LTM8031_15 Datasheet, PDF (12/22 Pages) Linear Technology – Ultralow Noise EMC 36V, 1A DC/DC Module Regulator

LTM8031
Applications Information
RUN
15k
0.22µF
RUN/SS
GND
I_L
0.5A/DIV
V_RUN/SS
2V/DIV
V_OUT
2V/DIV
2ms/DIV
8031 F01
Figure 1. To Soft-Start the LTM8031, Add a Resistor
and Capacitor to the RUN/SS Pin
V_IN
V_IN
V_OUT
V_OUT
RUN/SS
AUX
LTM8031
BIAS
ADJ
R_T SYNC GND
8031 F02
Figure 2. 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 LTM8031
Runs Only When the Input is Present
Synchronization
The internal oscillator of the LTM8031 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 LTM8031 oscillates 20% lower than the intended
synchronization frequency (see the Frequency Selection
section). The LTM8031 will not enter Burst Mode operation
while synchronized to an external clock, but will instead
skip pulses to maintain regulation.
Shorted Input Protection
Care needs to be taken in systems where the output will
be held high when the input to the LTM8031 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 LTM8031’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 LTM8031’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 LTM8031 can pull large currents from
the output through the V_IN pin, potentially damaging the
device. Figure 2 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 headaches associated with PCB layout have
been alleviated or even eliminated by the high level of
integration of the LTM8031. The LTM8031 is neverthe-
less a switching power supply and care must be taken to
minimize EMI and ensure proper operation. Even with the
high level of integration, you may fail to achieve specified
operation with a haphazard or poor layout. See Figure 3
for a suggested layout.
Ensure that the grounding and heat sinking are acceptable.
A few rules to keep in mind are:
1. Place the R_ADJ and R_T resistors as close as possible to
their respective pins.
2. Place the C_IN capacitor as close as possible to the V_IN
and GND connection of the LTM8031. If a capacitor
is connected to the FIN terminals, place it as close
as possible to the FIN terminals, such that its ground
connection is as close as possible to that of the C_IN
capacitor.
3. Place the C_OUT capacitor as close as possible to the
V_OUT and GND connection of the LTM8031.
8031fb
12