ltm8023 Linear Technology Corporation, ltm8023 Datasheet - Page 12

• Manufacturer Part Number: ltm8023
• Description: 2a, 36v Dc/dc ?module
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LTM8023
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.
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 fl oating. The resistor tied from the
R
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.
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
(either by a logic signal or because it is tied to V
the LTM8023’s internal circuitry will pull its quiescent
current through its internal power switch. This is fi ne if
your system can tolerate a few milliamps in this state. If
you ground the ⎯ S ⎯ H ⎯ D ⎯ N pin, the SW pin current will drop
to essentially zero. However, if the V
while the output is held high, then parasitic diodes inside
the LTM8023 can pull large currents from the output
through the V
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 LTM8023. The LTM8023 is neverthe-
less a switching power supply, and care must be taken to
minimize EMI and ensure proper operation. Even with the
12
T
IN
pin to ground should be chosen such that the LTM8023
pin is allowed to fl oat and the ⎯ S ⎯ H ⎯ D ⎯ N pin is held high
IN
pin. Figure 4 shows a circuit that will run
IN
pin is grounded
IN
), then
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.
high level of integration, you may fail to achieve specifi ed
operation with a haphazard or poor layout. See Figure 5
for a suggested layout.
Ensure that the grounding and heatsinking are acceptable.
A few rules to keep in mind are:
1. Place the R
2. Place the C
3. Place the C
Figure 5. Layout Showing Suggested External Components, GND
Plane and Thermal Vias
V
their respective pins.
and GND connection of the LTM8023.
V
IN
OUT
GND
and GND connection of the LTM8023.
V
OUT
ADJ
IN
OUT
capacitor as close as possible to the V
and R
capacitor as close as possible to the
C
OUT
V
RUN/SS
T
R
IN
T
resistors as close as possible to
SYNC
LTM8023
SHARE
GND
AUX
C
R
IN
T
BIAS
V
BIAS
AUX
8023 F04
ADJ
OUT
R
V
ADJ
IN
SYNC
PGOOD
RUN/SS
8023 F05
V
OUT
8023f
IN