Datasheet

LT3791-1
12
37911f
operaTion
The LT3791-1 is a current mode controller that provides an
output voltage above, equal to or below the input voltage.
The LT C proprietary topology and control architecture uses
a current sensing resistor in buck or boost operation. The
sensed inductor current is controlled by the voltage on
the V
C
pin, which is the output of the feedback amplifiers
A11 and A12. The V
C
pin is controlled by three inputs, one
input from the output current loop, one input from the
input current loop, and the third input from the feedback
loop. Whichever feedback input is higher takes precedence,
forcing the converter into either a constant-current or a
constant-voltage mode.
The LT3791-1 is designed to transition cleanly between
the two modes of operation. Current sense amplifier A1
senses the voltage between the IVINP and IVINN pins and
provides a pre-gain to amplifier A11. When the voltage
between IVINP and IVINN reaches 50mV, the output of A1
provides IVINMON_INT to the inverting input of A11 and
the converter is in constant-current mode. If the current
sense voltage exceeds 50mV, the output of A1 increases
causing the output of A11 to decrease, thus reducing the
amount of current delivered to the output. In this manner
the current sense voltage is regulated to 50mV.
The output current amplifier works similar to the input
current amplifier but with a 100mV voltage instead of
50mV. The output current sense level is also adjustable
by the CTRL pin. Forcing CTRL to less than 1.2V forces
ISMON_INT to the same level as CTRL, thus providing
current-level control. The output current amplifier provides
rail-to-rail operation. Similarly if the FB pin goes above
1.2V the output of A11 decreases to reduce the current
level and regulate the output (constant-voltage mode).
The LT3791-1 provides monitoring pins IVINMON and
ISMON that are proportional to the voltage across the
input and output current amplifiers respectively.
The main control loop is shut down by pulling the EN/
UVLO pin low. When the EN/UVLO pin is higher than 1.2V,
an internal 14µA current source charges soft-start capaci-
tor C
SS
at the SS pin. The V
C
voltage is then clamped a
diode voltage higher than the SS voltage while the C
SS
is
slowly charged during start-up. This soft-start clamping
prevents abrupt current from being drawn from
the input
power supply.
The
top MOSFET drivers are biased from floating boot-
strap capacitors C1 and C2, which are normally recharged
through an external diode when the top MOSFET is turned
off. A unique charge sharing technique eliminates top FET
refresh switching cycle in buck or boost operation.Schottky
diodes across the synchronous switch M4 and synchronous
switch M2 are not required, but they do provide a lower drop
during the dead time. The addition of the Schottky diode
typically improves peak efficiency by 1% to 2% at 500kHz.
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 LT3791-1 as a function of duty cycle D. The power
switches are properly controlled so the transfer between
regions is continuous. When V
IN
approaches V
OUT
, the
buck-boost region is reached.
Buck Region (V
IN
> V
OUT
)
Switch M4 is always on and switch M3 is always off during
this mode. At the start of every cycle, synchronous switch
M2 is turned on first. Inductor current is sensed when
synchronous switch M2 is turned on. After the sensed
inductor current
falls below the reference voltage, which
is
proportional to V
C
, synchronous switch M2 is turned off
and switch M1 is turned on for the remainder of the cycle.
Switches M1 and M2 will alternate, behaving like a typical
M1
SW1
V
IN
V
OUT
M2
TG2
BG2
M4
SW2
M3
TG1
BG1
R
SENSE
37911 F01
L1
D
MAX
BOOST
(BG2)
M1 ON, M2 OFF
PWM M3, M4 SWITCHES
BOOST REGION
M4 ON, M3 OFF
PWM M2, M1 SWITCHES
BUCK REGION
37911 F02
4-SWITCH PWMBUCK-BOOST REGION
D
MIN
BOOST
D
MAX
BUCK
(TG1)
D
MIN
BUCK
Figure 1. Simplified Diagram of the Output Switches
Figure 2. Operating Regions vs Duty Cycle