Datasheet LTC3785 (Analog Devices) - 8

LTC3785 operaTion
MAIN CONTROL LOOP
VIN VOUT The LTC3785 is a buck-boost voltage mode controller that provides an output voltage above, equal to or below the TG1 A D TG2 input voltage. SW1 L SW2 The LTC proprietary topology and control architecture also BG1 B C BG2 employs drain-to-source sensing (No RSENSE) for forward and reverse current limiting. The control er provides 3785 F01 all N-channel MOSFET output switch drive, facilitating single package multiple power switch technology along
Figure 1. Output Switch Configuration
with lower RDS(ON). The error amp output voltage (VC) determines the output duty cycle of the switches. Since the VC pin is a filtered signal, it provides rejection of high 90% D frequency noise. MAX BOOST A ON, B OFF BOOST REGION The FB pin receives the voltage feedback signal, which PWM C, D SWITCHES D is compared to the internal reference voltage by the er- MIN BOOST ror amplifier. The top MOSFET drivers are biased from a FOUR SWITCH PWM BUCK/BOOST REGION DMAX BUCK floating bootstrap capacitor, which is normally recharged D ON, C OFF during each off cycle through an external diode when the BUCK REGION PWM A, B SWITCHES top MOSFET turns off. Optional Schottky diodes can be DMIN BUCK 3785 F02 connected across synchronous switch B and D to provide a lower drop during the dead time and eliminate efficiency
Figure 2. Operation Mode vs VC Voltage
loss due to body diode reverse recovery. The main control loop is shut down by pulling the RUN/ the off time of switch A, synchronous switch B turns on for SS pin low. An internal 1µA current source charges the the remainder of the switching period. Switches A and B will RUN/SS pin and when the pin voltage is higher than 0.7V alternate similar to a typical synchronous buck regulator. the IC is enabled. The V As the control voltage increases, the duty cycle of switch C voltage is then clamped to the RUN/SS voltage minus 0.7V while C A increases until the max duty cycle of the converter in SS is slowly charged during start-up. This soft-start clamping prevents inrush buck mode reaches DMAX_BUCK, given by: current draw from the input power supply. DMAX_BUCK = 100 – D4(SW)% where D4(SW) = duty cycle % of the four switch range.
POWER SWITCH CONTROL
D4(SW) = (300ns • f) • 100% Figure 1 shows a simplified diagram of how the four power switches are connected to the inductor, V where f = operating frequency, Hz. IN, VOUT and GND. Figure 2 shows the regions of operation for the LTC3785 Beyond this point the four switch or buck-boost region as a function of duty cycle D. The power switches are is reached. properly controlled so that the transfer between modes is continuous.
Buck-Boost or Four Switch (VIN ~ VOUT)
When the error amp output voltage, V
Buck Region (V
C, is above ap-
IN > VOUT)
proximately 0.65V, switch pair AD remain on for duty Switch D is always on and switch C is always off during cycle DMAX_BUCK, and the switch pair AC begin to phase buck mode. When the error amp output voltage, VC, is ap- in. As switch pair AC phases in, switch pair BD phases proximately above 0.1V, output A begins to switch. During out accordingly. When the VC voltage reaches the edge of 3785fc Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Application Package Description Revision History Typical Application Related Parts