Datasheet

LTM8032
13
8032fg
For more information www.linear.com/LTM8032
Soft-Start
The RUN/SS pin can be used to soft-start the LTM8032,
reducing the maximum input current during start-up. The
RUN/SS pin is driven through an external RC network to
create a voltage ramp at this pin. Figure 2 shows the start-
up and shutdown waveforms with the soft-start circuit. By
choosing an appropriate RC time constant, the peak start-up
current can be reduced to the current that is required to
regulate the output, with no overshoot. Choose the value
of the resistor so that it can supply at least 20µA when
the RUN/SS pin reaches 2.5V.
applicaTions inForMaTion
Figure 2. To Soft-Start the LTM8032, Add a Resistor
and Capacitor to the RUN/SS Pin
Shorted Input Protection
Care needs to be taken in systems where the output will
be held high when the input to the LTM8032 is absent.
This may occur in battery charging applications or in
battery back-up systems where a battery or some other
supply is diode ORed with the LTM8032’s output. If the
V
IN
pin is allowed to float and the RUN/SS pin is held high
(either by a logic signal or because it is tied to V
IN
), then
the LTM8032’s internal circuitry will pull its quiescent
current through its internal power switch. This is fine if
your system can tolerate a few milliamps in this state. If
you ground the RUN/SS pin, the internal switch current
will drop to essentially zero. However, if the V
IN
pin is
grounded while the output is held high, then parasitic
diodes inside the LTM8032 can pull large currents from
the output through the V
IN
pin, potentially damaging the
device. Figure 3 shows a circuit that will run only when
the input voltage is present and that protects against a
shorted or reversed input.
Figure 3. The Input Diode Prevents a Shorted Input from
Discharging a Back-Up Battery Tied to the Output. It Also
Protects the Circuit from a Reversed Input. The LTM8032
Runs Only When the Input is Present
V
OUT
V
IN
RUN/SS
BIAS
R
T
ADJ
LTM8032
8032 F03
V
OUT
GND
V
IN
AUX
SYNC
Synchronization
The internal oscillator of the LTM8032 can be synchro-
nized by
applying an external 250kHz to 2MHz clock to
the
SYNC pin. Do not leave this pin floating. The resistor
tied from the RT pin to ground should be chosen such
that the LTM8032 oscillates 20% lower than the intended
synchronization frequency (see the Frequency Selection
section). The LTM8032 will not enter Burst Mode operation
while synchronized to an external clock, but will instead
skip pulses to maintain regulation.
8023 F02
I
L
1A/DIV
V
RUN/SS
2V/DIV
V
OUT
2V/DIV
RUN/SS
GND
0.22µF
RUN
15k
2ms/DIV