SC2612E 500kHz Step-Down DC/DC Converter POWER MANAGEMENT Description Features u u u u u u The SC2612E is a voltage mode switcher designed for low cost, “point of use” voltage conversion. SC2612E is available with fixed switching frequencies of 500kHz. The SC2612E 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 500kHz Input supply of 4.5V to 15V 0.5A Drive current for up to 10A output Output voltages down to 0.8V Overcurrent protection and soft start SO-8 package Applications u Graphics IC Power supplies u Embedded, low cost, high efficiency converters Typical Application Circuit 12V IN 5V IN R1 C10 C1 C2 U2 2 7 8 C5 4 C7 VCC COMP BST DH SS/EN DL GND FB 6 5 R2 3 R3 Q2 L1 1.5V OUT 1 Q3 R6 C3 SC2612E C9 R9 R10 Revision: October 12, 2004 1 www.semtech.com SC2612E 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 18 V VBST 20 V VDLO, VDHI -1 to +20 V DH to GND Negative Pulse (tpulse < 10ns) VDH_PULSE -4.5 V DL to GND Negative Pulse (tpulse < 20ns) VDL_PULSE -4.5 V Operating Ambient Temperature Range TA 0 to 70 °C Operating Junction Temperature TJ 125 °C TSTG -65 to 150 °C TLEAD 300 °C θJA 113 °C/W Thermal Resistance Junction to Case θJC 42 °C/W ESD Rating (Human Body Model) ESD 2 kV VCC Supply Voltage Boost Pin Voltage DL to GND , DH to GND (1) (1) Storage Temperature Lead Temperature (Soldering) 10s Thermal Resistance Junction to Ambient (2) Electrical Characteristics Unless specified: VCC = 4.5V to 12V; VFB = VO; BST = Vcc+5V; TA = 0 to 70°C Parameter Symbol Conditions Min Typ Max Units 15 V 10 mA 18 V 5 mA VCC Supply Voltage VCC VCC Quiescent Current IQVCC BST Supply Voltage VBST BST Quiescent Current IQBST VCC Under Voltage Lockout UVVCC 3.8 4.15 4.5 V BST Under Voltage Lockout UVBST 3.15 3.5 3.85 V 792 800 808 mV 0.7 V Output Voltage VOS Overcurrent trip voltage VITS Load Regulation 4.5 VCC = 5.0V, VBST = 12.0V, SS/EN = 0V 5 4.5 VCC = 5.0V, VBST = 12.0V, SS/EN = 0V IO = 10mA; VFB = VOS, TA = 25°C 0.4 IO = 0.2A to 4A Line Regulation Oscillator Frequency fOSC 400 Oscillator Max Duty Cycle δMAX 80 SS/EN Shutdown Voltage VSS 0.3 SS/EN Charge current ISS Vss = 0.8V 1 % ±0.5 % 500 600 kHz % 0.8 25 V µA 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 2004 Semtech Corp. 2 www.semtech.com SC2612E POWER MANAGEMENT Electrical Characteristics Unless specified: VCC = 4.5V to 12V; VFB = VO; BST = Vcc+5V; TA = 0 to 70°C Parameter Error Amplifier Transconductance Symbol (3) Error Amplifier Gain (3) Conditions gm A EA RCOMP = open Error Amplifier Source/Sink Current Modulator Gain (3) AM V C C = 5V Dead Time Min Typ Max Units 0.8 mS 45 dB ± 60 µA 19 dB 50 ns Notes: (1) See Gate Resistor selection recommendations. (2) 1square inch of FR4, double sided, 1oz. minimum copper weight. (3) Guaranteed by design, not tested in production. 2004 Semtech Corp. 3 www.semtech.com SC2612E POWER MANAGEMENT Pin Configuration Ordering Information TOP VIEW FB 1 8 SS/EN VCC 2 7 COMP DL 3 6 BST GND 4 5 DH Part Numbers (1) Frequency P ackag e SC2612ESTRT (2) 500kHz SO-8 Note: (1) Only available in tape and reel packaging. A reel contains 2500 devices. (2) Lead free product. This product is fully WEEE and RoHS compliant. (SO-8) Pin Descriptions Pin # Pin Name 1 FB 2 VC C Pin Function Switcher section feeedback input. Chip Supply Input Voltage. 3 DL 4 GND Switcher Low side FET drive output. 5 DH Switcher High side FET drive output. 6 BST Supply voltage for FET drives. 7 COMP Output of the Switcher section voltage error amplifier. 8 SS/EN Soft start and enable pin, controls the switcher output voltage ramp rate. Analog and Power Ground, connect directly to ground plane, see layout guidelines. 