LTM4657 datasheet(13/24 Pages) AD

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LTM4705

Rev. 0

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APPLICATIONS INFORMATION

or dynamic VOUT changes, the LTM4705’s PGOOD falling

edge includes a blanking delay of approximately 40μs.

Stability compensation

The LTM4705 module internal compensation loop is

designed and optimized for low ESR ceramic output

capacitors only applications. Please note that for appli-

cations that need to achieve high bandwidth control loop

compensation with enough phase margin, a feed forward

capacitor between 33pF – 100pF is recommended from

VOUT to VFB pin. Table 8 is provided for most application

requirements. The LTpowerCAD Design Tool is available

to download online for control loop optimization.

RUN Enable

Pulling the RUN pin to ground forces the LTM4705 into

its shutdown state, turning off both power MOSFETs and

most of its internal control circuitry. Tying the RUN pin

voltage above 1.27V will turn on the entire chip.

Pre-Biased Output Start-Up

There may be situations that require the power supply to

start up with a pre-bias on the output capacitors. In this

case, it is desirable to start up without discharging that

output pre-bias. The LTM4705 can safely power up into

a pre-biased output without discharging it.

The LTM4705 accomplishes this by forcing discontinu-

ous mode (DCM) operation until the TRACK/SS pin volt-

age reaches 0.6V reference voltage. This will prevent the

bottom MOSFET from turning on during the pre-biased

output start-up which would discharge the output.

Overtemperature Protection

The internal overtemperature protection monitors the

junction temperature of the module. If the junction

temperature reaches approximately 160°C, both power

switches will be turned off until the temperature drops

about 15°C cooler.

Input Overvoltage Protection

In order to protect the internal power MOSFET devices

against transient voltage spikes, the LTM4705 constantly

monitors each VIN pin for an overvoltage condition. When

VIN rises above 22.5V, the regulator suspends operation

by shutting off both power MOSFETs on the correspond-

ing channel. Once VIN drops below 21.5V, the regulator

immediately resumes normal operation. The regulator

executes its soft-start function when exiting an overvolt-

age condition.

Thermal Considerations and Output Current Derating

The thermal resistances reported in the Pin Configuration

section of the data sheet are consistent with those param-

eters 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 package mounted to a hardware test board—

also defined by JESD51-9 (“Test Boards for Area Array

Surface Mount Package Thermal Measurements”). The

motivation for providing these thermal coefficients in

found in JESD51-12 (“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 appli-

cation at various electrical and environmental operating

conditions to compliment any FEA activities. Without

FEA software, the thermal resistances reported in the

Pin Configuration 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 per-

taining 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 JESD 51-12; 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

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