LTM8045 datasheet(28/54 Pages) AD

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LTM4655

Rev. 0

For more information www.analog.com

Power Module Protection

The LTM4655’s current mode control architecture pro-

vides fast cycle-by-cycle current limit in an overcur-

rent condition, 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 and the internal oscil-

lator will then fold-back automatically to one-fifth of the

LTM4655’s programmed switching frequency—thereby

reducing the output current and affording the load a

chance to recover.

The LTM4655 ceases channel

n switching action if the

channel’s internal temperatures exceed 165°C. The chan-

nel’s 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 are not production

tested. This overtemperature protection is intended to

protect the device during momentary overload condi-

tions. The maximum rated junction temperature will be

exceeded when this overtemperature protection is active.

Continuous operation above the specified absolute maxi-

mum operating junction temperature may impair device

reliability or permanently damage the device. See Note 1

of the Electrical Characteristics table.

The LTM4655 does not feature any specialized output

overvoltage protection beyond what is inherent to the

control loop’s servo mechanism.

RUN Pin Enable

The RUN

n pin is used to enable the power module or

sequence the power module. The threshold is 1.2V. The

RUN

n pin can be used to provide an undervoltage lockout

(UVLO) function by connecting a resistor divider from

the input supply to the RUN

n pin, as shown in Figure 2.

Undervoltage lockout keeps channel

n of the LTM4655

in shutdown until the supply input voltage is above a

certain voltage programmed by the user. The RUN

n pin

hysteresis voltage prevents noise from falsely tripping

UVLO. Resistors are chosen by first selecting RBn (refer

to Figure 2 and Equation 14). Then:

RUN

n PIN

RAn

RBn

VSUPPLY

4655 F02

Figure 2. Undervoltage Lockout Resistive Divider

RAn =RBn •

VINn(ON)

1.2V

–1

⎛

⎝⎜

⎞

⎠⎟

(14)

where VINn(ON) is the input voltage at which the under-

voltage lockout is overcome and the supply turns on. The

VINn turn-off voltage, VINn(OFF) is given by Equation 15.

VINn(OFF) =1.07V •

RAn

RBn

⎛

⎝⎜

⎞

⎠⎟

(15)

If UVLO is not needed, RUN

n can be connected to

LTM4655’s LDOOUT pin.

When RUN

n is below its threshold, UVLO of channel n

is engaged, MTn and MBn are turned off, INTVCCn ceases

to be regulated, and ISET

na is discharged to SVOUTn– by

internal circuitry.

Loop Compensation

External loop compensation may be preferred for some

applications and can be implemented easily, as follows:

leave COMP

nb open circuit; connect a series-RC net-

work RTHn and CTHn from COMPnb to SVOUTn–; in some

instances, connect a capacitor (CTHPn) from COMPna to

SVOUTn– (paralleling the RTHn–CTHn series-RC network).

See Table 10 for suggested input and output capaci-

tances for a variety of operating conditions. Additionally,

the LTpowerCAD design tool is available for transient and

stability analysis.

APPLICATIONS INFORMATION

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