LTC1871-1 LINER [Linear Technology], LTC1871-1 Datasheet - Page 12

• Manufacturer Part Number: LTC1871-1
• Description: High Effi ciency, Synchronous, 4-Switch Buck-Boost Controller
• Manufacturer: LINER [Linear Technology]

OPERATION
LTC3780
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
V
compared to the internal reference voltage by the EA.
The top MOSFET drivers are biased from fl oating boost-
strap capacitors C
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 effi 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
C
SS voltage while C
This “soft-start” clamping prevents abrupt current from
being drawn from the input power supply.
POWER SWITCH CONTROL
Figure 1 shows a simplifi ed diagram of how the four
power switches are connected to the inductor, V
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
region is reached; the mode-to-mode transition time is
typically 200ns.
Buck Region (V
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 fi 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 V
12
OSENSE
SS
at the SS pin. The I
TH
pin receives the voltage feedback signal, which is
pin, which is the output of the amplifi er EA. The
IN
ITH
A
> V
and C
SS
, synchronous switch B is turned off
IN
OUT
is slowly charged during start-up.
approaches V
TH
B
)
(Figure 11), which are normally
voltage is then clamped to the
OUT
, the buck-boost
IN
, V
OUT
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
where D
range:
and f is the operating frequency in Hz.
Figure 3 shows typical buck mode waveforms. If V
approaches V
Buck-Boost (V
When V
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
D
D
MAX_BUCK
BUCK-BOOST
Figure 1. Simplifi ed Diagram of the Output Switches
BOOST
BOOST
IN
BUCK-BOOST
D
D
BUCK
BUCK
D
D
98%
MAX
MAX
MIN
MIN
3%
is close to V
BG2
TG2
Figure 2. Operating Mode vs Duty Cycle
OUT
= 100% – D
IN
A ON, B OFF
PWM C, D SWITCHES
FOUR SWITCH PWM
D ON, C OFF
PWM A, B SWITCHES
= (200ns • f) • 100%
V
, the buck-boost region is reached.
≅ V
IN
A
B
SW2
= duty cycle of the buck-boost switch
OUT
OUT
)
MAX_BUCK
L
, the controller is in buck-boost
BUCK-BOOST
R
3780 F01
SENSE
SW1
BUCK/BOOST REGION
BOOST REGION
V
D
C
BUCK REGION
OUT
, given by:
TG1
BG1
3780 F02
3780fe
IN