Datasheet

LTC4218
13
4218ff
APPLICATIONS INFORMATION
In this same fi gure the OV threshold is lowered from
15.05V to 13.5V. Decreasing the OV threshold requires
adding a resistor between V
DD
and OV. This resistor can
be calculated as follows:
R
SHUNT2
=
R
IN
()
•V
OLD
V
TH
()
V
NEW
–V
OV TH
()
()
V
OLD
–V
NEW
()
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
=
18k •15.05
1.235
13.5–1.235
()
15.05–13.5
()
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
= 1.736M
Use the equation for R
SHUNT1
for increasing the OV and
FB thresholds. Likewise, use the equation for R
SHUNT2
for
decreasing the UV and FB thresholds.
Design Example
Consider the following design example (Figure 5): V
IN
=
12V, I
MAX
= 7.5A. I
INRUSH
= 1A, C
L
= 330μF, V
UVON
= 9.88V,
V
OVOFF
= 15.05V, V
PWRGD
= 10.5V. A current limit fault trig-
gers an automatic restart of the power up sequence.
The selection of the sense resistor, (R
S
), is set by the
overcurrent threshold of 15mV:
R
S
= 15mV/I
MAX
= 15mV/7.5A = 0.002Ω
The MOSFET should be sized to handle the power dissi-
pation during the inrush charging of the output capacitor
C
OUT
. The method used to determine the power in Q1 is
the principal:
E
C
= Energy in C
L
= Energy in Q1
Thus:
E
C
= ½ CV
2
= ½ (330μF)(12)
2
= 0.024J
Calculate the time it takes to charge up C
OUT
:
t
CHARGUP
=
C
L
•V
IN
I
INRUSH
=
330μF • 12V
1A
= 4ms
The inrush current is set to 1A using C
GATE
:
C
GATE
= C
L
I
GATE(UP)
I
INRUSH
= 330μF
24μA
1A
≅ 0.01μF
The average power dissipated in the MOSFET:
P
DISS
= E
C
/t
CHARGUP
= 0.024J/4ms = 6W
The SOA (safe operating area) curves of candidate MOS-
FETs must be evaluated to ensure that the heat capacity
of the package can stand 6W for 4ms. The SOA curves of
the Vishay Siliconix Si7108DN provide 1.5A at 10V (15W)
for 100ms, satisfying the requirement.
Figure 5. 6A, 12V Card Resident Application
12V
C
T
0.1μF
ADC
C1
0.1μF
R3
20k
4218 F05
R2
10k
C
L
330μF
SENSE
–
GATE
SENSE
+
V
DD
UV
SOURCE
PG
GND
I
MON
LTC4218DHC-12
INTV
CC
TIMER
FLT
R1
10Ω
R
S
2mΩ
Q1
Si7108DN
12V
+
V
OUT
12V
6A
R
GATE
1k
C
GATE
0.01μF