LTM8031
8
8031fb
operaTion
applicaTions inForMaTion
The LTM8031 is a standalone nonisolated step-down
switching DC/DC power supply. It can deliver up to 1A of
DC output current with only bulk external input and output
capacitors. This module provides a precisely regulated
output voltage programmable via one external resistor
from 0.8VDC to 10VDC. The input voltage range is 3.6V
to 36V. Given that the LTM8031 is a step-down converter,
make sure that the input voltage is high enough to support
the desired output voltage and load current. A simplified
Block Diagram is given on the previous page.
The LTM8031 is designed with an input EMI filter and other
features to make its radiated emissions compliant with
several EMC specifications including EN55022 class B.
Compliance with conducted emissions requirements may
be obtained by adding a standard input filter.
The LTM8031 contains a current mode controller, power
switching element, power inductor, power Schottky diode
and a modest amount of input and output capacitance. The
LTM8031 is a fixed frequency PWM regulator. The switch-
ing frequency is set by simply connecting the appropriate
resistor value from the RT pin to GND.
An internal regulator provides power to the control circuitry.
The bias regulator can draw power from the V
IN
pin, but if
the BIAS pin is connected to an external voltage higher than
2.8V, bias power will be drawn from the external source
(typically the regulated output voltage). This improves
efficiency. The RUN/SS pin is used to place the LTM8031
in shutdown, disconnecting the output and reducing the
input current to less than 1µA.
To further optimize efficiency, the LTM8031 automatically
switches to Burst Mode operation in light load situations.
Between bursts, all circuitry associated with controlling the
output switch is shut down reducing the input supply cur-
rent to 50µA in a typical application. The oscillator reduces
the LTM8031’s operating frequency when the voltage at the
ADJ pin is low. This frequency foldback helps to control
the output current during start-up and overload.
The LTM8031 contains a power good comparator which
trips when the ADJ pin is at 90% of its regulated value.
The PGOOD output is an open-collector transistor that is
off when the output is in regulation, allowing an external
resistor to pull the PGOOD pin high. Power good is valid
when the LTM8031 is enabled and V
IN
is above 3.6V.
For most applications, the design process is straight
forward, summarized as follows:
1. Look at Table 1 and find the row that has the desired
input range and output voltage.
2. Apply the recommended C
IN
, C
OUT
, R
ADJ
and R
T
values.
3. Connect BIAS as indicated.
As the integrated input EMI filter may ring in response to an
application of a step input voltage, a bulk capacitance, series
resistance or some clamping mechanism may be required.
See the Hot-Plugging Safely section for details.
While these component combinations have been tested for
proper operation, it is incumbent upon the user to verify
proper operation over the intended system’s line, load and
environmental conditions.
Capacitor Selection Considerations
The C
IN
and C
OUT
capacitor values in Table 1 are the
minimum recommended values for the associated oper-
ating conditions. Applying capacitor values below those
indicated in Table 1 is not recommended, and may result
in undesirable operation. Using larger values is generally
acceptable, and can yield improved dynamic response, if
it is necessary. Again, it is incumbent upon the user to
verify proper operation over the intended system’s line,
load and environmental conditions.
Ceramic capacitors are small, robust and have very low
ESR. However, not all ceramic capacitors are suitable. X5R
and X7R types are stable over temperature and applied
voltage and give dependable service. Other types, includ-
ing Y5V and Z5U have very large temperature and voltage
coefficients of capacitance. In an application circuit they
