LTC2926IGN datasheet(11/28 Pages) LINER

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LTC2926

2926fa

RFA and RFB. The slave output voltage varies as a function

of the master signal with terms set by RTA and RTB. By

selecting appropriate values of RTA and RTB, it is possible

to generate any of the proﬁles in Figures 1 to 4.

Controlling the Ramp-Up and Ramp-Down Behavior

The operation of the LTC2926 is most easily understood by

referring to the simpliﬁed functional diagram in Figure 6.

When the ON pin is low, the remote sense switch is opened

and the MGATE pin is pulled to ground causing the master

signal to remain low. Since the current through RTB1 is at

its maximum when the master signal is low, the current

sourced by FB1 is also at its maximum. The current forces

the FB1 pin voltage above 0.8V, which pulls the SGATE1

pin low and disconnects the slave’s supply generator. The

minimum voltage across the slave load is a function of the

maximum FB1 current, the feedback divider resistors, and

the load resistance (see Load Requirements).

When the ON pin rises above 1.23V, the master signal

ramps up, and the slave supply tracks the master signal.

The master ramp rate is set by an external capacitor driven

by a 10µA current source from an internal charge pump. If

no external MOSFET is used for the master signal, the ramp

rate is set by tying the MGATE and RAMP pins together

at one terminal of the external capacitor (see Ratiometric

Tracking Example or Supply Sequencing Example). The

MGATE pin voltage will be limited to VCC + 1V (max) by

the weak internal clamp on the RAMP pin.

The rising master signal decreases the tracking current

mirrored out of the FB1 pin. The gate controller circuitry

maintains 0.8V at FB1 by driving the SGATE1 voltage and,

via the external MOSFET source-follower, the slave supply

output. When the slave supply output reaches the slave

supply module voltage, the FB1 pin will fall below 0.8V

and the gate controller will drive the SGATE1 pin above

VCC to fully enhance the MOSFET.

After the MGATE, SGATE1 and SGATE2 pins reach their

maximum voltages, the RSGATE pin is pulled up by a

10µA current source from an internal charge pump,

which closes the integrated remote sense switches. The

integrated remote sense switch allows the slave supply

generator to compensate for voltage drop across the slave’s

MOSFET (Q1).

When the ON pin falls below VON(TH) – ΔVON(TH), typically

1.16V, the remote sense switch opens and the MGATE pin

pulls down with 10µA. The master signal and the slave

supplies will fall at the same rate as they rose previously,

following the tracking or sequencing proﬁle in reverse.

The ON pin can be controlled by a digital I/O pin or it

can be used to monitor an input supply. By connecting a

resistive voltage divider from an input supply to the ON

pin, the supplies will ramp up only after the monitored

supply reaches a preset voltage.

Figure 5. Simpliﬁed Tracking Cell and Gate Controller Cell Combination

APPLICATIO S I FOR ATIO

RTB

RTA

2926 F05

LTC2926

GATE CONTROLLER CELL

SGATE

0.8V

µA

SUPPLY

TRACK

IFB

ITRACK

MASTER

RAMP

SLAVE

RFB

RFA

µA

0.8V

VCC

QEXT

VCC + 5V

–

TRACKING CELL

–

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