LTC3536_15 Datasheet, PDF (9/28 Pages) Linear Technology – 1A Low Noise, Buck-Boost DC/DC Converter

LTC3536
Operation
Introduction
The LTC3536 is a monolithic buck-boost converter that
can operate with input and output voltages from as low
as 1.8V to as high as 5.5V. A proprietary switch control
algorithm allows the buck-boost converter to maintain
output voltage regulation with input voltages that are
above, below or equal to the output voltage. Transitions
between these operating modes are seamless and free of
transients and subharmonic switching.
The LTC3536 can be configured to operate over a wide
range of switching frequencies, from 300kHz to 2MHz,
allowing applications to be optimized for board area and
efficiency. The LTC3536 has an internal fixed-frequency
oscillator with a switching frequency that is easily set by
a single external resistor. In noise sensitive applications,
the converter can also be synchronized to an external clock
via the MODE/SYNC pin. The operating frequency defaults
to 1.2MHz when RT is connected to V_IN eliminating the
external resistor.
The LTC3536 has been optimized to reduce input current
in shutdown and standby for applications that are sensi-
tive to quiescent current draw, such as battery-powered
devices. In Burst Mode operation, the no-load standby
current is only 32µA and in shutdown the total supply
current is reduced to less than 1µA.
PWM Mode Operation
With the MODE/SYNC pin forced low or driven by an ex-
ternal clock, the LTC3536 operates in a fixed-frequency
pulse-width modulation (PWM) mode using a voltage mode
control loop. This mode of operation maximizes the output
current that can be delivered by the converter, reduces out-
put voltage ripple, and yields a low noise fixed-frequency
switching spectrum. A proprietary switching algorithm
provides seamless transitions between operating modes
and eliminates discontinuities in the average inductor cur-
rent, inductor current ripple, and loop transfer function
throughout all regions of operation. These advantages
result in increased efficiency, improved loop stability, and
lower output voltage ripple in comparison to the traditional
4-switch buck-boost converter.
Figure 1 shows the topology of the LTC3536 power stage
which is comprised of two P-channel MOSFET switches
and two N-channel MOSFET switches and their associated
gate drivers. In response to the error amplifier output, an
internal pulse-width modulator generates the appropriate
switch duty cycles to maintain regulation of the output
voltage.
V_IN
V_OUT
PMOS A
SW1
L
SW2
PMOS D
NMOS B
NMOS C
3536 F01
Figure 1. Power Stage Schematic
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. Switch A and B are pulse-width modulated
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 90% 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. At
this point, switch A remains on continuously while switch
pair CD is pulse-width modulated to obtain the desired
output voltage. At this point, the converter is operating
solely in boost mode.
Oscillator and Phase-Locked Loop
The LTC3536 operates from an internal oscillator with a
switching frequency that can be configured by a single
external resistor between RT and ground. Tying RT to V_IN
sets the default internal operating frequency to typically
1.2MHz. If the RT pin is driven externally to a level higher
than V_IN, a current limiting resistor should be used. 1M for
6V on the RT pin limits the current to 6µA. Also, a Schottky
3536fa
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