LTC3785-1 Linear Technology, LTC3785-1 Datasheet - Page 8

• Manufacturer Part Number: LTC3785-1
• Description: Buck-Boost Controller
• Manufacturer: Linear Technology

www.DataSheet4U.com
OPERATION
LTC3785-1
MAIN CONTROL LOOP
The LTC3785-1 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
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
output duty cycle of the switches. Since the V
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 amplifi er. The top MOSFET drivers are biased from a
fl oating 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 effi ciency
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
RUN/SS voltage minus 0.7V while C
during start-up. This “soft-start” clamping prevents inrush
current draw 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
Figure 2 shows the regions of operation for the LTC3785-1
as a function of duty cycle D. The power switches are
properly controlled so that the transfer between modes
is continuous.
Buck Region (V
Switch D is always on and switch C is always off during
buck mode. When the error amp output voltage, V
approximately above 0.1V, output A begins to switch. During
the off time of switch A, synchronous switch B turns on for
8
DS(ON)
. The error amp output voltage (V
IN
> V
OUT
C
)
voltage is then clamped to the
SS
SENSE
is slowly charged
IN
C
) determines the
, V
C
pin is a fi ltered
OUT
) for forward
and GND.
C
, is
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
where D4(SW) = duty cycle % of the four switch range.
where f = operating frequency, Hz.
Beyond this point the “four switch” or buck-boost region
is reached.
Buck-Boost or Four Switch (V
When the error amp output voltage, V
proximately 0.65V, switch pair AD remain on for duty
cycle D
in. As switch pair AC phases in, switch pair BD phases
out accordingly. When the V
the buck-boost range, approximately 0.7V, the AC switch
pair completely phase out the BD pair, and the boost phase
begins at duty cycle, D4(SW).
D
D4(SW) = (300ns • f) • 100%
MAX_BUCK
MAX_BUCK
BOOST
BOOST
BG1
TG1
D
D
BUCK
BUCK
D
D
90%
MAX
MAX
MIN
MIN
Figure 1. Response Time Test Circuit
Figure 1. Response Time Test Circuit
= 100 – D4(SW)%
A ON, B OFF
PWM C, D SWITCHES
D ON, C OFF
PWM A, B SWITCHES
FOUR SWITCH PWM
V
, and the switch pair AC begin to phase
IN
A
B
SW1
MAX_BUCK
L
C
voltage reaches the edge of
IN
, given by:
SW2
BUCK/BOOST REGION
~ V
37851 F01
BOOST REGION
BUCK REGION
V
OUT
D
C
OUT
)
C
, is above ap-
TG2
BG2
37851 F02
37851f