Datasheet

LTM8023
13
8023fj
For more information www.linear.com/LTM8023
Soft-Start
The RUN/SS pin can be used to soft-start the LTM8023,
reducing the maximum input current during start-up.
The
RUN/SS pin is driven through an external RC filter
to create a voltage ramp at this pin. Figure 3 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 4. The Input Diode Prevents a Shorted Input from
Discharging a Backup Battery Tied to the Output. It Also Protects
the Circuit from a Reversed Input. The LTM8023 Runs Only
When the Input is Present.
Shorted Input Protection
Care needs to be taken in systems where the output will be
held high when the input to the LTM8023 is absent. This
may occur in battery charging applications or in battery
backup systems where a battery or some other supply is
diode OR-ed with the LTM8023’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
LTM8023’s internal circuitry will pull its quiescent cur-
rent 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 power switch cur-
rent will drop to essentially zero. However, if the V
IN
pin
is grounded while the output is held high, then parasitic
diodes inside the LTM8023 can pull large currents from
the output through the V
IN
pin. Figure 4 shows a circuit
that will run only when the input voltage is present and
that protects against a shorted or reversed input.
V
OUT
V
IN
RUN/SS
BIAS
R
T
ADJ
LTM8023
8023 F04
V
OUT
GND
V
IN
AUX
SYNC
Figure 3. To Soft-Start the LTM8023, Add a
Resistor and Capacitor to the RUN/SS Pin
8023 F03
I
L
1A/DIV
V
RUN/SS
2V/DIV
V
OUT
2V/DIV
RUN/SS
GND
0.22µF
RUN
15k
2ms/DIV
Synchronization
The internal oscillator of the LTM8023 can be synchronized
by applying an external 250kHz to 2MHz clock to the SYNC
pin.
Do not leave this pin floating . The resistor tied from the
R
T
pin to ground should be chosen such that the LTM8023
oscillates 20% lower than the intended synchronization
frequency (see the Frequency Selection section).
The LTM8023 will not enter Burst Mode operation while
synchronized to an external clock, but will instead skip
pulses to maintain regulation.
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