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LTC2926

2926fa

Considerations when Using Remote Sense Switches

Consider the supply and sense path detail functional

diagram and equivalent circuit in Figure 11. For proper

compensation of the I • R drop across the external control

MOSFET Q0 by the supply module, the voltage at its sense

pin input must be equal to the supply voltage at the load.

Solving for VSENSE in the equivalent circuit yields:

SENSE

XSW

SUPPLY

XSW

⎛

⎝⎜

⎞

⎠⎟

•+

⎛

⎝⎜

⎞

⎠⎠⎟

• VOUT

For the best compensation, i.e., VSENSE ≈ VSUPPLY, choose

RX >> RSW.

The remote sense switch is intended to be a low-current

voltage feedback path. The control MOSFET (Q0 in Figure

11a) should carry all but a tiny fraction of the entire load

current. The remote sense switch current is:

LOAD

XSW

=•

⎛

⎝⎜

⎞

⎠⎟

To minimize switch current, choose RX >> RDS. In appli-

cations that use the LTC2926’s integrated remote sense

switches, ISW must not exceed the Absolute Maximum

Ratings for switch pin currents.

It is recommended design practice to satisfy both

resistance value conditions.

SGATE Voltage at Ramp Start/End

When the master ramp is 0V (before ramp up or after

ramp down), the control MOSFET ideally conducts no

current. If the tracking proﬁle has no delay or offset, the

gate control loops may force the SGATE pins either to

ground or to just below the MOSFET threshold voltage,

depending on reference offsets, resistor mismatches and

the load resistance. In both cases the slave load will be at

about 0V, but if a known state of SGATE is desired, include

an offset in the tracking proﬁle.

To guarantee grounding of the SGATE pins at RAMP = 0V,

include a positive offset, VOS, based on the maximum slave

supply voltage, VSLAVE(max), and the tracking/feedback

resistor tolerance. Note that at the start of ramp up, the

gate capacitance of the MOSFET must be charged to the

threshold voltage before the source begins ramping. The

SGATE pins do provide extra current to speed the initial

charging.

Calculate the required VOS from:

VOS ≥ k • VSLAVE(max)

For 1% resistors k = 1/8, for 5% resistors k = 1/4, for

10% resistors k = 2/5.

To guarantee the SGATE pins sit at the MOSFET threshold

voltages at RAMP = 0V, include a negative offset. Note that

when the master ramp goes to 0V, the slave supplies will

remain above ground by the magnitude of the offset.

Calculate the required VOS from:

VOS ≤ –k • VSLAVE(max)

Figure 11. Supply and Sense Path Detail, (a) Functional Diagram

and (b) Equivalent Circuit When Remote Sense Switch Is Closed

APPLICATIO S I FOR ATIO

LOAD

2926 F11

OUT

SENSE

MGATE

RSGATE

(a)

ISW

SUPPLY

MODULE

+ VDS –

MASTER

SUPPLY

VOUT

VSENSE

(b)

ISW

RSW

+ VDS –

RDS

VSUPPLY

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