LTC2926IGN datasheet(17/28 Pages) LINER

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LTC2926

2926fa

Three-Step Design Procedure

The following three-step design procedure allows one

to choose the FBn resistors, RFAn and RFBn, the TRACKn

resistors, RTAn and RTBn, and the master ramp capacitor,

CMGATE, that give any of the tracking or sequencing proﬁles

shown in Figures 1 to 4. A three-supply application circuit

is shown in Figure 12.

1. Set the ramp rate of the master signal.

Solve for the value of CMGATE, the capacitor on the MGATE

pin, based on the desired ramp rate (volts per second)

of the master ramp signal, SM, and the MGATE pull-up

current IMGATE, which is nominally 10µA.

MGATE

(1)

If the master ramp signal is a master supply, consider

the gate capacitance of the required external N-channel

MOSFET. If the gate capacitance is comparable to CMGATE,

reduce the external capacitor’s value to compensate for

the gate capacitance of the MOSFET.

If the master ramp signal is not a master supply, tie the

RAMP pin to the MGATE pin.

2. Choose the feedback resistors based on the slave

supply voltage and slave load.

It is important that the feedback resistors are signiﬁcantly

larger than the load resistance, especially as the slave

voltage nears ground (see Load Requirements).

First determine the effective slave load resistance, RL (not

shown), at low slave voltage levels, and select the value

of the top feedback resistor, RFB, to satisfy:

RFB ≥ 100 • RL (recommended),

RFB ≥ 23 • RL (required)

(2)

Second, determine a value for the lower feedback resistor,

RFA, that will ensure that the LTC2926 fully enhances the

gate of the slave control MOSFET at the end of ramping.

Select RFA based on RFB, the resistor tolerance, TOLR, and

the maximum slave supply voltage, VSLAVE(max):

TOL

SLAVE

<• −

⎛

⎝⎜

⎞

⎠⎟

0 784

(max)

−−

⎛

⎝⎜

⎞

⎠⎟

(3)

Note: Choose the value of VSLAVE(max) to cover all slave

supply voltage tolerances by a good margin. Exceeding the

VSLAVE(max) voltage used for this calculation can result in

triggering a Power Good Fault unintentionally.

If the slave generator has an accessible resistive divider

and a ground-based voltage reference, it may be able to

be controlled without a series MOSFET. In that case, let

the generator’s design set RFA and RFB, substitute the

generator’s reference voltage for VFB(REF) in step 3, and

see the subsection Slave Control Without MOSFETs.

3. Solve for the tracking resistors that set the desired

ramp rate and voltage offset or time delay of the slave

supply.

Choose a ramp rate for the slave supply, SS. If the slave

supply tracks coincidently with the master supply or with

only a ﬁxed offset or delay, then the slave ramp rate equals

the master ramp rate. Be sure that the slave ramp rate and

its offset or delay allows the slave voltage to ﬁnish ramping

Figure 12. Three-Supply Application

APPLICATIO S I FOR ATIO

RTB1

RTA1

2926 F12

LTC2926

FB1

RAMPBUF

TRACK1

TRACK2

GND

PGTMR

VIN

RTB2

RTA2

FAULT

10k

CPGTMR

CMGATE

MGATE

0.1

µF

Ω

VCC

RAMP

VIN

STATUS

STATUS/PGI

ON/OFF

10k

VIN

SGATE2

Ω

SGATE1

Ω

MASTER

RFB1

RFA1

FB2

RFB2

RFA2

FAULT

OUT

SENSE

RX1

SUPPLY MODULE

VIN

SLAVE1

OUT

SENSE

RX2

SUPPLY MODULE

VIN

SLAVE2

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