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LTC4211CMS-PBF

Hot Swap Controller with Multifunction Current Control

Linear Technology 

LTC4211

APPLICATIONS INFORMATION

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

circuitry 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 volt-

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

the voltage on the FILTER pin is pulled higher than its

1.156V 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

operation, 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. FAULT is asserted

low (but not latched) to indicate a start-up failure. The

start-up sequence only resumes with the second timing

cycle at Time Point 5 when the input overvoltage condition

is removed. At this point in time, the GATE pin voltage is

allowed to ramp up, FAULT is pulled to logic high and the

circuit breaker is armed. At any time after Time Point 5,

should a supply overvoltage condition develop (VFILTER >

1.236V), the electronic circuit breaker will trip, the GATE

will be pulled low to turn off the external MOSFET and

FAULT will be asserted low and latched. This sequence is

shown in detail at Time Point B.

5V

FAULT

RESET

ON/OFF

BACKPLANE PCB EDGE

CONNECTOR CONNECTOR

(FEMALE) (MALE)

LONG

SHORT

SHORT

SHORT

R4

10k

Z1*

R3

10Ω

C1

0.1μF

R5 Z2

10k 6.2V

R6 R7

10k 10k

CFILTER

47pF

LONG

GND

LTC4211

1 RESET FAULT 10

2 ON

VCC 9

3 FILTER SENSE 8

4 TIMER GATE 7

CTIMER

10nF

5

GND

FB 6

RSENSE

0.007Ω

Q1

Si4410DY

R1

36k +

R2

15k

VOUT

5V

5A

CLOAD

4211 F23

Z1 = 1SMA10A OR SMAJ10A

Z2 = 1N4691

* OPTIONAL

Figure 23. LTC4211MS High Side Overvoltage Protection Implementation

4211fb

31

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