LTC3785 Datasheet, PDF (8/20 Pages) Linear Technology – 10V, High Effi ciency, Synchronous, No RSENSE Buck-Boost Controller

LTC3785
U
OPERATIO
MAIN CONTROL LOOP
The LTC3785 is a buck-boost voltage mode controller
that provides an output voltage above, equal to or below
the input voltage.
The LTC proprietary topology and control architecture also
employs drain-to-source sensing (No R_SENSE) for forward
and reverse current limiting. The controller provides
all N-channel MOSFET output switch drive, facilitating
single package multiple power switch technology along
with lower R_DS(ON). The error amp output voltage (V_C)
determines the output duty cycle of the switches. Since
the V_C pin is a filtered signal, it provides rejection of high
frequency noise.
The FB pin receives the voltage feedback signal, which
is compared to the internal reference voltage by the er-
ror amplifier. The top MOSFET drivers are biased from a
floating bootstrap capacitor, which is normally recharged
during each off cycle through an external diode when the
top MOSFET turns off. Optional Schottky diodes can be
connected across synchronous switch B and D to provide
a lower drop during the dead time and eliminate efficiency
loss due to body diode reverse recovery.
The main control loop is shut down by pulling the RUN/
SS pin low. An internal 1µA current source charges the
RUN/SS pin and when the pin voltage is higher than 0.7V
the IC is enabled. The V_C voltage is then clamped to the
RUN/SS voltage minus 0.7V while C_SS is slowly charged
during start-up. This “soft-start” clamping prevents inrush
current draw from the input power supply.
POWER SWITCH CONTROL
Figure 1 shows a simplified diagram of how the four power
switches are connected to the inductor, V_IN, V_OUT and GND.
Figure 2 shows the regions of operation for the LTC3785
as a function of duty cycle D. The power switches are
properly controlled so that the transfer between modes
is continuous.
Buck Region (V_IN > V_OUT)
Switch D is always on and switch C is always off during
buck mode. When the error amp output voltage, V_C, is ap-
proximately above 0.1V, output A begins to switch. During
8
V_IN
V_OUT
TG1
A
D
TG2
SW1 ^L SW2
BG1
B
C
BG2
3785 F01
Figure 1. Output Switch Configuration
90%
D_MAX
BOOST
D_MIN
BOOST
D_MAX
BUCK
D_MIN
BUCK
A ON, B OFF
PWM C, D SWITCHES
FOUR SWITCH PWM
D ON, C OFF
PWM A, B SWITCHES
BOOST REGION
BUCK/BOOST REGION
BUCK REGION
3785 F02
Figure 2. Operation Mode vs V_C Voltage
the off time of switch A, synchronous switch B turns on for
the remainder of the switching period. Switches A and B will
alternate similar to a typical synchronous buck regulator.
As the control voltage increases, the duty cycle of switch
A increases until the max duty cycle of the converter in
buck mode reaches D_{MAX_BUCK}, given by:
D_{MAX_BUCK} = 100 – D4(SW)%
where D4(SW) = duty cycle % of the four switch range.
D4(SW) = (300ns • f) • 100%
where f = operating frequency, Hz.
Beyond this point the “four switch” or buck-boost region
is reached.
If during the rectification phase (switch pair BD on) the
inductor current becomes discontinuous, then switch B is
turned off and a damping impedance is connected across
the inductor to prevent ringing.
Buck-Boost or Four Switch (V_IN ~ V_OUT)
When the error amp output voltage, V_C, is above ap-
proximately 0.65V, switch pair AD remain on for duty
cycle D_{MAX_BUCK}, and the switch pair AC begin to phase
in. As switch pair AC phases in, switch pair BD phases out
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