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LTC3780_12 Datasheet, PDF (12/28 Pages) Linear Technology – High Effi ciency, Synchronous, 4-Switch Buck-Boost Controller | |||
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LTC3780
OPERATION
MAIN CONTROL LOOP
The LTC3780 is a current mode controller that provides an
output voltage above, equal to or below the input voltage.
The LTC proprietary topology and control architecture em-
ploys a current-sensing resistor in buck or boost modes.
The sensed inductor current is controlled by the voltage
on the ITH pin, which is the output of the ampliï¬er EA. The
VOSENSE pin receives the voltage feedback signal, which is
compared to the internal reference voltage by the EA.
The top MOSFET drivers are biased from ï¬oating boost-
strap capacitors CA and CB (Figure 11), which are normally
recharged through an external diode when the top MOSFET
is turned off. Schottky diodes across the synchronous
switch D and synchronous switch B are not required, but
provide a lower drop during the dead time. The addition of
the Schottky diodes will typically improve peak efï¬ciency
by 1% to 2% at 400kHz.
The main control loop is shut down by pulling the RUN
pin low. When the RUN pin voltage is higher than 1.5V, an
internal 1.2μA current source charges soft-start capacitor
CSS at the SS pin. The ITH voltage is then clamped to the
SS voltage while CSS is slowly charged during start-up.
This âsoft-startâ clamping prevents abrupt current from
being drawn from the input power supply.
POWER SWITCH CONTROL
Figure 1 shows a simpliï¬ed diagram of how the four
power switches are connected to the inductor, VIN, VOUT
and GND. Figure 2 shows the regions of operation for the
LTC3780 as a function of duty cycle D. The power switches
are properly controlled so the transfer between modes is
continuous. When VIN approaches VOUT, the buck-boost
region is reached; the mode-to-mode transition time is
typically 200ns.
Buck Region (VIN > VOUT)
Switch D is always on and switch C is always off during
this mode. At the start of every cycle, synchronous switch
B is turned on ï¬rst. Inductor current is sensed when
synchronous switch B is turned on. After the sensed in-
ductor current falls below the reference voltage, which is
proportional to VITH, synchronous switch B is turned off
12
VIN
VOUT
TG2
A
D
TG1
SW2 L SW1
BG2
B
C
BG1
RSENSE
3780 F01
Figure 1. Simpliï¬ed Diagram of the Output Switches
98%
DMAX
BOOST
DMIN
BOOST
DMAX
BUCK
3%
DMIN
BUCK
A ON, B OFF
PWM C, D SWITCHES
BOOST REGION
FOUR SWITCH PWM BUCK/BOOST REGION
D ON, C OFF
PWM A, B SWITCHES
BUCK REGION
3780 F02
Figure 2. Operating Mode vs Duty Cycle
and switch A is turned on for the remainder of the cycle.
switches A and B will alternate, behaving like a typical
synchronous buck regulator. The duty cycle of switch A
increases until the maximum duty cycle of the converter
in buck mode reaches DMAX_BUCK, given by:
DMAX_BUCK = 100% â DBUCK-BOOST
where DBUCK-BOOST = duty cycle of the buck-boost switch
range:
DBUCK-BOOST = (200ns ⢠f) ⢠100%
and f is the operating frequency in Hz.
Figure 3 shows typical buck mode waveforms. If VIN
approaches VOUT, the buck-boost region is reached.
Buck-Boost (VIN â
VOUT)
When VIN is close to VOUT, the controller is in buck-boost
mode. Figure 4 shows typical waveforms in this mode.
Every cycle, if the controller starts with switches B and D
turned on, switches A and C are then turned on. Finally,
switches A and D are turned on for the remainder of the
time. If the controller starts with switches A and C turned
3780fe
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