LTC3521 Datasheet, PDF (10/20 Pages) Linear Technology – Wide VIN, 1A Buck-Boost DC/DC and Dual 600mA Buck DC/DC Converters

LTC3521
Operation
Soft-Start
The buck converters have an internal voltage mode
soft-start circuit with a nominal duration of 500μs. The
converters remain in regulation during soft-start and will
therefore respond to output load transients which occur
during this time. In addition, the output voltage rise time
has minimal dependency on the size of the output capaci-
tor or load current.
Error Amplifier and Compensation
The LTC3521 buck converters utilize an internal transcon-
ductance error amplifier. Compensation of the feedback
loop is performed internally to reduce the size of the
application circuit and simplify the design process. The
compensation network has been designed to allow use of
a wide range of output capacitors while simultaneously
ensuring rapid response to load transients.
PGOOD Comparators
The PGOOD2 and PGOOD3 pins are open-drain outputs
which indicate the status of the buck converters. If
the buck output voltage falls 9% below the regulation
voltage, the respective PGOOD open-drain output will
pull low. The output voltage must rise 2% above the
falling threshold before the pull-down will turn off. In
addition, there is a 60μs typical deglitching delay in
the flag in order to prevent false trips due to voltage
transients on load steps. The respective PGOOD output
will also pull low during overtemperature shutdown,
undervoltage lockout or if the respective buck con-
verter SHDN pin is pulled low to indicate these fault
conditions.
Buck-Boost Converter Operation
PWM Mode Operation
When the PWM pin is held high, the LTC3521 buck-boost
converter operates in a constant-frequency PWM mode
with voltage mode control. A proprietary switching algo-
rithm allows the converter to switch between buck, buck-
boost and boost modes without discontinuity in inductor
current or loop characteristics. The switch topology for
the buck-boost converter is shown in Figure 1.
When the input voltage is significantly greater than the
output voltage, the buck-boost converter operates in
buck mode. Switch D turns on continuously and switch
C remains off. Switches A and B are pulse width modu-
lated to produce the required duty cycle to support the
output regulation voltage. As the input voltage decreases,
switch A remains on for a larger portion of the switching
cycle. When the duty cycle reaches approximately 85%,
the switch pair AC begins turning on for a small fraction
of the switching period. As the input voltage decreases
further, the AC switch pair remains on for longer durations
and the duration of the BD phase decreases proportionally.
As the input voltage drops below the output voltage, the
AC phase will eventually increase to the point that there is
no longer any BD phase. At this point, switch A remains on
continuously while switch pair CD is pulse width modu-
lated to obtain the desired output voltage. At this point,
the converter is operating solely in boost mode.
This switching algorithm provides a seamless transition
between operating modes and eliminates discontinuities
in average inductor current, inductor current ripple and
loop transfer function throughout all three operational
L
PV_{IN1 A} SW1A
SW1B _D V_OUT1
B
C
LTC3521
PGND1A PGND1B
3521 F01
Figure 1. Buck-Boost Switch Topology
3521f
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