SC2612A/C 600kHz/200kHz Step-Down DC/DC Converter POWER MANAGEMENT Description Features u u u u u u The SC2612 is a voltage mode switcher designed for low cost, "point of use" voltage conversion. SC2612 is available with fixed switching frequencies of 600kHz (SC2612A) and 200kHz (SC2612C). The SC2612 has soft start and enable functions and is short circuit protected. The output of the switcher may be set anywhere between 0.8V and 75% of Vin. Short circuit protection is disabled during start-up to allow the output capacitors time to fully charge. Operating frequency of 600kHz or 200kHz Input supply of 3V to 8V 0.5A Drive current for up to 10A output Output voltages down to 0.8V Overcurrent protection and soft start MSOP-8 package Applications u Graphics IC Power supplies u Embedded, low cost, high efficiency converters Typical Application Circuit 12V IN 3.3V IN R1 C10 C1 C2 U2 4 1 2 C5 5 C7 R9 VCC BST COMP DH SS/EN DL GND FB 8 7 R2 6 R3 3 Q2 L1 1.5V OUT Q3 SC2612 C3 R6 C9 R10 August 18, 2004 1 www.semtech.com SC2612A/C POWER MANAGEMENT Absolute Maximum Ratings Exceeding the specifications below may result in permanent damage to the device, or device malfunction. Operation outside of the parameters specified in the Electrical Characteristics section is not implied. Parameter Symbol Maximum Units VCC 15 V VBST 20 V VDLO, VDHI -1 to +20 V Operating Ambient Temperature Range TA 0 to 70 C Operating Junction Temperature TJ 125 C TSTG -65 to 150 C TLEAD 300 C JA 113 C/W Thermal Resistance Junction to Case JC 42 C/W ESD Rating (Human Body Model) ESD 2 kV Input Supply Voltage Boost Pin Voltage DL to GND (2) , DH to GND (2) Storage Temperature Lead Temperature (Soldering) 10s Thermal Resistance Junction to Ambient (3) Electrical Characteristics Unless specified: VCC = 3V to 12V; VFB = VO; BST = Vcc+5V; TA = 0 to 70C Parameter Symbol Conditions Min Typ Max Units 15 V 10 mA 20 V 5 mA VCC Supply Voltage VCC VCC Quiescent Current IQVCC BST Supply Voltage VBST BST Quiescent Current IQBST VCC Under Voltage Lockout UVVCC 2.3 2.6 2.9 V BST Under Voltage Lockout UVBST 7.0 8.0 9.0 V 792 800 808 mV 0.7 V Output Voltage VOS Overcurrent trip voltage VITS Load Regulation 3.0 VCC = 5.0V, VBST = 12.0V, SS/EN = 0V 5 11 VCC = 5.0V, VBST = 12.0V, SS/EN = 0V IO = 0; VFB = VOS, TA = 25C 0.4 IO = 0.2A to 4A Line Regulation Oscillator Frequency fOSC Oscillator Max Duty Cycle MAX SS/EN Shutdown Voltage VSS SS/EN Charge current ISS 1 % 0.5 % SC2612A 480 600 720 SC2612C 160 200 240 SC2612A, SC2612C 80 % 0.3 Vss = 0.8V kHz 0.8 25 V A Peak DH Sink/Source Current BST - DH = 4.5V, DH - GND = 3.3V DH - GND = 1.5V 0.5 50 A mA Peak DL Sink/Source Current BST - DL = 4.5V, DL - GND = 3.3V DL - GND = 1.5V 0.5 50 A mA 2003 Semtech Corp. 2 www.semtech.com SC2612A/C POWER MANAGEMENT Electrical Characteristics Unless specified: VCC = 3V to 12V; VFB = VO; BST = Vcc+5V; TA = 0 to 70C Parameter Symbol Error Amplifier Transconductance gm Error Amplifier Gain A EA Conditions RCOMP = open Error Amplifier Source/Sink Current Modulator Gain AM V C C = 5V Dead Time Min Typ Max Units 0.8 mS 45 dB 60 A 19 dB 50 ns Notes: (1) See Gate Resistor selection recommendations (2) 1 square inch of FR4, double sided, 1oz. minimum copper weight. 2003 Semtech Corp. 3 www.semtech.com SC2612A/C POWER MANAGEMENT Pin Configuration Ordering Information Part Numbers TOP VIEW COMP 1 8 BST SS/EN 2 7 DH FB 3 6 DL VCC 4 5 GND (1) Frequency SC2612AMSTR 600kHz SC2612AMSTRT (2) 600kHz SC2612CMSTR 200kHz SC2612CMSTRT (2) 200kHz P ackag e MSOP-8 Note: (1) Only available in tape and reel packaging. A reel contains 2500 devices. (2) Lead free products. (MSOP-8) Pin Descriptions Pin # Pin Name Pin Function 1 COMP Output of the Switcher section voltage error amplifier. 