Datasheet
LT4363
16
4363fa
APPLICATIONS INFORMATION
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
CC
pin
will clamp the voltage spikes.
Another way to limit transients above 100V at the V
CC
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
CC
pin
is below 100V. The inclusion of R7 in series with the V
CC
pin will increase the minimum required voltage at V
IN
due
to the 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
SNS
in Figure 8) is recom-
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.
Note that 1oz copper exhibits a sheet resistance of about
530µΩ/square. Small resistances can cause large errors in
Figure 8. Overvoltage Regulator with Input Voltage Detection
C
TMR
47nF
R
SNS
10mΩ
Q1
FDB33N25
V
IN
V
OUT
4363 F08
LT4363DE-2
GND TMR
9 12
OUT
2
SNS
3
FB
1
C2
0.1µF
D1*
SMAJ58A
R2
4.99k
R1
100k
R7
1k
GATE
4
V
CC
5
R3
10Ω
*DIODES INC.
**SANYO 25CE22GA
SHDN
6
UV
8
OV
7
DC/DC
CONVERTER
GND
SHDN
V
CC
FLT
ENOUT
10
11
C
L
**
22µF
FAULT
R5
90.9k
R6
10k
R4
374k
C1
47nF
C
TMR
0.1µF
R
SNS
10mΩ
Q1
IRLR2908
Q2
SI7461DP
V
IN
12V
V
OUT
12V, 3A
CLAMPED
AT 16V
4363 F07
LT4363DE-2
GND TMR
9 12
OUT
2
SNS
3
V
CC
5
FB
1
D1*
SMAJ58CA
R2
4.99k
R1
57.6k
GATE
4
R7
10k
D2
1N5245
15V
R3
10Ω
*DIODES INC.
SHDN
6
UV
8
OV
7
FLT
ENOUT
10
11
C1
47nF
Figure 7. Overvoltage Regulator with P-channel MOSFET
Reverse Input Protection