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LTC2921/LTC2922 Series

29212fa

APPLICATIO S I FOR ATIO

Figure 5. Basic Monitor Connection

Setting the Supply Monitor Levels

The LTC2921 and LTC2922 series both feature low 0.5V

monitoring thresholds with tight 1% accuracy. To set a

supply monitoring level tightly, design a precision ratio

resistive divider to relate the lowest valid supply voltage to

the maximum specified monitor threshold voltage. Use

resistors with 1% tolerance or better to limit the error due

to mismatch. The basic resistive divider connection for

supply monitoring is shown in Figure 5.

2921/22 F05

LTC2922

GND

GATE

VQ1

CGATE

LOAD

IMON

IA1

RB1

RY1

RZ1

RA1

VL1

VV1

VSRC1

RG1

Ω

VOUT

VFB

GND

DC/DC

CONVERTER

+–

±0.1µA

First, divide the nominal monitor threshold voltage by an

acceptable bias current (IA1), and choose a nearby stan-

dard value for resistor RA1 (see Equation 1).

Next, calculate the bounds on the value of RB1 that

guarantee that the divided minimum supply voltage ex-

ceeds the maximum specified monitor threshold voltage,

and that the minimum specified overvoltage threshold

exceeds the divided maximum supply voltage. Use Equa-

tions 2 and 3 to calculate RB1(MAX) and RB1(MIN) from RA1,

the resistor tolerance (RTOL), the supply voltage, the

monitor threshold and overvoltage specifications, and the

monitor pin leakage current specification.

When the integrated remote sensing switch is closed, the

DC/DC converter will compensate for the IR drop from

drain to source of the external N-channel FET (VQ1(ON)) by

increasing the supply voltage by the same amount. Calcu-

late with VQ1(ON)(MAX) = 0V if the remote sense switch is

not used.

0 500

(1)

RTOL

VA R

B MAX

SRC MIN

0 505

0 1

()

•

–

•

–.

•









+µ









(2)

RTOL

VA R

B MIN

SRC MAX

Q ON MAX

0 665

0 1

()

(

)()

•

–

•

–.

.– .

•

















(3)

Choose a standard resistor value for RB1 that satisfies the

inequality of Equation 4.

RB1(MIN) ≤ RB1 ≤ RB1(MAX)

(4)

When several standard values meet the requirement,

choose the value closest to RB1(MAX) to set the tightest

monitor threshold. This also allows more headroom for

larger VQ1(ON)(MAX). Alternatively, choose the standard

value closest to RB1(MIN) to set the tightest overvoltage

threshold.

All four monitor input voltages must be between the

monitor threshold and the overvoltage threshold for the

turn-on sequence to begin. Connect unneeded monitor

input pins to any of the utilized monitor input pins.

Selecting the External N-Channel MOSFETs

The GATE pin drives the gate of external N-channel

MOSFETs above VCC to connect the supplies to the loads.

The GATE drive voltage provided by the LTC2921/LTC2922

series is best suited to logic-level and sublogic-level

power MOSFETs. To achieve the lowest switch resistance,

the VCC pin must be connected to the highest supply

voltage.

Consider the application requirements for current, turnoff

speed, on-resistance, gate-source voltage specification,

etc. Refer to the Electrical Specifications and Typical

Performance Curves to determine the GATE voltages for

given VCC voltages over the required range of conditions.

Calculate the minimum gate drive voltage for each moni-

tored supply for use in selecting the FETs. Check the

maximum GATE voltage against the FETs’ gate-source

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