2 SS/EN Soft start and enable pin, controls the switcher output voltage ramp rate. 3 FB 4 VCC Chip Supply Input Voltage. Switcher section feeedback input. 5 GND Analog and Power Ground, connect directly to ground plane, see layout guidelines. 6 DL Switcher Low side FET drive output. 7 DH Switcher High side FET drive output. 8 BST Supply voltage for FET drives. Block Diagram VCC VREF UVLO + LEVEL SHIFT AND HIGH SIDE DRIVE - SHDN Q - DH R + FB BST UVLO & REF S - SHOOT -T HRU CONT ROL + COMP VREF R Q 25uA S OSCILLAT OR SS/EN + + SSOVER DL GND - 2003 Semtech Corp. SYNCHRONOUS MOSFET DRIVE 4 www.semtech.com SC2612A/C POWER MANAGEMENT Theory of Operation The SC2612 is a step down DC/DC controller designed for minimum cost and size without sacrificing accuracy and protection. Overcurrent protection is implemented by a simple undervoltage detection scheme and is disabled until soft start has been completed to eliminate false trips due to output capacitor charging. The SS/EN pin is held low, as are the DH and DL pins, until the undervoltage lockout points are exceeded. Once the VCC and BST pins both rise above their undervoltage lockout points, the SS capacitor begins to charge, controlling the duty cycle of the switcher, and therefore slowly ramping up the switcher output voltage. Once the SS capacitor is charged, the current limit circuitry is enabled. If a short circuit is applied , the output will be pulled down below it's trip point and shut down. The device may be restarted by either cycling power, or momentarily pulling SS/EN low. Component Selection OUTPUT INDUCTOR - A good starting point for output filter component selection is to choose an inductor value that will give an inductor ripple current of approximately 20% of max. output current. Inductor ripple current is given by:- IL RIPPLE ae V o VO x cc1 - O // e VIN o = L x fOSC So choose inductor value from:ae V o 5 x VO x cc1 - O // e VIN o L= IO x fOSC OUTPUT CAP ACIT OR(S) - The output capacitors should CAPA CITOR(S) be selected to meet output ripple and transient response criteria. Output ripple voltage is caused by the inductor ripple current flowing in the output capacitor's ESR (There is also a component due to the inductor ripple current charging and discharging the output capacitor itself, but this component is usually small and can often be ignored). Given a maximum output voltage ripple requirement, ESR is given by:- RESR ae V o VO x VRIPPLE x cc1 - O // e VIN o < L x fOSC Output voltage transient excursions are a function of load current transient levels, input and output voltages and inductor and capacitor values. Capacitance and RESR values to meet a required transient condition can be calculated from:RESR < C> VT IT L x IT2 2 x VT x VA where VA = VIN - VO for negative transients (load application) and VA = VO for positive transients (load release) values for positive and negative transients must be calculated seperately and the worst case value chosen. For Capacitor values, the calculated value should be doubled to allow for duty cycle limitation and voltage drop issues. 2003 Semtech Corp. 5 www.semtech.com SC2612A/C POWER MANAGEMENT Calculate the filter double pole frequency (Fp(lc)) COMPENSA TION COMPONENTS - Once the filter comCOMPENSATION ponents have been determined, the compensation components can be calculated. The goal of compensation is to modify the frequency response characteristics of the error amplifier to ensure that the closed loop feedback system has the highest gain and bandwidth possible while maintaining stability. A simplified stability criteria states that the open loop gain of the converter should fall through 0dB at 20dB/ decade at a frequency no higher than 20-25% of the switching frequency. This objective is most simply met by generating asymptotic bode plots of the small signal response of the various