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LTM4649

4649fc

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TIME

MASTER OUTPUT

SLAVE OUTPUT

4649 F06

Figure 6. Output Coincident Tracking Characteristic

APPLICATIONS INFORMATION

down with the same resistor values used to set the slave’s

output, then the slave will coincident track with the master

until it reaches its final value. The master will continue to

its final value from the slave’s regulation point. Voltage

tracking is disabled when VTRACK is more than 0.6V. RTA

in Figure 5 will be equal to RFB for coincident tracking.

Ratiometric tracking can be achieved by a few simple

calculations and the slew rate value applied to the mas-

ter’s TRACK/SS pin. As mentioned above, the TRACK/SS

pin has a control range from 0V to 0.6V. The master’s

TRACK/SS pin slew rate is directly equal to the master’s

output slew rate in Volts/Time. The equation for RTB:

• 10k

= RTB

where MR is the master’s output slew rate and SR is the

slave’s output slew rate in Volts/Time. When coincident

tracking is desired, then MR and SR are equal, thus RTB

is equal to 10k. RTA is derived from equation:

RTA =

0.6V

VFB

10k

+ VFB

RFB

−

VTRACK

RTB

where VFB is the feedback voltage reference of the regula-

tor, and VTRACK is 0.6V. Since RTB is equal to the 10k 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 = 10k, and RTA = 10k in Figure 5.

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 it final value before the master output.

Each of the TRACK/SS pins will have the 1.3µA current

source on when a resistive divider is used to implement

trackingonthatspecificchannel.Thiswillimposeanoffset

on the TRACK/SS pin input. Smaller value resistors with

the same ratios as the resistor values calculated from the

previous equations can be used. For example, where the

10k value is calculated then a 1.0k can be used to reduce

the TRACK/SS pin offset to a negligible value.

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. A pull-up

resistor can be connected from PGOOD to a supply volt-

age no greater than 6V.

Stability Compensation

The module has already been internally compensated for

alloutputvoltages.Table3isprovidedformostapplication

requirements. LTpowerCAD is available for other control

loop optimization.

Run Enable

The RUN pin has an enable threshold of 1.4V maximum,

typically 1.25V with 150mV of hysteresis. It controls the

turn-on of the µModule. The RUN pin can be pulled up to

VIN for 5V operation, or a 5V Zener diode can be placed

on the pin and a 10k to 100k resistor can be placed up to

higher than 5V input for enabling the µModule. The RUN

pin can also be used for output voltage sequencing.

In parallel operation the RUN pins can be tied together and

controlled from a single control. See the Typical Applica-

tion circuits in Figures 19 and 20. The RUN pin can also

be left floating. The RUN pin has a 1µA pull-up current

source that increases to 4.5µA during ramp-up.

Differential Remote Sense Amplifier

An accurate differential remote sense amplifier is provided

in the LTM4649 to sense low output voltages accurately

at the remote load points. This is especially applicable for

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