LTM8049 datasheet(16/32 Pages) LINER

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LTM4653

Rev 0

For more information www.analog.com

Power Module Protection

TheLTM4653’scurrentmodecontrolarchitectureprovides

fastcycle-by-cyclecurrentlimitinanovercurrentcondition,

as shown in the Typical Performance Characteristics sec-

tion. If the output voltage collapses sufficiently due to an

overload or short-circuit condition, minimum on-time will

be violated and the internal oscillator will then fold-back

automatically to one-fifth of the LTM4653’s programmed

switchingfrequency—therebyreducingtheoutputcurrent

and affording the load a chance to recover.

TheLTM4653featuresinputovervoltageshutdownprotec-

tion: when VIN > 68V, switching action ceases (with 4V of

hysteresis)—however, be advised that this protection is

onlyactiveoutsidetheLTM4653’ssafeoperatingarea(see

Note 1 and Note 4 of the Electrical Characteristics table).

The LTM4653 ceases switching action if internal tempera-

tures exceed 165°C. The control IC resumes operation

after a 10°C cool-down hysteresis. Note that these typical

parameters are based on measurements in a lab oven and

arenotproductiontested.Thisovertemperatureprotection

is intended to protect the device during momentary over-

loadconditions.Themaximumratedjunctiontemperature

will be exceeded when this overtemperature protection is

active. Continuous operation above the specified absolute

maximum operating junction temperature may impair

device reliability or permanently damage the device.

The LTM4653 does not feature any specialized output

overvoltage protection beyond what is inherent to the

control loop’s servo mechanism.

RUN Pin Enable

The RUN pin is used to enable the power module or se-

quence the power module. The threshold is 1.2V. The RUN

pincanbeusedtoprovideanundervoltagelockout(UVLO)

function by connecting a resistor divider from the input

supply to the RUN pin, as shown in Figure 2. Undervoltage

lockout keeps the LTM4653 in shutdown until the supply

input voltage is above a certain voltage programmed by

the user. The RUN pin hysteresis voltage prevents noise

APPLICATIONS INFORMATION

from falsely tripping UVLO. Resistors are chosen by first

selecting RB (refer to Figure 2). Then:

RA =RB •

VIN(ON)

1.2V

– 1

⎛

⎝

⎜

⎞

⎠

⎟

where VIN(ON) is the input voltage at which the undervolt-

age lockout is overcome and the supply turns on. RA may

be replaced with a hardwired connection from VD to RUN.

The VIN turn-off voltage, VIN(OFF) is given by:

VIN(OFF) = 1.07V •

⎛

⎝

⎜

⎞

⎠

⎟

IfUVLOisnotneeded,RUNcanbeconnectedtoLTM4653’s

VD or VIN pins.

When RUN is below its threshold, UVLO is engaged, MT

and MB are turned off, INTVCC ceases to be regulated, and

ISETa is discharged to SGND by internal circuitry.

Loop Compensation

External loop compensation may be preferred for some

applications and can be implemented easily, as follows:

leave COMPb open circuit; connect a series-RC network

(RTH and CTH) from COMPa to SGND; in some instances,

connect a capacitor (CTHP) from COMPa to SGND (paral-

leling the RTH-CTH series-RC network). See Table 7 for

suggested input and output capacitances for a variety of

operatingconditions.Additionally,theLTpowerCADdesign

tool is available for transient and stability analysis.

RUN PIN

VSUPPLY

4653 F02

Figure 2. Undervoltage Lockout Resistive Divider

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