LTC4211
31
4211fc
For more information www.linear.com/LTC4211
APPLICATIONS INFORMATION
1
2
3
4
5
10
9
8
7
6
FAULT
V
CC
SENSE
GATE
FB
RESET
ON
FILTER
TIMER
GND
LTC4211
Z1 = 1SMA10A OR SMAJ10A
Z2 = 1N4691
* OPTIONAL
Q1
Si4410DY
R1
36k
V
OUT
5V
5A
R2
15k
C
LOAD
4211 F23
R
SENSE
0.007Ω
+
C
TIMER
10nF
C
FILTER
47pF
Z2
6.2V
LONG
5V
GND
FAULT
LONG
PCB EDGE
CONNECTOR
(MALE)
BACKPLANE
CONNECTOR
(FEMALE)
SHORT
SHORT
SHORT
RESET
ON/OFF
R3
10Ω
Z1*
C1
0.1µF
R5
10k
R6
10k
R7
10k
R4
10k
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 V
FILTER
> 1.156V, or when an external fault condition
(V
FILTER
> 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 V
CC
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 FIL
TER 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 sup
-
ply 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
V
FILTER
> 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 V
FILTER
< 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 (V
FILTER
>
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.
Figure 23. LTC4211MS High Side Overvoltage Protection Implementation