sections of the converter. Fp(lc ) = and calculate ESR Zero frequency (Fz(esr)) Fz( esr ) = Type 2 Example As an example of type 2 compensation, we will use the Evaluation board schematic. MODULATOR + EA FB - L OUT VOUT Vin=5V SC2612 AND FETS Ra COMP REF Zf Co Zs 1 2p x Co x Re sr Choose an open loop crossover frequency (Fco) no higher than 20% of the switching frequency (Fs). The proximity of Fz(esr) to the crossover frequency Fco determines the type of compensation required, if Fz(esr)>Fco/4, use type 3 compensation, otherwise use type 2. Type 1 compensation is not appropriate and is not discussed here. SC2612 AND FETS REF 1 2p LCo Zp Resr MODULATOR + EA FB - 3.3uH OUT VOUT Rb 6.98k COMP 3000uF Cs Cp 22mOhm 8.06k Rs It is convenient to split the converter into two sections, the Error amp and compensation components being one section and the Modulator, output filter and divider being the other. First calculate the DC Filter+Modulator+Divider gain The DC filter gain is always 0dB, the Modulator gain is 19dB at 5V in and is proportional to Vin, so modulator gain at any input voltage is. The total Filter+Modulator+Divider DC Gain is aeV o GMOD = 19 + 20 x Logc IN / e 5 o Fp(lc ) = the divider gain is given by This is point B in Fig2. ae R8 G DIV = 20 x Log cc e R5 + R8 Fz(esr ) = 8.06 ae5o ae o GFMD = 19 + 20 x Logc / + 20 x Logc / = 13.6dB e5o e 6.98 + 8.06 o This is drawn as the line A-B in Fig2 o // o 1 = 2.4kHz 2p x 3000 x 10 - 6 x 22 x 10 -3 This is point C in Fig2., the line joining B-C slopes at 40dB/decade, the line joining C-D slopes at -20dB/decade. For 600kHz switching frequency, crossover is designed for 100kHz. Since Fz(esr)<<Fco/4 Type 2 compensation is appropriate. So the total Filter+Modulator+Divider DC Gain is ae RB o aeV o // GFMD = 19 + 20 x Logc IN / + 20 x Logcc e 5 o e R A + RB o 2003 Semtech Corp. 1 1 = 1.6kHz 2p LCo 2p 3.3 x 10 -6 x 3000 x 10 -6 6 www.semtech.com SC2612A/C POWER MANAGEMENT Having plotted the line ABCD, and confirmed the type of compensation necessary, compensation component values can be determined. At Fco, the line ABCD shows a gain of -27.5dB and a slope of -20dB/decade. In order for the total open loop gain to be 0dB with a -20dB/decade slope at this frequency, the compensated error amp gain at Fco must be +27.5dB with a 0dB slope. This is the line FG on the plot below. Since open loop DC gain should be as high as possible to minimize errors, a zero is placed at F and to minimize high frequency gain and switching interference a pole is placed at G. The zero at F should be no higher than Fco/4 and the pole at G no lower than 4*Fco. The equations to set the gain and the pole and zero locations are: A 10 20 Rs = where A = gain at Fco (in dB) gm Cs = 1 2p x Fz1 x Rs Cp = 1 2p x Fp1 x Rs For this example, this results in the following values. 27.5 10 20 Rs = = 29.6kW 30kW 0.8 Cs 1 = 0.22nF 6 x 25 x 103 x 30 x 10 3 Cp 1 = 14pF (unecessar y due to EA rolloff ) 6 x 400 x 10 3 x 30 x 10 3 100 80 E 60 Compensated Error Amp gain Gain (dB) 40 G F H 20 A Fz1 B Fp1 C 0 Total open loop gain Fp(lc) Fz(esr) -20 Filter+modulator +divider gain Fco -40 -60 100.0E+0 1.0E+3 10.0E+3 100.0E+3 D 1.0E+6 Frequency (Hz) Fig2: Type 2 Error Amplifier Compensation 2003 Semtech Corp. 7 www.semtech.com SC2612A/C POWER MANAGEMENT Layout Guidelines Careful attention to layout requirements are necessary for successful implementation of the SC2612 PWM controller. High currents switching at high frequency are present in the application and their effect on ground plane voltage differentials must be understood and minimized. 1). The high power parts of the circuit should be laid out first. A ground plane should be used, the number and position of ground plane interruptions should be such as to not unnecessarily compromise ground plane integrity. Isolated or semi-isolated areas of the ground plane may be deliberately introduced to constrain ground currents to particular areas, for example the input capacitor and bottom FET ground. 