LTC2926IGN datasheet(23/28 Pages) LINER

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LTC2926

2926fa

APPLICATIO S I FOR ATIO

Slave Control Without MOSFETs

The LTC2926 can control tracking and sequencing of a

slave supply without a MOSFET if the supply generator’s

output voltage is set by an accessible resistive voltage

divider and if its voltage reference is ground-based. Track-

ing currents mirrored to the FB pins are injected into the

feedback nodes of the supply generators to control the

output voltage. When master ramp signal has reached

it maximum voltage, the FB pin current is zero, and the

LTC2926 has no effect on the output voltage accuracy,

transient response or stability of the generator.

To control a supply generator (e.g., DC/DC converter) with

a feedback reference voltage VFB(GEN) of 0.75V or less,

connect the FB pin of the LTC2926 to the tap point of the

generator’s resistive divider, as shown in Figure 21a. Fol-

low steps 1 and 3 of the Three-Step Design Procedure to

set the ramp rates and tracking proﬁle. Use the feedback

resistor values required by the supply generator for RFA

and RFB in step 3.

A generator with VFB(GEN)>0.75Vmaybecontrolledwithout

a MOSFET if the slave voltage is large enough (see Figure

22). First follow steps 1 and 3 of the Three-Step Design

Procedure to set the ramp rates and tracking proﬁle.

Use the feedback resistor values required by the supply

generator in step 3. Next, split resistor RFA as in Figure

21b, so that

and

FAA

FB GEN

≤

•

()

075

RFAB = RFA – RFAA

and tie the LTC2926’s FB pin to the node in between. The

new tap point allows the LTC2926’s FB pin to see <0.75V

at the end of ramp-up. Finally, scale the track resistors to

match RFAA:

′ =

•

FAA

FAB

TA and

′ =

•

FAA

FAB

Voltage regulators that force their reference voltage be-

tween their output and feedback nodes do not employ a

ground-based reference, and thus will not be controllable

by the LTC2926 without a series MOSFET.

Figure 21. Slave Supply Control Without a MOSFET

(a) Generator Reference VFB(GEN) ≤ 0.75V and (b) Generator Reference VFB(GEN) > 0.75V

OUT

SUPPLY GENERATOR

GND

VFB(GEN)

VIN

SLAVE

RTB

RTA

LTC2926

RAMPBUF

TRACK

GND

SGATE

(a)

RFA

RTB

RTA

2926 F21

LTC2926

RAMPBUF

TRACK

GND

SGATE

OUT

SUPPLY GENERATOR

GND

VFB(GEN)

VIN

SLAVE

(b)

RFAB

RFB

RFAA

RFB

′

VFB(GEN) (V)

0.25

0.50

0.75

1.00

2926 F22

1.25

1.50

CONTROL

VIA FB PIN

CONTROL VIA

FB PIN AND

SPLIT RFA

RESISTOR

MOSFET

CONTROL ONLY

Figure 22. Regions of Possible

Slave Control Without a MOSFET

Slave Supply Control Without

a MOSFET

LTC2926IGN Datasheet(PDF) 23 Page - Linear Technology

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