LTC4261IGN datasheet(18/32 Pages) LINER

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LTC4261/LTC4261-2

42612fb

to turn on again immediately (without delay) unless the

overvoltage auto-retry has been disabled by clearing reg-

ister bit D0.

Undervoltage Comparator and Undervoltage Fault

The LTC4261/LTC4261-2 provide two undervoltage pins,

UVH and UVL, for adjustable UV threshold and hyster-

esis. The UVH and UVL pins have the following accurate

thresholds:

For UVH rising, VUVH(TH) = 2.56V, turn on

For UVL falling, VUVL(TH) = 2.291V, turn off

Both UVH and UVL pins have a minimum hysteresis of

δVUV (15mV typical). In either a rising or a falling input

supply, the undervoltage comparator works in such a way

that both the UVH and the UVL pins have to cross their

thresholds for the comparator output to change state.

The UVH, UVL, and OV threshold ratio is designed to

match the standard telecom operating range of 43V to

71V and UV hysteresis of 4.5V when UVH and UVL are

tied together as in Figure 1, where the built-in UV hyster-

esis referred to the UVL pin is:

ΔVUV(HYST) = VUVH(TH) – VUVL(TH) = 0.269V

Using R1 = 11.8k, R2 = 16.9k and R3 = 453k as in Figure 1

gives a typical operating range of 43.0V to 70.7V, with

an undervoltage shutdown threshold of 38.5V and an

overvoltage shutdown threshold of 72.3V.

The UV hysteresis can be adjusted by separating the

UVH and the UVL pins with a resistor RH (Figure 7). To

increase the UV hysteresis, the UVL tap should be placed

above the UVH tap as in Figure 7a. To reduce the UV hys-

teresis, place the UVL tap under the UVH tap as in Figure

7b. UV hysteresis referred to the UVL pin is given by:

for VUVL ≥

UVH

UVL HYST

UV HYST

.•

()

∆∆

256

UVH

UVL HYST

or for VUVL

()

(

∆∆

HYST

) –.

•

256

For VUVL < VUVH, the minimum UV hysteresis allowed is

the minimum hysteresis at UVH and UVL:

δVUV = 15mV

when RH(MAX) = 0.11 • (R1 + R2)

The design of the LTC4261/LTC4261-2 protects the UV

comparator from chattering even when RH is larger than

RH(MAX).

An undervoltage fault occurs when the UVL pin falls be-

low 2.291V and the UVH pin falls below 2.56V –

δVUV.

This activates the FET turn-off and sets the undervoltage

RTN – VEE

TMR

GATE

VOUT

SENSE

PGIO

36V

2.56V

VGATEH

LOAD

42612 F06

50mV

LOAD + INRUSH

FET VTH

72V

Figure 6. –36V to –72V Step Response

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