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LTC4261/LTC4261-2

42612fd

For more information www.linear.com/LTC4261

overvoltage auto-retry has been disabled by clearing reg-

ister bit D0.

Undervoltage Comparator and Under

voltage 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, V

UVH(TH)

= 2.56V, turn on

For UVL falling, V

UVL(TH)

= 2.291V, turn off

Both UVH and UVL pins have a minimum hysteresis of

(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:

UV(HYST)

= V

UVH(TH)

– V

UVL(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 R

(Figure 8). To

increase the UV hysteresis, the UVL tap should be placed

above the UVH tap as in Figure 8a. To reduce the UV hys

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

8b. UV hysteresis referred to the UVL pin is given by:

for V

UVL

≥ V

UVH

UVL(HYST)

= DV

UV(HYST)

+ 2.56V •

R1+ R2

or for V

UVL

< V

UVH

UVL(HYST)

= DV

UV(HYST)

– 2.56V •

R1+ R2 + R

For V

UVL

< V

UVH

, the minimum UV hysteresis allowed is

the minimum hysteresis at UVH and UVL: dV

= 15mV

when R

H(MAX)

= 0.11 • (R1 + R2)

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

comparator from chattering even when R

is larger than

H(MAX)

An undervoltage fault occurs when the UVL pin falls

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

This activates the FET turn-off and sets the undervoltage

Figure 7. –36V to –72V Step Response

APPLICATIONS INFORMATION

RTN – V

TMR

GATE

OUT

SENSE

PGIO

36V

2.56V

GATEH

LOAD LOAD

42612 F07

50mV

LOAD + INRUSH

FET V

72V

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