LTC4211CS8 datasheet(30/36 Pages) LINER

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LTC4211

4211f

SUPPLY OVERVOLTAGE DETECTION/

PROTECTION USING FILTER PIN

In addition to using external protection devices around the

LTC4211 for large scale transient protection, low power

Zener diodes can be used with the LTC4211’s FILTER pin

to act as a supply overvoltage detection/protection circuit

on either the high side (input) or low side (output) of the

external pass transistor. Recall that internal control cir-

cuitry keeps the LTC4211 GATE voltage from ramping up

if VFILTER > 1.156V, or when an external fault condition

(VFILTER > 1.236V) causes FAULT to be asserted low.

High Side (Input) Overvoltage Protection

As shown in Figure 23, a low power Zener diode can be

used to sense an overvoltage condition on the input

(high) side of the main 5V supply. In this example, a low

bias current 1N4691 Zener diode is chosen to protect the

system. Here, the Zener diode is connected from VCC to

the LTC4211’s FILTER pin (Pin 3 MS). If the input voltage

to the system is greater than 6.8V during start-up, the

voltage on the FILTER pin is pulled higher than its 1.156V

APPLICATIO S I FOR ATIO

threshold. As a result, the GATE pin is not allowed to ramp

and the second timing cycle will not commence until the

supply overvoltage condition is removed. Should the

supply overvoltage condition occur during normal op-

eration, internal control logic would trip the electronic

circuit breaker and the GATE would be pulled to ground,

shutting OFF the external pass transistor. If a lower

supply overvoltage threshold is desired, use a Zener

diode with a smaller breakdown voltage.

A timing diagram for illustrating LTC4211 operation under

a high side overvoltage condition is shown in Figure 24.

The start-up sequence in this case (between Time Points

1 and 2) is identical to any other start-up sequence under

normal operating conditions. At Time Point 2A, the input

supply voltage causes the Zener diode to conduct thereby

forcing VFILTER > 1.156V. At Time Point 3, FAULT is

asserted low and the TIMER pin voltage ramps down. At

Time Point 4, the LTC4211 checks if VFILTER < 1.156V.

Figure 23. LTC4211MS High Side Overvoltage Protection Implementation

FAULT

VCC

SENSE

GATE

RESET

FILTER

TIMER

GND

LTC4211

Z1 = 1SMA10A OR SMAJ10A

Z2 = 1N4691

* OPTIONAL

Si4410DY

36k

VOUT

15k

CLOAD

4211 F23

RSENSE

0.007

Ω

CTIMER

10nF

CFILTER

47pF

6.2V

LONG

GND

FAULT

LONG

PCB EDGE

CONNECTOR

(MALE)

BACKPLANE

CONNECTOR

(FEMALE)

SHORT

RESET

ON/OFF

Ω

Z1*

0.1

µF

10k

LTC4211CS8 Datasheet(PDF) 30 Page - Linear Technology

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