SC2608A

Simple, Synchronous Voltage Mode PWM Controller POWER MANAGEMENT Description The SC2608A is a versatile voltage-mode PWM controller designed for use in step down DC/DC power supply applications. A simple, fixed frequency, highly efficient buck regulator can be implemented using the SC2608A with minimal external components. The input voltage range is from +5V to +12V. Internal level shift and drive circuitry eliminates the need for an expensive P-channel, high-side MOSFET. The small device footprint allows for compact circuit design. SC2608A features include temperature compensated voltage reference, triangle wave oscillator, current limit comparator, and an externally compensated error amplifier. Current limit is implemented by sensing the voltage drop across the bottom MOSFET RDS(ON). The SC2608A operates at a fixed frequency of 250kHz providing an optimum compromise between efficiency , external component size, and cost. SC2608A has a thermal protection circuit, which is activated if the junction temperature exceeds 150 OC. SC2608A Features +5V or +12V input voltage 250kHz operation High efficiency (>90%) 1.5% Reference voltage accuracy Hiccup mode over current protection Robust output drive RDS(ON) Current sensing for protection Industrial temperature range SO-8 package Integrated boot strap diode Thermal Shut down Fully WEEE and RoHS Compliant Applications Termination supplies Low cost microprocessor supplies Peripheral card supplies Industrial power supplies High density DC/DC conversion Typical Application Circuit SC2608 SC2608A sense COMP/SS SENSE +12V/+5V/+3.3V +12V GND VCC DL PHASE DH BST VOUT sense Figure 1 Revision 2: April, 2006 1 www.semtech.com SC2608A 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. P ar am e t e r VCC to GND BST to PHASE BST to GND PHASE to GND (note1) DH to PHASE (note1) S y m b ol M ax i m u m +20 +15 +35 -1 to +24 +15 -1 to +15 +7 +7 Un i ts V V V V V V V V O O DL to GND (note2) COMP/SS to GND SENSE to GND Thermal Resistance Junction to Case Thermal Resistance Junction to Ambient Op erating Temp erature Range Storage Temp erature Range ESD Rating (Human Body Model) qJC qJA TJ TSTG ESD 40 120 -40 to +125 -65 to +150 2 C/W C/W O O C C kV Note 1: Under pulsing condition, the peak negative voltage can not be lower than -3.6V with less than 20nS from 50% to 50%. Note 2: Under pulsing condition, the peak negative voltage can not be lower than -5V with less than 20nS from 50% to 50%. Electrical Characteristics Unless specified: VCC = 12V, VBST - VPhase = 12 V, VOUT = 3.3V, TJ = TA = 25oC. P a r a m et er P ow er Su p p l y Supply Voltage Supply Current E r r or A m p l i f i er Feedback Voltage Sym b ol C on d i t i on s Mi n Typ Ma x Un i ts VCC ICC VCOMP < 0.4V 4.5 6 14 V mA V FB 4.75V 400nS VGS = 4.5V (src) VGS = 2.5V (snk) CL = 3000p F, See Fig. 2 CL = 4000p F, See Fig. 2 See Fig. 2 4.75V < Vcc < 12.6V 0.6 0.6 0.8 0.7 50 50 80 400 A A ns ns ns ns V T R IP 4.75V < Vcc < 12.6V Vtrip = VPHASE - GND -400 -350 -300 mV ISRC ISNK VCOMP < 2.5V VCOMP > 0.5V 6 -6 uA uA V th -40< TJ < 85OC 3.9 4.1 4.3 V TOTP 150 o C © 2006 Semtech Corp. 3 www.semtech.com SC2608A POWER MANAGEMENT Gate Drive Timing Diagram Figure 2 Block Diagram VCC BST DH PHASE OSC S + REF 0.8V R Q LEVEL SHIFT E/A + NON-OVERLAP TIMING 0 PWM VCC OCP & UVLO SENSE - Vcc DL GND 0 OCP PHASE + 0 0 COMP/SS Figure 3 © 2006 Semtech Corp. 4 www.semtech.com SC2608A POWER MANAGEMENT Pin Configuration Top View Ordering information Device (1 ) Package SO-8 Evaluation Board Temp Range (TJ) -40 to 125OC SC2608ASTRT(2) BST DH GND DL 1 2 3 4 (8-Pin SO-8) 8 7 6 5 PHASE COMP/SS SENSE VCC SC2608AEVB Notes: (1) Only available in tape and reel packaging. A reel contains 2500 devices. (2) This device is fully WEEE and RoHS Compliant Pin Descriptions Pin # 1 2 3 4 5 6 P i n N am e BST DH GN D DL VCC Sense COMP/SS PHASE Bootstrap for high side driver. High side driver outp ut. Ground. Low side driver outp ut. Chip bias sup p ly p in. Outp ut voltage sense inp ut. Error amp lifier outp ut. Connect comp ensation network to GN D. The comp ensation cap acitor serves as soft star t cap acitor. By p ulling this p in low will disable the outp ut. Connect this p in to the switching node between the MOSFETs. P i n Fu n c t i o n 7 8 © 2006 Semtech Corp. 5 www.semtech.com SC2608A POWER MANAGEMENT Theory of Operation Synchronous Buck Converter The output voltage of the synchronous converter is set and controlled by the output of the error amplifier. The inverting input of the error amplifier receives its voltage from the SENSE pin. The non-inverting input of the error amplifier is connected to an internal 0.8V reference. The error amplifier output is connected to the compensation pin. The error amplifier generates a current proportional to (0.8V-Vsense), which is the COMP pin output current (Transconductance ~ 7mS). The voltage on the COMP pin is the integral of the error amplifier current. The COMP voltage is the non-inverting input of the PWM comparator and controls the duty cycle of the MOSFET drivers. The compensation network controls the stability and transient response of the regulator. The larger capacitor, the slower COMP voltage changes, and slower the duty cycle changes. The non-inverting input voltage of the PWM comparator is the triangular ramp signal generated from the oscillator. The peak-to-peak voltage of the ramp is 1V, this is a parameter used in control loop calculation. When the oscillator ramp signal rises above the COMP voltage, the comparator output goes high and the PWM latch is reset. This pulls DH low, turning off the high-side MOSFET. After a short delay (dead time), DL is pulled high, turning on the low-side MOSFET. The oscillator also produces a set pulse for the PWM latch to turn off the low-side MOSFET, After a delay time, DH is pulled high to turn on the highside MOSFET. The delay time is determined by a monostable on the chip. The triangle wave minimum is about 0.8V, and the maximum is about 1.8V. Thus, if Vcomp = 0.7V, high side duty cycle is the minimum (~0%) , but if Vcomp is 1.8V, duty cycle is at maximum ( ~90%).The internal oscillator uses an on-chip capacitor and trimmed precision current sources to set the oscillation frequency to 250kHz. Figure 1 shows a 2.5V output converter. If the Vout 2.5V, the error amplifier will sink current and reduce the COMP voltage, so that duty cycle will decrease.The circuit will be in steady state when Vout =2.5V , Vsense = 0.8V, Icomp = 0. The COMP voltage and duty cycle depend on Vin. outputs remain in the off state whenever the supply voltage drops below the set threshold. Lockout occurs if VCC falls below 4.1V typ. Soft Start The SC2608A provides a soft start function to prevent large inrush currents upon power-up or hiccup retry. If both COMP and SENSE pins are low (