Linear Technology LT4363 Datasheet page 16

LT4363
applicaTions inForMaTion
Q2
SI7461DP
V
IN
12V
D2
D1*
1N5245
SMAJ58CA
15V
R7
C1
10k
47nF
5
V
CC
6
SHDN
8
UV
7
OV
GND
*DIODES INC.
Figure 7. Overvoltage Regulator with P-channel MOSFET
Reverse Input Protection
Shutdown
The LT4363 can be shut down to a low current mode when
the voltage at the SHDN pin is pulled below the shutdown
threshold of 0.4V. The quiescent current drops down to
7µA with internal circuitry turned off.
The SHDN pin can be pulled up to 100V or below GND by
up to 60V without damage. Leaving the pin open allows
an internal current source to pull it up and turn on the part
while clamping the pin to 2.2V. The leakage current at the
pin should be limited to no more than 1µA if no pull up
device is used to help turn it on.
Supply Transient Protection
The LT4363 is tested to operate to 80V and guaranteed to
be safe from damage up to 100V. Nevertheless, voltage
transients above 100V may cause permanent damage.
During a short-circuit condition, the large change in cur-
rent flowing through power supply traces and associated
wiring can cause inductive voltage transients which could
exceed 100V. To minimize the voltage transients, the power
trace parasitic inductance should be minimized by using
wide traces. A small RC filter, in Figure 8, at the V
will clamp the voltage spikes.
16
R
Q1
SNS
10m
IRLR2908
V
12V, 3A
CLAMPED
AT 16V
R3
10
R1
57.6k
4 3 2
GATE SNS OUT
1
FB
R2
4.99k
LT4363DE-2
11
ENOUT
10
FLT
TMR
9 12 4363 F07
C
TMR
0.1µF
CC
For more information
OUT
V
IN
R7
1k
C2
0.1µF
D1*
SMAJ58A
R4
374k
R5
90.9k
R6
10k
Figure 8. Overvoltage Regulator with Input Voltage Detection
Another way to limit transients above 100V at the V
pin is to use a Zener diode and a resistor, D1 and R7 in
Figure 8. The Zener diode limits the voltage at the pin while
the resistor limits the current through the diode to a safe
level during the surge. However, D1 can be omitted if the
filtered voltage, due to R7 and C1, at the V
100V. The inclusion of R7 in series with the V
increase the minimum required voltage at V
extra voltage drop across it. This voltage drop is due to the
supply current of the LT4363 and the leakage current of D1.
A total bulk capacitance of at least 22µF low ESR electro-
lytic is required close to the source pin of MOSFET Q1. In
addition, the bulk capacitance should be at least 10 times
larger than the total ceramic bypassing capacitor on the
input of the DC/DC converter.
Layout Considerations
To achieve accurate current sensing, Kelvin connection
to the current sense resistor (R
mended. The minimum trace width for 1 oz copper foil is
0.02" per amp to ensure the trace stays at a reasonable
temperature. 0.03" per amp or wider is recommended.
pin
Note that 1oz copper exhibits a sheet resistance of about
530µΩ/square. Small resistances can cause large errors in
high current applications. Noise immunity will be improved
www.linear.com/LT4363
R
Q1
SNS
10m
FDB33N25
C1
R3
47nF
10
R1
5 4 3 2
100k
V GATE SNS OUT
CC
1
FB
R2
4.99k
6
SHDN
LT4363DE-2
8
UV
11
ENOUT
7 10
OV FLT
GND TMR
9 12 4363 F08
C
TMR
*DIODES INC.
47nF
**SANYO 25CE22GA
in Figure 8) is recom-
SNS
V
OUT
C **
L
22µF
V
CC
DC/DC
CONVERTER
SHDN
FAULT
GND
CC
pin is below
CC
pin will
CC
due to the
IN
4363fb