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LTM4651

4651f

For more information www.linear.com/LTM4651

Power Module Protection

TheLTM4651’scurrentmodecontrolarchitectureprovides

fast cycle-by-cycle current limit in an overcurrent condi-

tion, as shown in the Typical Performance Characteristics

section. If the output voltage collapses sufficiently due to

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

will be violated (EQ. 3) and the internal oscillator will then

fold-back automatically to one-fifth of the LTM4651’s

programmed switching frequency—hereby reducing the

output current and affording the load a chance to recover.

The LTM4651 features input overvoltage shutdown

protection: when VIN+|VOUT–| > 68V, switching action

ceases (with 4V of hysteresis)—however, be advised that

this protection is only active outside the LTM4651’s safe

operating area (see Note 1 and Note 4 of the Electrical

Characteristics table).

The LTM4651 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 LTM4651 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 3. Undervoltage

lockout keeps the LTM4651 in shutdown until the supply

input voltage is above a certain voltage programmed by

the user. The RUN pin hysteresis voltage prevents noise

from falsely tripping UVLO. Resistors are chosen by first

selecting RB (refer to Figure 3). Then:

RA =RB •

VIN(ON)

1.2V

–1

















(15)

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 •

















(16)

IfUVLOisnotneeded,RUNcanbeconnectedtoLTM4651’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 SVOUT– 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 SVOUT–; in some instances,

connect a capacitor (CTHP) from COMPa to SVOUT– (par-

alleling the RTH-CTH series-RC network). See Table 8 for

suggested input and output capacitances for a variety of

operatingconditions.Additionally,theLTpowerCADdesign

tool is available for transient and stability analysis.

APPLICATIONS INFORMATION

Figure 3. Undervoltage Lockout Resistive Divider

RUN PIN

VSUPPLY

4651 F03

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