ltm8021 Linear Technology Corporation, ltm8021 Datasheet - Page 10

• Manufacturer Part Number: ltm8021
• Description: 36vin, 500ma Step-down Dc/dc ?module
• Manufacturer: Linear Technology Corporation

LTM8021
APPLICATIONS INFORMATION
Soft-Start
The RUN/SS pin can be used to soft-start the LTM8021,
reducing the maximum input current during start-up.
The RUN/SS pin is driven through an external RC fi lter
to create a voltage ramp at this pin. Figure 2 shows the
soft-start circuit. By choosing a large 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 80μA
when the RUN/SS pin reaches 2V.
10
Figure 2. To Soft-Start the LTM8021, Add a
Resisitor and Capacitor to the RUN/SS Pin
0.22μF
RUN
15k
RUN/SS
GND
8021 F02
Shorted Input Protection
Care needs to be taken in systems where the output will be
held high when the input to the LTM8021 is absent. This
may occur in battery charging applications or in battery
backup systems where a battery or some other supply is
diode ORed with the LTM8021’s output. If the V
allowed to fl oat and the RUN/SS pin is held high (either
by a logic signal or because it is tied to V
LTM8021’s internal circuitry will pull its quiescent cur-
rent through its SW pin. This is fi ne if your system can
tolerate a few milliamps in this state. If the RUN/SS pin
is grounded, the SW pin current will drop to essentially
zero. However, if the V
is held high, then parasitic diodes inside the LTM8021
can pull large currents from the output through the SW
pin and the V
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 Backup Battery Tied to the Output. It Also Protects
the Circuit from a Reversed Input. The LTM8021 Runs Only When
the Input is Present
4V TO 36V
V
IN
IN
pin. Figure 3 shows a circuit that will run
IN
pin is grounded while the output
C
IN
V
RUN/SS
IN
GND
LTM8021
RT
V
BIAS
OUT
R
ADJ
8021 F03
IN
), then the
C
V
IN
OUT
OUT
pin is
8021fa