LTM4658EVPBF datasheet(13/26 Pages) AD

13 / 26 page

LTM4658

Rev. 0

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Once the soft-start cycle has completed and the output

power good flag thrown, the SSTT pin reports the die

junction temperature. The LTM4658 regulates the SSTT

pin to a voltage proportional to the junction temperature.

While reporting the temperature, the SSTT voltage is not

valid below 1V. The junction temperature is calculated

with Equation 6.

TJ (°C) =

VSSTT

4mV

– 273

(6)

The following procedure is used for a more accurate mea-

surement of the junction temperature:

1. Measure the ambient temperature TA.

2. Measure the SSTT voltage while in pulse-skipping

mode with the VOUT pulled up slightly higher than the

regulated VOUT.

3. Calculate the slope of the temperature sensing circuit

using Equation 7.

Slope mV

°C

⎛

⎝

⎜

⎞

⎠

⎟=

VSSTT

TA + 273

(7)

4. Calculate the junction temperature with the new cali-

brated slope.

When the output voltage goes out of regulation and the

power good pin is pulled low, the soft-start pin no longer

reports the temperature.

Power Good

The PGOOD pins are open drain pins that can be used to

monitorvalidoutputvoltageregulation.Thispinmonitorsa

−3/+10%windowaroundtheregulationpoint.Aresistorcan

be pulled up to a particular supply voltage for monitoring.

To prevent unwanted PGOOD glitches during transients

or dynamic VOUT changes, the LTM4658’s PGOOD falling

edgeincludesablankingdelayofapproximately100µs.The

PGOOD is also actively pulled low during fault conditions:

RUN pin is low, VIN is too low or in thermal shutdown.

Stability Compensation

The LTM4658 module’s internal compensation loop is

designed and optimized for low ESR ceramic output

capacitors only application. A 10pF to 15pF phase boost

cap between VOUT and FB pins may be required to help

maintainingahighphasemargin.TheLTpowerCADdesign

tool is available to download for control loop optimization.

RUN Threshold Programming

The LTM4658 has a precision threshold RUN pin to en-

able or disable switching. Pulling the RUN pin to ground

forces the LTM4658 into its shutdown state, turning off

both power MOSFETs and most of its internal control

circuitry. Bringing the RUN pin above 0.4V will turn on

the entire chip.

The rising threshold of the RUN comparator is 400mV

with 60mV of hysteresis. Connect the RUN pin to VIN if

the shutdown feature is not used. Adding a resistor divider

from VIN to RUN programs the LTM4658 to regulate the

output only when VIN is above a desired voltage (see the

Block Diagram). Typically, this threshold, VIN(RUN),isused

in situations where the input supply is current limited or

has a relatively high source resistance. A switching regu-

lator draws constant power from the source, so source

current increases as 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(RUN) threshold prevents the

regulator from operating at source voltages where prob-

lems may occur. This threshold can be adjusted by setting

the values R1 and R2 such that they satisfy Equation 8.

VIN(RUN) =

⎛

⎝

⎜

⎞

⎠

⎟• 400mV

(8)

where the LTM4658 will remain off until VIN is above

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

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

Alternatively, a resistor divider from an output of another

regulator to the enable RUN pin of the LTM4658 provides

event-based power up sequencing, enabling the LTM4658

when the output of the other regulator reaches a prede-

termined level.

APPLICATIONS INFORMATION

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