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 - 2004 Semtech Corp. SYNCHRONOUS MOSFET DRIVE 4 www.semtech.com SC2612E POWER MANAGEMENT Theory of Operation The SC2612E 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 æ V ö VO × çç1 - O ÷÷ è VIN ø = L × fOSC So choose inductor value from:æ V ö 5 × VO × çç1 - O ÷÷ è VIN ø L= IO × 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 æ V ö VO × VRIPPLE × çç1 - O ÷÷ è VIN ø < L × 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 × IT2 2 × VT × 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. 2004 Semtech Corp. 5 www.semtech.com SC2612E 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. MODULAT OR + EA FB - L OUT VOUT Vin=5V SC2612E AND FETS Ra COMP REF Zf Co Zs 1 2p × Co × 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. SC2612E AND FETS REF 1 2p LCo Zp Resr MODULAT OR + 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 æV ö GMOD = 19 + 20 × Logç IN ÷ è 5 ø Fp(lc ) = the divider gain is given by This is point B in Fig2. æ R8 G DIV = 20 × Log çç è R5 + R8 Fz(esr ) = 8.06 æ5ö æ ö GFMD = 19 + 20 × Logç ÷ + 20 × Logç ÷ = 13.6dB è5ø è 6.98 + 8.06 ø This is drawn as the line A-B in Fig2 ö ÷÷ ø 1 = 2.4kHz 2p × 3000 × 10 - 6 × 22 × 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 500kHz 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 æ RB ö æV ö ÷÷ GFMD = 19 + 20 × Logç IN ÷ + 20 × Logçç è 5 ø è R A + RB ø 2004 Semtech Corp. 1 1 = » 1.6kHz 2p LCo 2p 3.3 × 10 -6 × 3000 × 10 -6 6 www.semtech.com SC2612E 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 × Fz1 × Rs Cp = 1 2p × Fp1 × 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 × 25 × 103 × 30 × 10 3 Cp » 1 = 14pF (unecessar y due to EA rolloff ) 6 × 400 × 10 3 × 30 × 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 2004 Semtech Corp. 7 www.semtech.com SC2612E POWER MANAGEMENT Layout Guidelines Careful attention to layout requirements are necessary for successful implementation of the SC2612E 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 2 7 8 0.1uF 4 VCC BST COMP DH SS/EN DL GND FB 6 Q1 5 3 Vout Cin 1 Q2 L Cout SC2612E GND 2004 Semtech Corp. 8 www.semtech.com SC2612E 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 SC2612E 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 SC2612E should be supplied from the 5V supply through a 10Ω resistor, the Vcc pin should be decoupled directly to AGND by a 0.1µF ceramic capacitor, trace lengths should be as short as possible. Currents in Power Section Vin + Vout + 2004 Semtech Corp. 9 www.semtech.com SC2612E 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 2004 Semtech Corp. 6 8 10 IO (A) SS/EN Voltage (V) Typical Load Regulation 10 www.semtech.com SC2612E POWER MANAGEMENT Outline Drawing - SOIC-8 A D e N DIM E1 1 E 2 ccc C 2X N/2 TIPS .069 .053 .010 .004 .065 .049 .012 .020 .010 .007 .189 .193 .197 .150 .154 .157 .236 BSC .050 BSC .010 .020 .016 .028 .041 (.041) 8 8° 0° .004 .010 .008 A A1 A2 b c D E1 E e h L L1 N 01 aaa bbb ccc 2X E/2 e/2 B D DIMENSIONS MILLIMETERS INCHES MIN NOM MAX MIN NOM MAX aaa C h A2 A SEATING PLANE C A1 bxN bbb 1.75 1.35 0.25 0.10 1.65 1.25 0.31 0.51 0.25 0.17 4.80 4.90 5.00 3.80 3.90 4.00 6.00 BSC 1.27 BSC 0.25 0.50 0.40 0.72 1.04 (1.04) 8 0° 8° 0.10 0.25 0.20 h H C A-B D c GAGE PLANE 0.25 SEE DETAIL L (L1) A DETAIL SIDE VIEW 01 A NOTES: 1. CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES). 2. DATUMS -A- AND -B- TO BE DETERMINED AT DATUM PLANE -H- 3. DIMENSIONS "E1" AND "D" DO NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. 4. REFERENCE JEDEC STD MS-012, VARIATION AA. Land Pattern - SOIC-8 X DIM (C) G DIMENSIONS INCHES MILLIMETERS C G P X Y Z Z Y (.205) .118 .050 .024 .087 .291 (5.20) 3.00 1.27 0.60 2.20 7.40 P NOTES: 1. THIS LAND PATTERN IS FOR REFERENCE PURPOSES ONLY. CONSULT YOUR MANUFACTURING GROUP TO ENSURE YOUR COMPANY'S MANUFACTURING GUIDELINES ARE MET. 2. REFERENCE IPC-SM-782A, RLP NO. 300A. Contact Information Semtech Corporation Power Management Products Division 200 Flynn Rd., Camarillo, CA 93012 Phone: (805)498-2111 FAX (805)498-3804 2004 Semtech Corp. 11 www.semtech.com