LTC3532_15 Datasheet, PDF (7/16 Pages) Linear Technology – Micropower Synchronous Buck-Boost DC/DC Converter

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OPERATIO
The LTC3532 provides high efficiency, low noise power
for applications such as portable instrumentation, digital
cameras, and MP3 players. The LTC proprietary topology
allows input voltages above, below or equal to the output
voltage by properly phasing the output switches. The error
amp output voltage on V_C determines the output duty cycle
of the switches. Since V_C is a filtered signal, it provides
rejection of frequencies well below the switching frequency.
The low R_DS(ON), low gate charge synchronous switches
provide high frequency pulse width modulation control at
high efficiency. Schottky diodes across the synchronous
switch D and synchronous switch B are not required, but
provide a lower voltage drop during the break-before-make
time (typically 15ns). Schottky diodes will improve peak
efficiency by typically 1% to 2%. High efficiency is achieved
at light loads when Burst Mode operation is entered and
the IC’s quiescent current drops to a low 35μA.
LOW NOISE FIXED FREQUENCY OPERATION
Oscillator
The frequency of operation is programmed by an external
resistor from R_T to ground, according to the following
equation:
f(kHz) = ^48,000
R_T (kΩ)
Error Amp
The error amplifier is a voltage mode amplifier. The loop
compensation components are configured around the
amplifier (from FB to V_C) to obtain stability of the converter.
For improved bandwidth, an additional RC feedforward
network can be placed across the upper feedback divider
resistor. The voltage on SHDN/SS clamps the error amp
output, V_C, to provide a soft-start function.
Internal Current Limit
There are two different current limit circuits in the LTC3532.
They have internally fixed thresholds which vary inversely
with V_IN. The first circuit is a high speed peak current limit
comparator that will shut off switch A if the current exceeds
1.1A typical. The delay to output of this amplifier is typi-
LTC3532
cally 50ns. A second amplifier will begin to source current
into the FB pin to drop the output voltage once the peak
input current exceeds 1A typical. This method provides a
closed loop means of clamping the input current. During
conditions where V_OUT is near ground, such as during a
short-circuit or during startup, this threshold is cut in half
providing a fold back feature. For this current limit feature
to be most effective, the Thevenin resistance from FB to
ground should be greater than 100k.
Reverse Current Limit
During fixed frequency operation, the LTC3532 operates in
forced continuous conduction mode. The reverse current
limit amplifier monitors the inductor current from the out-
put through switch D. Once the negative inductor current
exceeds 340mA typical, the IC will shut off switch D.
4-Switch Control
Figure 1 shows a simplified diagram of how the four internal
switches are connected to the inductor, V_IN, V_OUT and GND.
Figure 2 shows the regions of operation for the LTC3532 as
a function of the internal control voltage, V_CI. Depending
on the control voltage, the IC will operate in either buck,
buck/boost or boost mode. The V_CI voltage is a level shifted
voltage from the output of the error amp (V_C) (see Figure
5). The four power switches are properly phased so the
transfer between operating modes is continuous, smooth
and transparent to the user. When V_IN approaches V_OUT
the buck/boost region is reached where the conduction
time of the 4-switch region is typically 150ns. Referring
to Figures 1 and 2, the various regions of operation will
now be described.
V_IN
7
PMOS A
SW1
3
NMOS B
V_OUT
6
PMOS D
SW2
4
NMOS C
3532 F01
Figure 1. Simplified Diagram of Output Switches
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