LTC4371
13
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 ∆V
SD
of each channel as measured
across SA – DA and SB – DB. If ∆V
SD
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 ∆V
SD
include excessive load current (I
LOAD
×
R
DS(ON)
> 200mV), an open circuit MOSFET or an open
fuse placed in series with the MOSFET. A high ∆V
SD
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
masked since the gate must also be high for fault detection.
The ∆V
SD
monitor can be used to detect open fuses, as
shown in Figure11. An open fuse gives the same signa-
ture as an open MOSFET: ∆
V
SD
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 V
SS
.
Diodes D1 and D2 clamp the DA, DB pins from exceeding
the absolute maximum of –40V with respect to V
SS
.
Figure12 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 V
SS
in the positive direction, and
700mV below V
SS
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 V
GS(TH)
/R
DA
(100μA for R
DA
= 20kΩ),
a value that is indefinitely sustainable.
FAULTB Pin
The open drain FAULTB pin pulls low when the ∆V
SD
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 V
SS
in the high state
and cannot be pulled up to return except in low voltage
applications.
In Figure13, 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.
Figure14 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,
the maximum operating voltage is limited to 100V, the LED
4371 F11
LTC4371
DA DB GA GB SA SB V
SS
R
DA
20k
R
DB
20k
M1
M2
V
A
–36V TO
–72V
V
B
–36V TO
–72V
V
OUT
D1
1N4148W
D2
1N4148W
F1
F2
4371 F12
LTC4371
DA GA SA V
SS
M2*
M3*
M1
V
A
V
OUT
R
DA
20k
82V
*M2, M3: BSP135 (600V) DEPLETION NMOS
