LTM8021
10
8021fd
applicaTions inForMaTion
Figure 2. To Soft-Start the LTM8021, Add a
Resistor and Capacitor to the RUN/SS Pin
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 filter
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.
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
V
OUT
V
IN
RUN/SS
BIAS
GND
LTM8021
8021 F03
V
OUT
R
ADJ
C
OUT
RT
V
IN
4V TO 36V
C
IN
C
IN
V
IN
C
OUT
FB
GND
V
OUT
RUN/SS
R
ADJ
BIAS
PLANE
8021 F04
Figure 4. Layout Showing Suggested External Components,
GND Plane and Thermal Vias
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
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 LTM8021’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
the RUN/SS pin is grounded, the internal power 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 LTM8021 can pull large currents from
the output through the internal power switch and the V
IN
pin. Figure 3 shows a circuit that will run only when the
input voltage is present and that protects against a shorted
or reversed input.
PCB Layout
Most of the problems associated with the PCB layout
have been alleviated or eliminated by the high level of
integration of the LTM8021. The LTM8021 is nevertheless
a switching power supply, and care must be taken to
minimize EMI and ensure proper operation. Even with the
high level of integration, one may fail to achieve a specified
operation with a haphazard or poor layout. See Figure 4
for a suggested layout.
Ensure that the grounding and heatsinking are acceptable.
A few rules to keep in mind are:
1. Place the C
IN
capacitor as close as possible to the V
IN
and GND connection of the LTM8021.
2. Place the C
OUT
capacitor as close as possible to the
V
OUT
and GND connection of the LTM8021.
3. Place the C
IN
and C
OUT
capacitors such that their ground
currents flow directly adjacent to, or underneath the
LTM8021.
8021 F02
RUN/SS
GND
0.22µF
RUN
15k