Datasheet

LTC3533
8
3533fc
For more information www.linear.com/LTC3533
The LTC3533 provides high efficiency, low noise power
for a wide variety of handheld electronic devices. The LTC
proprietary topology allows input voltages above, below
or equal to the output voltage by properly phasing the
output switches. The error amplifier 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. High
efficiency is achieved at light loads when Burst Mode
operation is entered and the LTC3533’s quiescent current
drops to a low 40µA.
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) = 33,170/R
T
(k)
Error Amplifier
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 feed-forward
network can be placed across the upper feedback divider
resistor. The voltage on the RUN/SS pin clamps the error
amplifier output, V
C
, to provide a soft-start function.
Supply Current Limits
There are two different supply current limit circuits in the
LTC3533, working consecutively, each having internally
fixed thresholds which vary inversely with V
IN
.
The first circuit is a current limit amplifier, sourcing cur-
rent into FB to drop the output voltage, should the peak
input current exceed 4.5A 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 startup, this threshold is cut to 750mA,
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.
Should the peak input current exceed 7A typical, the second
circuit, a high speed peak current limit comparator, shuts
off PMOS switch A. The delay to output of this comparator
is typically 50ns.
Reverse Current Limit
During fixed frequency operation, the LTC3533 operates in
forced continuous conduction mode. The reverse current
limit comparator monitors the inductor current from the
output through switch D. Should this negative inductor
current exceed 500mA typical, the LTC3533 shuts off
switch D.
Four-Switch Control
Figure 1 shows a simplified diagram of how the four in
-
ternal switches are connected to the inductor, V
IN
, V
OUT
and GND.Figure 2 shows the regions of operation for the
LTC3533 as a function of the control voltage, V
C
.
Dependent on V
C
’s magnitude, the LTC3533 will operate
in either buck, buck/boost or boost mode. 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 entered, where the conduction time
of the four switch region is typically 150ns. Referring
to Figures 1 and 2, the various regions of operation will
now be described.
operation
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