2). The loop formed by the Input Capacitor(s) (Cin), the Top FET (Q1) and the Bottom FET (Q2) must be kept as small as possible. This loop contains all the high current, fast transition switching. Connections should be as wide and as short as possible to minimize loop inductance. Minimizing this loop area will a) reduce EMI, b) lower ground injection currents, resulting in electrically "cleaner" grounds for the rest of the system and c) minimize source ringing, resulting in more reliable gate switching signals. 3). The connection between the junction of Q1, Q2 and the output inductor should be a wide trace or copper region. It should be as short as practical. Since this connection has fast voltage transitions, keeping this connection short will minimize EMI. The connection between the output inductor and the output capacitors should be a wide trace or copper area, there are no fast voltage or current transitions in this connection and length is not so important, however adding unnecessary impedance will reduce efficiency. 12V IN Vin 10 10uF U1 4 1 0.1uF 2 5 VCC BST COMP DH SS/EN GND DL FB 8 Q1 7 6 Vout Cin 3 Q2 L Cout SC2612 GND 2003 Semtech Corp. 8 www.semtech.com SC2612A/C POWER MANAGEMENT Layout Guidelines (Cont.) 4) The Output Capacitor(s) (Cout) should be located as close to the load as possible, fast transient load currents are supplied by Cout only, and connections between Cout and the load must be short, wide copper areas to minimize inductance and resistance. 5) The SC2612 is best placed over a quiet ground plane area, avoid pulse currents in the Cin, Q1, Q2 loop flowing in this area. PGNDH and PGNDL should be returned to the ground plane close to the package. The AGND pin should be connected to the ground side of (one of) the output capacitor(s). If this is not possible, the AGND pin may be connected to the ground path between the Output Capacitor(s) and the Cin, Q1, Q2 loop. Under no circumstances should AGND be returned to a ground in- side the Cin, Q1, Q2 loop. 6) Vcc for the SC2612 should be supplied from the 5V supply through a 10 resistor, the Vcc pin should be decoupled directly to AGND by a 0.1F ceramic capacitor, trace lengths should be as short as possible. Currents in Power Section Vin + Vout + 2003 Semtech Corp. 9 www.semtech.com SC2612A/C POWER MANAGEMENT Typical Characteristics 100% 1.500 95% 1.498 VIN = 5V 90% 12V 1.496 VO (V) Efficiency (%) IO = 2.00A; VBST = 18V VBST = 12V for VIN = 5V VBST = 18V for VIN = 12V 85% 1.494 80% 1.492 75% 1.490 70% 0 2 4 6 8 4 10 5 6 7 8 Typical Efficiency 10 11 12 Typical Line Regulation 0.0% 100% 80% VBST = 12V for VIN = 5V VBST = 18V for VIN = 12V -0.5% VIN = 12V 60% VO (V) Duty Cycle (%) (No Feedback) 9 VIN (V) Output Current (A) 5V -1.0% 40% -1.5% 20% -2.0% 0% 0.0 0.2 0.4 0.6 0.8 1.0 0 1.2 2 4 SS/EN Control of duty cycle 2003 Semtech Corp. 6 8 10 IO (A) SS/EN Voltage (V) Typical Load Regulation 10 www.semtech.com SC2612A/C POWER MANAGEMENT Evaluation Board Schematic & Layout GND J5 12V IN J1 3.3V - -12V 3.3V 5 VININ J2 10 C1 C3 1500uF 0.1uF C4 1500uF C5 1500uF C2 10uF GND J6 U1 R1 4 1 C8 2 EN J11 5 220pF BST COMP DH SS/EN DL GND FB 8 7 6 R3 2.2 R2 2.2 EMPTY C9 Q1 Si4410DY L1 1uH 3 Q2 Si4410DY SC2612 C11 R7 30k VCC R5 6.98k 1.5V OUT J4 C6 1500uF D1 EMPTY C10 R8 8.06k 1500uF 0.1uF GND GND J8 J7 2003 Semtech Corp. 11 www.semtech.com SC2612A/C POWER MANAGEMENT Outline Drawing - MSOP-8 Land Pattern - MSOP-8 Contact Information Semtech Corporation Power Management Products Division 200 Flynn Road, Camarillo, CA 93012 Phone: (805)498-2111 FAX (805)498-3804 2003 Semtech Corp. 12 www.semtech.com