SC2610A/B 600kHz/1MHz Step-Down DC/DC Converter POWER MANAGEMENT Description Features The SC2610 is a voltage mode switcher and LDO combo designed for low output voltages and tracking power supplies. The SC2610 is available with fixed switching frequencies of 600kHz (SC2610A) and 1MHz (SC2610B). The SC2610 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. The LDO controller drives an external FET to provide an output voltage which can be set between 1.25V and 90% of Vin. It is RDS(ON) overcurrent protected. Short circuit protection is disabled during startup to allow the output capacitors time to fully charge. Operating frequency of 600kHz or 1Mhz Input supply of 3V to 15V 0.5A Drive current for up to 10A output Output voltages down to 0.8V Overcurrent protection on both outputs Soft Start for switcher output MSOP-10 package Applications Graphics IC Power supplies Tracking Power supplies VTT Supply and reference Typical Application Circuit 12V IN 3.3V IN R1 C10 C1 C2 U1 5 Q1 3 2.5V OUT 4 R5 C4 10 C5 2 C7 R8 Revision 4, March 2003 R9 VCC LDOG LDFB BST DH DL COMP FB SS/EN GND 9 8 R2 7 R3 1 6 Q2 L1 1.5V OUT Q3 C3 R6 SC2610 C9 R10 1 www.semtech.com SC2610A/B 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 Input Supply Voltage VCC 15 V Boost Pin Voltage VBST 20 V VDLO, VDHI -1 to +20 V Operating Ambient Temperature Range TA 0 to 70 C Operating Junction Temperature TJ 125 C Storage Temperature TSTG -65 to 150 C Lead Temperature (Soldering) 10s TLEAD 300 C JA 113 C/W ESD 2 kV DL to GND , DH to GND (1) (1) Thermal Resistance Junction to Ambient (2) ESD Rating (Human Body Model) 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 General VCC Supply Voltage VCC VCC Quiescent Current IQVCC BST Supply Voltage VBST BST Quiescent Current IQBST 3.0 VCC = 5.0V, VBST = 12.0V, SS/EN = 0V 5 11 VCC = 5.0V, VBST = 12.0V, SS/EN = 0V VCC Under Voltage Lockout UVVCC 2.3 2.6 2.9 V BST Under Voltage Lockout UVBST 7.0 8.0 9.0 V 1.242 1.255 1.268 V Linear Section Output Voltage VOL LDFB = VOL, TA = 25C Gain AOLL LDFB to LDOG 90 dB IO = 0 to 4A 0.5 % 0.5 % Load Regulation Line Regulation Output Impedance VGATE = 6.5V Overcurrent Trip Voltage VITL SS/EN Shutdown voltage V ss Gate Pulldown Impedance A t LD F B pi n LDOG to GND; VCC = VBST = 0V LDFB Input Impedance 2003 Semtech Corp. 1 1.5 k 0.6 0.8 1.0 V 0.2 0.4 0.6 V 80 300 750 k 10 2 k www.semtech.com SC2610A/B POWER MANAGEMENT Electrical Characteristics (Continued) Unless specified: VCC = 3V to 12V; VFB = VO; BST = Vcc+5V; TA = 0 to 70C Parameter Symbol Conditions Min Typ Max Units 792 800 808 mV 0.7 V Sw itching Section Output Voltage VOS Overcurrent Trip Voltage VITS Load Regulation IO = 500mA; 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 % S C 2610A 480 600 720 S C 2610B 800 1000 1200 S C 2610A 80 S C 2610B 70 % 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 Error Amplifier Transconductance gm Error Amplifier Gain A EA RCOMP = open Error Amplifier Source/Sink Current Modulator Gain AM V C C = 5V Dead Time 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 SC2610A/B POWER MANAGEMENT Pin Configuration Ordering Information TOP VIEW FB 1 10 SS/EN 2 9 BST LDOG 3 8 DH LDFB 4 7 DL VCC 5 6 GND COMP Part Numbers Frequency P ackag e SC2610AMSTR(1) 600kHz MSOP-10 SC2610BMSTR(1) 1MHz MSOP-10 Note: (1) Only available in tape and reel packaging. A reel contains 2500 devices. (MSOP-10) Pin Descriptions Pin # Pin Name Pin Function 1 FB 2 SS/EN Soft start and enable pin, controls the switcher output voltage ramp rate. 3 LDOG Connect to the Gate of the LDO FET. 4 LD F B LDO Feedback Input. Switcher section feeedback input. 5 VC C Chip Supply Input Voltage. 6 GND Analog and Power Ground, connect directly to ground plane, see layout guidelines. 7 DL Switcher Low side FET drive output. 8 DH Switcher High side FET drive output. 