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LTM4612

4612f

APPLICATIONS INFORMATION

Fault Conditions: Current Limit and

Overcurrent Foldback

LTM4612 has a current mode controller, which inherently

limits the cycle-by-cycle inductor current not only in steady

state operation, but also in transient.

To further limit current in the event of an overload condi-

tion, the LTM4612 provides foldback current limiting. If the

output voltage falls by more than 50%, then the maximum

output current is progressively lowered to about one sixth

of its full current limit value.

Soft-Start and Tracking

The TRACK/SS pin provides a means to either soft-start

the regulator or track it to a different power supply. A

capacitor on this pin will program the ramp rate of the

output voltage. A 1.5μA current source will charge up the

external soft-start capacitor to 80% of the 0.6V internal

voltage reference minus any margin delta. This will control

the ramp of the internal reference and the output voltage.

The total soft-start time can be calculated as:

tV V

µA

SOFTSTART

OUT MARGIN

≅

()

.•

–

•

()

If the RUN pin falls below 2.5V, then the soft-start pin

is reset to allow for the proper soft-start again. Current

foldback and force continuous mode are disabled during

the soft-start process. The soft-start function can also

be used to control the output ramp rising time, so that

another regulator can be easily tracked.

Output Voltage Tracking

Output voltage tracking can be programmed externally

using the TRACK/SS pin. The output can be tracked up

and down with another regulator. The master regulator’s

output is divided down with an external resistor divider

that is the same as the slave regulator’s feedback divider.

Figure 6 shows an example of coincident tracking. Ra-

tiometric modes of tracking can be achieved by selecting

different resistor values to change the output tracking ratio.

The master output must be greater than the slave output

for the tracking to work. Figure 7 shows the coincident

output tracking.

RUN Enable

The RUN pin is used to enable the power module. The

pin has an internal 5.1V zener to ground. The pin can be

driven with 5V logic levels.

The RUN pin can also be used as an undervoltage lockout

(UVLO) function by connecting a resistor divider from

the input supply to the RUN pin. The equation for UVLO

threshold:

UVLO =

•.

where R1 is the top resistor, and R2 is the bottom resistor.

Power Good

The PGOOD pin is an open-drain pin that can be used to

monitor valid output voltage regulation. This pin monitors

a ±10% window around the regulation point, and tracks

with margining.

Figure 6. Coincident Tracking

Figure 7. Coincident Output Tracking

OUTPUT

VOLTAGE

TIME

4612 F07

MASTER OUTPUT

SLAVE OUTPUT

PGOOD

RUN

COMP

INTVCC

DRVCC

TRACK/SS

fSET

VOUT

VFB

FCB

MARG0

MARG1

MPGM

TRACK

CONTROL

PLLIN

LTM4612

5.23k

100k

10μF

MASTER

OUTPUT

100k

COUT

SLAVE

OUTPUT

4612 F06

CIN

VIN

PGND

SGND

VIN

5.23k

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