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LTM4646

4646f

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APPLICATIONS INFORMATION

(hysteresis applies), or in undervoltage lockout (UVLO).

In an overvoltage (OV) condition, MT is turned off and MB

is turned on immediately without delay and held on until

the overvoltage condition clears. This happens regardless

of any other condition as long as the RUN pin is enabled.

For example, upon enabling the RUN1 pin, if VOUT is prebi-

ased at more than 7.5% above the programmed regulated

voltage, the OV stays triggered and BG forced on until

VOUT is pulled a ~2.5% hysteresis below the 7.5% OV

threshold.

Stability Compensation

The module has already been internally compensated for

all output voltages. Table 5 is provided for most applica-

tion requirements. LTpowerCAD will be provided for other

control loop optimization. Use LTpowerCAD when tying

output in parallel for higher current. External compensa-

tion may be necessary.

Run Enable

The RUN pins have an enable threshold of 1.3V maximum,

typically 1.2V with 100mV of hysteresis. They control the

turn on each of the channels and DRVCC and INTVCC. A

100k resistor to ground is internal, and can be used with

a pull-up resistor to VIN to turn on the module using the

external and internal resistor to program under voltage

lock out. Otherwise an external enable signal or source

can drive these pins directly below the 6V max. The RUN

pins can also be used for output voltage sequencing.

In parallel operation the RUN pins can be tie together

and controlled from a single control. See the Typical

Application circuits in Figure 21.

DRVCC, INTVCC, and EXTVCC

The LTM4646 module has an internal 5.3V low dropout

regulator (DRVCC) that is derived from the input voltage

through the CPWR (control power) pin. This regulator is

used to power the INTVCC control circuitry and the power

MOSFET drivers. This regulator can source up to 100mA,

and typically uses ~50mA for powering the device at the

maximum frequency. This internal 5.3V supply is enabled

by either RUN1 or RUN2.

EXTVCC allows an external 5V supply to power the

LTM4646 and reduce power dissipation from the internal

low dropout 5V regulator. The power loss savings can be

calculated by:

(CPWR – 5V) • 50mA = PLOSS

EXTVCC has a threshold of 4.6V for activation, and a maxi-

mum rating of 6V. When using a 5V input, connect this

5V input to EXTVCC also to maintain a 5V gate drive level.

EXTVCC must sequence on after CPWR, and EXTVCC must

sequence off before CPWR.

CPWR (Control Power)

The LTM4646 module has a CPWR pin that is biased with

a supply voltage minimum of 4.5V, and up to VIN maxi-

mum in normal operation. When operating at lower input

voltages below the 4.5V minimum, this pin can biased

with an alternate source to power the controller section

while operating down to the 2.375V minimum.

For example, if 3.3V is supplied to VIN, and a 5V bias with

a 50mA capability was used to source the CPWR pin, then

3.3V input power conversion can be implemented. Even

though the CPWR can operate from 4.5V to 20V, a lower

bias will lower the power loss if the module. See Figure 21

for an example.

Output Remote Sense

The LTM4646’s differential output sensing schemes are

distinct from conventional schemes where the regulated

output and its ground reference are directly sensed with

a difference amplifier whose output is then divided down

with an external resistor divider and fed into the error

amplifier input. This conventional scheme is limited by

the common mode input range of the difference amplifier

and typically limits differential sensing to the lower range

of output voltages.

The LTM4646 allows for seamless differential output

sensing by sensing the resistively divided feedback volt-

age differentially. This allows for differential sensing in the

full output range from 0.6V to 5.5V. Channel 1’s difference

amplifier (DIFFAMP) has a bandwidth of around 8MHz,

and channel 2’s feedback amplifier has a bandwidth of

around 4MHz, both high enough so as to not affect main

loop compensation and transient behavior.

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