LT3791
12
3791fb
For more information www.linear.com/LT3791
operaTion
The LT3791 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 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 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 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 SS can also be used as a fault timer
whenever an open or shorted LED is detected.
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. 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 diodes 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 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.
M1
SW1
IN
OUT
M2
M4
SW2
M3
TG1
R
SENSE
L1
MAX
(BG2)
M1 ON, M2 OFF
PWM M3, M4 SWITCHES
BOOST REGION
M4 ON, M3 OFF
PWM M2, M1 SWITCHES
BUCK REGION
4-SWITCH PWMBUCK-BOOST REGION
D
MIN
D
MAX
BUCK
(TG1)
D
MIN
Figure 1. Simplified Diagram of the Output Switches
Figure 2. Operating Regions vs Duty Cycle
