Datasheet LTC3785-1 (Analog Devices) - 8

LTC3785-1
OPERATION MAIN CONTROL LOOP
V V IN OUT The LTC3785-1 is a buck-boost voltage mode controller TG1 A D TG2 that provides an output voltage above, equal to or below L the input voltage. SW1 SW2 The LTC proprietary topology and control architecture also BG1 B C BG2 employs drain-to-source sensing (No RSENSE) for forward 37851 F01 and reverse current limiting. The controller provides all N-channel MOSFET output switch drive, facilitating
Figure 1. Response Time Test Circuit
single package multiple power switch technology along with lower R 90% DS(ON). The error amp output voltage (VC) DMAX determines the output duty cycle of the switches. Since BOOST A ON, B OFF the V BOOST REGION C pin is a fi ltered signal, it provides rejection of high PWM C, D SWITCHES frequency noise. DMIN BOOST FOUR SWITCH PWM BUCK/BOOST REGION The FB pin receives the voltage feedback signal, which DMAX BUCK is compared to the internal reference voltage by the er- D ON, C OFF BUCK REGION ror amplifi er. The top MOSFET drivers are biased from a PWM A, B SWITCHES DMIN fl oating bootstrap capacitor, which is normally recharged BUCK 37851 F02 during each off cycle through an external diode when the
Figure 1. Response Time Test Circuit
top MOSFET turns off. Optional Schottky diodes can be connected across synchronous switch B and D to provide the off time of switch A, synchronous switch B turns on for a lower drop during the dead time and eliminate effi ciency the remainder of the switching period. Switches A and B will loss due to body diode reverse recovery. alternate similar to a typical synchronous buck regulator. As the control voltage increases, the duty cycle of switch The main control loop is shut down by pulling the RUN/ A increases until the max duty cycle of the converter in SS pin low. An internal 1μA current source charges the buck mode reaches DMAX_BUCK, given by: RUN/SS pin and when the pin voltage is higher than 0.7V the IC is enabled. The VC voltage is then clamped to the DMAX_BUCK = 100 – D4(SW)% RUN/SS voltage minus 0.7V while CSS is slowly charged where D4(SW) = duty cycle % of the four switch range. during start-up. This soft-start clamping prevents inrush current draw from the input power supply. D4(SW) = (300ns • f) • 100% where f = operating frequency, Hz.
POWER SWITCH CONTROL
Beyond this point the four switch or buck-boost region Figure 1 shows a simplifi ed diagram of how the four power is reached. switches are connected to the inductor, VIN, VOUT and GND. Figure 2 shows the regions of operation for the LTC3785-1
Buck-Boost or Four Switch (VIN ~ VOUT)
as a function of duty cycle D. The power switches are When the error amp output voltage, VC, is above ap- properly controlled so that the transfer between modes proximately 0.65V, switch pair AD remain on for duty is continuous. cycle DMAX_BUCK, and the switch pair AC begin to phase in. As switch pair AC phases in, switch pair BD phases
Buck Region (VIN > VOUT)
out accordingly. When the VC voltage reaches the edge of Switch D is always on and switch C is always off during the buck-boost range, approximately 0.7V, the AC switch buck mode. When the error amp output voltage, VC, is pair completely phase out the BD pair, and the boost phase approximately above 0.1V, output A begins to switch. During begins at duty cycle, D4(SW). 37851fa 8