LTC3780EG Linear Technology, LTC3780EG Datasheet - Page 11

• Manufacturer Part Number: LTC3780EG
• Description: IC,SMPS CONTROLLER,CURRENT-MODE,CMOS,SSOP,24PIN,PLASTIC
• Manufacturer: Linear Technology

OPERATIO
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
employs 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 floating booststrap
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 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
OSENSE
SS
at the SS pin. The I
TH
pin, which is the output of the amplifier EA. The
pin receives the voltage feedback signal, which is
A
and C
U
B
(Figure 11), which are normally
TH
voltage is then clamped to the
Figure 1. Simplified Diagram of the Output Switches
BOOST
BOOST
D
D
BUCK
BUCK
D
D
Figure 2. Operating Mode vs Duty Cycle
98%
MAX
MAX
MIN
MIN
3%
BG2
TG2
A ON, B OFF
PWM C, D SWITCHES
FOUR SWITCH PWM
D ON, C OFF
PWM A, B SWITCHES
V
IN
A
B
SW2
L
R
3780 F01
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 simplified diagram of how the four power
switches are connected to the inductor, V
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 first. Inductor current is sensed when
Synchronous Switch B is turned on. After the sensed
inductor current falls below the reference voltage, which is
proportional to V
SENSE
BUCK/BOOST REGION
SW1
BOOST REGION
BUCK REGION
V
D
C
OUT
TG1
BG1
3780 F02
IN
ITH
> V
SS
, Synchronous Switch B is turned off
IN
OUT
is slowly charged during start-up.
approaches V
)
OUT
, the Buck-Boost
LTC3780
IN
, V
OUT
11
and
3780f