LTM4650 datasheet(13/28 Pages) AD

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LTM4660

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

The Typical Applications on the first page is a basic

LTM4660 application circuit.

INTVCC/EXTVCC Power

Power for power switch drivers and most internal circuitry

is derived from the INTVCC pin. When the EXTVCC pin is

grounded or tied to a voltage less than 7V, an internal

5.8V linear regulator supplies INTVCC power from VIN. If

EXTVCC is taken above 8V, this linear regulator is turned

OFF and another 5.8V linear regulator turns ON to provide

the INTVCC power from EXTVCC. Using the EXTVCC pin

allows the INTVCC power to be derived from a high effi-

ciency external source, resulting in an overall increase in

LTM4660 efficiency. For LTM4660 applications where VOUT

is greater than 8V, EXTVCC can be directly tied to VOUT.

Shutdown and Start-Up

When RUN pin is below 1.1V, INTVCC linear regulator

along with all the internal circuitry including the main

control loop enters shutdown mode. Releasing the RUN

pin will allow a internal 1μA current source to pull this pin

up, thus enabling LTM4660. RUN pin can also be driven

directly by logic if this voltage does not exceed the abso-

lute maximum rating of 6V.

The slew rate of the output voltage VOUT can be controlled by

the voltage on the TRACK/SS pin. When voltage on TRACK/

SS is less than internal reference voltage of 0.8V (or EXT_

REF if this feature is utilized), the LTM4660 regulates the VFB

voltage to the TRACK/SS voltage instead of to the reference

voltage. This allows TRACK/SS pin to be used to program

the soft-start period by connecting an external capacitor

from the TRACK/SS pin to SGND. After capacitor balancing

phase of LTM4660 is completed, an internal 10μA pull-up

current charges the soft-start capacitor, creating a voltage

ramp. As the voltage on this pin rises linearly from 0V to

the reference voltage (and beyond), output voltage VOUT

rises smoothly from zero to the final set value. Note that

soft-start is achieved not by limiting the maximum output

current of LTM4660 but by controlling the output ramp volt-

age according to the ramp rate at the TRACK/SS pin. The

total soft-start time can be calculated with Equation 2.

tSOFT-START = 0.8V or VEXT_REF •

Css

10µA

(2)

A 0.1µF capacitor connected between TRACK/SS pin and

SGND would be sufficient for most typical IBC applica-

tions with LTM4660.

Output Voltage Tracking

Alternatively, TRACK/SS pin allows start-up of VOUT to track

that of another supply. Typically, this requires connecting

to the TRACK/SS pin an external resistor divider from the

other supply to ground. Tracking can be configured to be

either coincident or ratiometric as shown in Figure 3. In

the following discussions, VOUT1 refers to another supply’s

output while VOUT2 refers to the LTM4660 output that tracks

VOUT1. To implement the coincident tracking in Figure 3a,

connect an additional resistive divider to VOUT1 and connect

its midpoint to the TRACK/SS pin of the LTM4660. The ratio

of this divider should be the same as that of the slave chan-

nel’s feedback divider shown in Figure 4a. In this tracking

mode, VOUT1 must be higher than VOUT2. This ensures the

final voltage on TRACK/SS pin is greater than 0.8V.

To implement ratiometric tracking shown in Figure 3b,

connect a resistor divider R1 and R2 from external supply

to TRACK/SS pin of LTM4660. Select R1 and R2 such that

when external supply reaches steady state, final voltage

on TRACK/SS is less than 0.8V. Use this final voltage to

select FB resistor R4 (see Figure 4b). Output voltage VOUT2

of LTM4660 which ratiometrically tracks external supply

voltage VOUT1 is given by Equation 3.

VOUT2=

VOUT1 1+

60.4k

(3)

In order to track down another supply after the soft-start

has successfully reached 82.5% of 0.8V or VEXT_REF, it is

recommended to set the LTM4660 into forced continu-

ous mode operation by setting the MODE/PLLIN = 0V. By

selecting different resistors, the LTM4660 can achieve dif-

ferent modes of tracking including the two in Figure 3. The

ratio-metric mode has lesser output accuracy on VOUT2

but is fully coupled to any variations in VOUT1. In both

modes, there is an error in output voltage setting caused

by the pin current of TRACK/SS. To minimize this error,

use smaller resistor values in the divider.

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