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 I
TH
pin, which is the output of the amplifier EA. The
V
OSENSE
pin receives the voltage feedback signal, which is
compared to the internal reference voltage by the EA.
The top MOSFET drivers are biased from floating boost-
strap capacitors C
A
and C
B
(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 efficiency
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
C
SS
at the SS pin. The I
TH
voltage is then clamped to the
SS voltage while C
SS
is slowly charged during start-up.
This “soft-start” clamping prevents abrupt current from
being drawn 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
LTC3780 as a function of duty cycle D. The power switches
are properly controlled so the transfer between modes is
continuous. When V
IN
approaches V
OUT
, the buck-boost
region is reached; the mode-to-mode transition time is
typically 200ns.
Buck Region (V
IN
> V
OUT
)
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 first. 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 V
ITH
, synchronous switch B is turned off
TG2
V
IN
A
SW2
BG2
B
L
V
OUT
D
SW1
C
BG1
TG1
R
SENSE
3780 F01
Figure 1. Simplified Diagram of the Output Switches
98%
D
MAX
BOOST
D
MIN
BOOST
D
MAX
BUCK
3%
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
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 D
MAX_BUCK
, given by:
D
MAX_BUCK
= 100% – D
BUCK-BOOST
where D
BUCK-BOOST
= duty cycle of the buck-boost switch
range:
D
BUCK-BOOST
= (200ns • f) • 100%
and f is the operating frequency in Hz.
Figure 3 shows typical buck mode waveforms. If V
IN
approaches V
OUT
, the buck-boost region is reached.
Buck-Boost (V
IN
≅
V
OUT
)
When V
IN
is close to V
OUT
, 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
12
LINEAR [ LINEAR INTEGRATED SYSTEMS ]
