LTC4371 datasheet(13/22 Pages) LINER

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LTC4371

4371f

For more information www.linear.com/LTC4371

applicaTions inForMaTion

Figure 11. Fuse and Open MOSFET Detection

Figure 12. Back-to-Back Drain Pin Limiter for ±600V

Fuse and Open MOSFET Detection

The LTC4371 monitors ∆VSDofeachchannelasmeasured

across SA – DA and SB – DB. If ∆VSD of either channel

exceeds 200mV and the associated gate pin is driven fully

on, FAULTB pulls low to indicate a fault. Conditions lead-

ing to high ∆VSD include excessive load current (ILOAD ×

RDS(ON) > 200mV), an open circuit MOSFET or an open

fuse placed in series with the MOSFET. A high ∆VSD fault

is detected on only the highest voltage input supply, i.e.

the path that should be supplying power is, as a result of

one of the aforementioned conditions, unable to do so.

Temporary conditions, such as the initial 700mV drop

experienced when an input first rises to the point of sup-

plying current but before the gate has been driven on, are

maskedsincethegatemustalsobehighforfaultdetection.

The ∆VSD monitor can be used to detect open fuses, as

shown in Figure 11. An open fuse gives the same signa-

ture as an open MOSFET: ∆VSD increases beyond 200mV

when the affected input surpasses the opposing channel.

The connection shown in Figure 11 introduces a new

problem: an open fuse and open MOSFET exposes the DA

and DB pins to high negative voltage with respect to VSS.

Diodes D1 and D2 clamp the DA, DB pins from exceeding

the absolute maximum of –40V with respect to VSS.

Figure 12 shows a protection method that extends DA and

DB pin operation to ±600V. The drain pins are clamped

by an 82V Zener diode. As shown, the DA pin is clamped

at 82V with respect to VSS in the positive direction, and

700mV below VSS in the negative direction. When a high

input voltage of either polarity is present, back-to-back

depletion mode N-channel MOSFETs limit the current in

the Zener diode to VGS(TH)/RDA (100μA for RDA = 20kΩ),

a value that is indefinitely sustainable.

FAULTB Pin

The open drain FAULTB pin pulls low when the ∆VSD of

either channel exceeds 200mV, while its gate is driven

fully on. FAULTB can sink 5mA to drive an LED for visual

indication, or an opto isolator to communicate across an

isolation barrier. The FAULTB pin voltage is limited to 17V

absolute maximum with respect to VSS in the high state

and cannot be pulled up to return except in low voltage

applications.

In Figure 13, the FAULTB pin is used to shunt current away

from a green LED; the LED indicates (illuminates when) no

fault condition is present. The operating voltage is limited

at the low end by the minimum acceptable LED current,

and at the high end by the FAULTB pin’s 5mA capability.

Figure 14 shows a simple implementation driving a red

LED; the LED indicates a fault condition is present. While

this simple configuration works well in –48V applications,

themaximumoperatingvoltageislimitedto100V,theLED

4371 F11

LTC4371

GA GB

SA SB VSS

RDA

20k

RDB

20k

–36V TO

–72V

–36V TO

–72V

VOUT

1N4148W

4371 F12

LTC4371

VSS

M2*

M3*

VOUT

RDA

20k

82V

*M2, M3: BSP135 (600V) DEPLETION NMOS

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