LTM4658EVPBF datasheet(14/26 Pages) AD

14 / 26 page

LTM4658

Rev. 0

For more information www.analog.com

Output Overvoltage Protection

During an output overvoltage event, when the FB pin volt-

age is greater than 110% of nominal, the LTM4658 top

power switch will be turned off. If the output remains out

of regulation for more than 100μs, the PGOOD pin will be

pulledlow.Anoutputovervoltageeventshouldnothappen

under normal operating conditions.

Output Short-Circuit Protection and Recovery

The peak inductor current at which the current compara-

tor shuts off the top power switch is controlled by the

voltage on the COMP pin. If the output current increases,

the error amplifier raises the COMP pin voltage until the

average inductor current matches the new load current. In

normal operation, the LTM4658 clamps at the maximum

COMP pin voltage.

When the output is shorted to ground, the inductor current

decays very slowly during the switch off time because of

the low voltage across the inductor. To keep the current

in control, a secondary limit is also imposed on the valley

inductorcurrent.Iftheinductorcurrentmeasuredthrough

the bottom power switch increases beyond IVALLEY(MAX),

the top power switch will be held off and switching cycles

will be skipped until the inductor current is reduced.

Recovery from a short circuit can be abrupt and because

the output is shorted and below regulation the regulator

is requesting the maximum current to charge the output.

When the short circuit condition is removed, the induc-

tor current could cause an extreme voltage overshoot in

the output. The LTM4658 addresses this potential issue

by regulating the SSTT voltage just above the FB volt-

age anytime the output is out of regulation. Therefore,

a recovery from an output short circuit goes through a

soft-start cycle. The output ramp is controlled and the

overshoot is minimized.

Thermal Considerations and Output Current Derating

The thermal resistances reported in the Pin Configuration

section are consistent with those parameters defined by

JESD51-9 and are intended for use with finite element

analysis (FEA) software modeling tools that leverage the

outcome of thermal modeling, simulation, and correlation

to hardware evaluation performed on a µModule pack-

age mounted to a hardware test board—also defined by

JESD51-9 (“Test Boards for Area Array Surface Mount

Package Thermal Measurements”). The motivation for

providingthesethermalcoefficientsinfoundinJESD51-12

(“Guidelines for Reporting and Using Electronic Package

Thermal Information”).

Many designers may opt to use laboratory equipment

and a test vehicle such as the demo board to anticipate

the µModule regulator’s thermal performance in their ap-

plication at various electrical and environmental operating

conditions to compliment any FEA activities. Without FEA

software, the thermal resistances reported in the Pin Con-

figuration section are in-and-of themselves not relevant to

providing guidance of thermal performance; instead, the

derating curves provided in the data sheet can be used in

a manner that yields insight and guidance pertaining to

one’s application-usage, and can be adapted to correlate

thermal performance to one’s own application.

The Pin Configuration section typically gives four thermal

coefficients explicitly defined in JESD51-12; these coef-

ficients are quoted or paraphrased below:

1. θJA, the thermal resistance from junction to ambient,

is the natural convection junction-to-ambient air ther-

mal resistance measured in a one cubic foot sealed

enclosure. This environment is sometimes referred to

as “still air” although natural convection causes the

air to move. This value is determined with the part

mounted to a JESD51-9 defined test board, which

does not reflect an actual application or viable operat-

ing condition.

2. θJCbottom, the thermal resistance from junction to bot-

tom of the product case, is determined with all of the

component power dissipation flowing through the bot-

tom of the package. In the typical module regulator,

the bulk of the heat flows out the bottom of the pack-

age, but there is always heat flow out into the ambient

environment. As a result, this thermal resistance value

maybe useful for comparing packages but the test con-

ditions don’t generally match the user’s application.

APPLICATIONS INFORMATION

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