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LTM8031

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.

Aninternalregulatorprovidespowertothecontrolcircuitry.

The bias regulator can draw power from the VIN pin, but if

theBIASpinisconnectedtoanexternalvoltagehigherthan

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.

Betweenbursts,allcircuitryassociatedwithcontrollingthe

output switch is shut down reducing the input supply cur-

rent to 50µA in a typical application. The oscillator reduces

theLTM8031’soperatingfrequencywhenthevoltageatthe

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 VIN 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 CIN, COUT, RADJ and RT

values.

3. Connect BIAS as indicated.

AstheintegratedinputEMIfiltermayringinresponsetoan

applicationofastepinputvoltage,abulkcapacitance,series

resistanceorsomeclampingmechanismmayberequired.

See the Hot-Plugging Safely section for details.

Whilethesecomponentcombinationshavebeentestedfor

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 CIN and COUT 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

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