAT Vishay 150pF/50V, 0603, NPO 1 Notes: 1. Micrel.Inc 408-944-0800 2. Vishay corp 206-452-5664 3. Diodes. Inc 805-446-4800 4. Sumida 408-321-9660 5. TDK 847-803-6100 6. muRata 800-831-9172 7. AVX 843-448-9411 8. International Rectifier 847-803-6100 9. Fairchild Semiconductor 207-775-8100 10. Cooper Electronic 561-752-5000 11. Coilcraft 1-800-322-2645 12. Central Semi 631-435-1110 April 2010 19 M9999-041210-B Micrel, Inc. MIC2169B Bill of Materials (continued) Item Part Number Manufacturer Description R8 CRCW06034021FRT1 Vishay 4.02k, 0603, 1/16W, 1% 1 R9 CRCW120610R0FRT1 Vishay 10, 1/8W, 1206, 1% 1 R10 CRCW12062R00FRT1 Vishay 2, 1/8W, 1206, 1% 1 R12 CRCW12061R40FRT1 Vishay 1.4, 1/8W, 1206, 1% R14 J1, J3, J4, J5 2551-2-00-01-00-00-07-0 MillMax Qty. Open 0 Turrent Pins 4 Notes: 13. Micrel.Inc 408-944-0800 14. Vishay corp 206-452-5664 15. Diodes. Inc 805-446-4800 16. Sumida 408-321-9660 17. TDK 847-803-6100 18. muRata 800-831-9172 19. AVX 843-448-9411 20. International Rectifier 847-803-6100 21. Fairchild Semiconductor 207-775-8100 22. Cooper Electronic 561-752-5000 23. Coilcraft 1-800-322-2645 24. Central Semi 631-435-1110 April 2010 20 M9999-041210-B Micrel, Inc. MIC2169B MIC2169B PCB Layout MIC2169B Top Layer April 2010 MIC2169B Bottom Layer 21 M9999-041210-B Micrel, Inc. MIC2169B Package Information 10-Pin ePad MSOP (MME) April 2010 22 M9999-041210-B Micrel, Inc. MIC2169B Package Information (continued) 10-Pin MSOP (MM) April 2010 23 M9999-041210-B Micrel, Inc. MIC2169B MIC2169B Land Patterns Recommended Land Pattern for 10-Pin MSOP April 2010 24 M9999-041210-B Micrel, Inc. MIC2169B MIC2169B Land Patterns (continued) Recommended Land Pattern for ePad 10-Pin MSOP MICREL, INC. 2180 FORTUNE DRIVE SAN JOSE, CA 95131 USA TEL +1 (408) 944-0800 FAX +1 (408) 474-1000 WEB http://www.micrel.com The information furnished by Micrel in this data sheet is believed to be accurate and reliable. However, no responsibility is assumed by Micrel for its use. Micrel reserves the right to change circuitry and specifications at any time without notification to the customer. Micrel Products are not designed or authorized for use as components in life support appliances, devices or systems where malfunction of a product can reasonably be expected to result in personal injury. Life support devices or systems are devices or systems that (a) are intended for surgical implant into the body or (b) support or sustain life, and whose failure to perform can be reasonably expected to result in a significant injury to the user. A Purchaser's use or sale of Micrel Products for use in life support appliances, devices or systems is a Purchaser's own risk and Purchaser agrees to fully indemnify Micrel for any damages resulting from such use or sale. (c) 2006 Micrel, Incorporated. April 2010 25 M9999-041210-B