9 BST Supply voltage for FET drives, usually 12V 10 COMP Output of the Switcher section voltage error amplifier. Block Diagram LDOG LDFB VCC + VREF UVLO + BST UVLO & REF LEVEL SHIFT AND HIGH SIDE DRIVE + DH - SHDN R + FB Q - - S + COMP R VREF Q S OSCILLATOR 25uA SS/EN SHOOT-THRU CONTROL + + SYNCHRONOUS MOSFET DRIVE DL GND SSOVER 100mV 2003 Semtech Corp. 4 www.semtech.com SC2610A/B POWER MANAGEMENT Theory of Operation The SC2610 is a switcher and LDO controller combo, designed for minimum cost and size without sacrificing accuracy and protection. Overcurrent protection on both outputs 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 LDO does not employ soft start, it's output voltage starts up as soon as the VCC and BST undervoltage lockouts are exceeded. The SS/EN pin is also 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 limits for both the switcher and the LDO are enabled. If a short circuit is applied to either output, that output will be pulled down below it's trip point, shutting down both outputs. 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 V VO 1 - O V IN = L fOSC So choose inductor value from: V 5 VO 1 - O V IN 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 < L fOSC VRIPPLE V VO 1 - O V IN 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 applicatio n) 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 SC2610A/B POWER MANAGEMENT Component Selection (Continued.) Calculate the filter double pole frequency (Fp(lc)) COMPENSATION COMPONENTS - Once the filter components 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 Vin=5V SC2610 AND FETS VOUT Ra COMP REF Zf Zp Resr MODULATOR + EA FB Co Zs 1 2 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. SC2610 AND FETS REF 1 2 LCo - 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 V GMOD = 19 + 20 Log IN 5 Fp(lc ) = the divider gain is given by This is point B in Fig2. R8 G DIV = 20 Log R5 + R8 Fz( esr ) = 8.06 5 GFMD = 19 + 20 Log + 20 Log = 13.6dB 5 6.98 + 8.06 This is drawn as the line A-B in Fig2 1 = 2.4kHz 2 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 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 RB V GFMD = 19 + 20 Log IN + 20 Log 5 R A + RB 2003 Semtech Corp. 1 1 = 1.6kHz -6 2 LCo 2 3.3 10 3000 10-6 6 www.semtech.com SC2610A/B POWER MANAGEMENT Component Selection (Continued.) 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 2 Fz1 Rs Cp = 1 2 Fp1 Rs For this example, this results in the following values. 27.5 10 20 Rs = = 29.6k 30k 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 2003 Semtech Corp. 7 www.semtech.com SC2610A/B POWER MANAGEMENT Typical Characteristics 0.4% 0.2% VCC < 7V; VBST=12V VCC > 7V; VBST = VCC + 5V TA = 25OC VCC = 5V TA = 25OC 0.1% Switcher 0.2% VBST Line Reg. (%) VCC Line Reg. (%) 0.3% LDO 0.1% 0.0% -0.1% 0.0% LDO -0.1% Switcher -0.2% -0.3% -0.2% -0.4% -0.3% 0 2 4 6 8 10 12 5 14 7 9 11 Typical VCC Line Regulation 15 17 19 Typical VBST Line Regulation 0.0% 0.02% VCC = 5V IOS = 0A O TA = 25 C Switcher -0.02% LDO Load Reg (%) Switcher Output Loaded 0.00% Load Reg (%) Linear Output Loaded 13 VBST (V) VCC (V) LDO -0.04% -0.06% -0.08% -0.2% -0.4% Switcher -0.6% VCC = 5V VBST = 12V IOS = 0A O TA = 25 C -0.8% -1.0% -0.10% 0.0 0.5 1.0 1.5 2.0 0 2.5 2 4 6 8 10 12 IOS (A) IOL (A) Typical Load Regulation with Linear loaded Typical Load Regulation with Switcher loaded 100% VCC = 5V VBST = 12V VOS = 1.5V O TA = 25 C Efficiency (%) 90% 80% 70% 60% 0 2 4 6 8 10 12 IOS (A) Typical Switcher Efficiency 2003 Semtech Corp. Typical Switcher Ripple Voltage 8 www.semtech.com SC2610A/B POWER MANAGEMENT Typical Characteristics (Continued) Duty Cycle (%) (No Feedback) 100% 80% 60% 40% 20% 0% 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.8 2.0 SS/EN Voltage (V) SS/EN control of duty cycle 100 Duty Cycle (%) 80 60 40 20 0 0.8 1.0 1.2 1.4 1.6 Vcomp (V) COMP Pin control of duty cycle 2003 Semtech Corp. 9 www.semtech.com SC2610A/B POWER MANAGEMENT Evaluation Board Schematic & Layout GND J5 12V IN J1 3.3V - 5V IN J2 C1 0.1uF R1 10 C12 1uF C3 1500uF C4 1500uF C5 1500uF C2 10uF GND J6 Si4410DY Q1 5 R11 2.5V OUT J3 0 3 4 R4 8.06k 10 C7 220uF C8 220pF R6 8.06k GND J7 2003 Semtech Corp. R7 30k Q2 Si4410DY U1 C11 2 VCC BST LDOG DH LDFB DL COMP FB SS/EN GND C9 9 R2 2.2 R3 2.2 8 7 1 6 Q3 Si4410DY L1 3.3uH R9 R5 No Stuff 6.98k 1.5V OUT J4 C6 1500uF SC2610 C13 No Stuff 0.1uF D1 No Stuff C10 R8 8.06k 1500uF GND J8 C11 = NO STUFF 10 www.semtech.com SC2610A/B POWER MANAGEMENT Outline Drawing - MSOP-10 Land Pattern - MSOP-10 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. 11 www.semtech.com