LTM4643 datasheet(13/24 Pages) AD

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LTM4710-1

Rev. 0

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mode. The rising threshold of the RUN comparator is

400mV, with 75mV of hysteresis. It can be tied to VIN if

the shutdown feature is not used. Adding a resistor divider

from VIN to RUN programs the LTM4710-1 to regulate the

output only when VIN is above a desired voltage. Typically,

this threshold, VIN(EN), is used in situations where the

input supply is current limited or has a relatively high

source resistance. A switching regulator draws constant

power from the source, so the source current increases

as the source voltage drops. This looks like a negative

resistance load to the source and can cause the source

to current limit or latch low under low source voltage

conditions. The VIN(EN) threshold prevents the regulator

from operating at source voltages where problems may

occur. This threshold can be adjusted by setting the values

R1 and R2 such that they satisfy Equation 6.

VIN(EN) =

⎛

⎝⎜

⎞

⎠⎟

• 400mV

(6)

where the LTM4710-1 will remain off until VIN is above

VIN(EN). Due to the comparator’s hysteresis, switching will

not stop until the input falls slightly below VIN(EN).

Alternatively, a resistor divider from an output of another

channel to the RUN pin of the LTM4710-1 provides event-

based power-up sequencing, enabling the LTM4710-1

when the output of the other regulator reaches a

predetermined level.

Thermal Considerations and Output Current Derating

The thermal resistances reported in the Pin Configuration

section of this data sheet are consistent with those param-

eters defined by JESD51-12 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.

The motivation for providing these thermal coefficients

in found in JESD51-12 (“Guidelines for Reporting and

Using Electronic Package Thermal Information”).

Many designers may 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 on 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 three ther-

mal coefficients explicitly defined in JESD 51-12; these

coefficients are quoted or paraphrased below.

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

the natural convection junction-to-ambient air thermal

resistance measured in a one cubic foot sealed enclo-

sure. 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 the demo board DC3164A-B.

2. θJCbottom, the thermal resistance from junction to

bottom of the product case, is determined with all 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

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

3. θJCtop, the thermal resistance from the junction to the

top of the product case, is determined with nearly all

of the component power dissipation flowing through

the top of the package. As the electrical connections of

the typical µModule are on the bottom of the package,

it is rare for an application to operate such that most of

the heat flows from the junction to the top of the part.

As in the case of θJCbottom, this value may be useful

for comparing packages, but the test conditions don’t

generally match the user’s application.

A graphical representation of the aforementioned thermal

resistances is shown in Figure 3; blue resistances are

contained within the μModule regulator, whereas green

resistances are external to the µModule.

APPLICATIONS INFORMATION

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