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LTM4627

4627fc

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top feedback resistor of the slave regulator in equal slew

rate or coincident tracking, then RTA is equal to RFB with

VFB = VTRACK. Therefore RTB = 60.4k, and RTA = 60.4k in

Figure 4.

In ratiometric tracking, a different slew rate maybe desired

for the slave regulator. RTB can be solved for when SR is

slower than MR. Make sure that the slave supply slew

rate is chosen to be fast enough so that the slave output

voltage will reach its final value before the master output.

For example, MR = 1.5V/ms, and SR = 1.2V/ms. Then RTB

= 75k. Solve for RTA to equal 51.1k.

For applications that do not require tracking or sequenc-

ing, simply tie the TRACK/SS pin to INTVCC to let RUN

control the turn on/off. When the RUN pin is below its

threshold or the VIN undervoltage lockout, then TRACK/

SS is pulled low.

Figure 5. Output Voltage Coincident Tracking

APPLICATIONS INFORMATION

Overcurrent and Overvoltage Protection

The LTM4627 has overcurrent protection (OCP) in a

short circuit. The internal current comparator threshold

folds back during a short to reduce the output current.

An overvoltage condition (OVP) above 10% of the regu-

lated output voltage will force the top MOSFET off and

the bottom MOSFET on until the condition is cleared.

4627 F05

TIME

SLAVE OUTPUT

MASTER OUTPUT

OUTPUT

VOLTAGE

An input electronic circuit breaker or fuse can be sized to

be tripped or cleared when the bottom MOSFET is turned

on to protect against the overvoltage. Foldback current

limiting is disabled during soft-start or tracking start-up.

Run Enable

The RUN pin is used to enable the power module or

sequence the power module. The threshold is 1.25V, and

the pin has an internal 5.1V Zener to protect the pin. The

RUN pin can be used as an undervoltage lockout (UVLO)

function by connecting a resistor divider from the input

supply to the RUN pin:

VUVLO = ((R1+R2)/R2) • 1.25V.

See the Block Diagram for the example of use.

INTVCC Regulator

The LTM4627 has an internal low dropout regulator from

VIN called INTVCC. This regulator output has a 4.7µF

ceramic capacitor internal. This regulator powers the

internal controller and MOSFET drivers. The gate driver

current is ~20mA for 750kHz operation. The regulator loss

can be calculated as:

(VIN– 5V)• 20mA =PLOSS

EXTVCC external voltage source ≥ 4.7V can be applied to

thispintoeliminatetheinternalINTVCCLDOpowerlossand

increase regulator efficiency. A 5V supply can be applied

to run the internal circuitry and power MOSFET driver. If

unused, leave pin floating. EXTVCC must be less than VIN

at all times during power-on and power-off sequences.

Stability Compensation

TheLTM4627hasalreadybeeninternallycompensatedfor

alloutputvoltages.Table4isprovidedformostapplication

requirements. LTpowerCAD is available for other control

loop optimization.

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