Datasheet

LTC4215-1/LTC4215-3
18
421513fc
large changes in current fl owing through power supply
traces may cause inductive voltage spikes which exceed
24V. To minimize such spikes, the power trace inductance
should be minimized by using wider traces or heavier
trace plating. Also, a snubber circuit dampens inductive
voltage spikes. Build a snubber by using a 100Ω resistor
in series with a 0.1µF capacitor between V
DD
and GND.
A surge suppressor, Z1 in Figure 1, at the input can also
prevent damage from voltage surges.
Design Example
As a design example, take the following specifi cations:
V
IN
= 12V, I
MAX
= 5A, I
INRUSH
= 1A, dI/dt
INRUSH
= 10A/ms,
C
L
= 330µF, V
UV(ON)
= 10.75V, V
OV(OFF)
= 14.0V, V
PWRGD(UP)
= 11.6V, and I
2
C ADDRESS = 1001011. This completed
design is shown in Figure 1.
Selection of the sense resistor, R
S
, is set by the overcurrent
threshold of 25mV:
R
S
=
25mV
I
MAX
= 0.005
The MOSFET is sized to handle the power dissipation dur-
ing inrush when output capacitor C
L
is being charged. A
method to determine power dissipation during inrush is
based on the principle that:
Energy in C
L
= Energy in Q1
This uses:
Energy in C
L
=
1
2
CV
2
=
1
2
0.33mF
()
12
()
2
or 0.024 Joules. Calculate the time it takes to charge up
C
OUT
:
t
STARTUP
= C
L
•
V
DD
I
INRUSH
= 0.33mF •
12V
1A
= 4ms
The power dissipated in the MOSFET:
P
DISS
=
Energyin C
L
t
STARTUP
= 6W
The SOA (safe operating area) curves of candidate MOSFETs
must be evaluated to ensure that the heat capacity of the
package tolerates 6W for 4ms. The SOA curves of the
Fairchild FDC653N provide for 2A at 12V (24W) for 10ms,
satisfying this requirement. Since the FDC653N has less
than 8nF of gate capacitance and we are using a GATE
RC network, the short circuit stability of the current limit
should be checked and improved by adding a capacitor
from GATE to SOURCE if needed.
The inrush current is set to 1A using C1:
C1= C
L
•
I
GATE
I
INRUSH
C1= 0.33mF •
20µA
1A
or C1= 6.8nF
The inrush dI/dt is set to 10A/ms using C
SS
:
C
SS
=
I
SS
dI / dt
A
s






• 0.0375 •
1
R
SENSE
=
10µA
10000
• 0.0375 •
1
5m
= 7.5nF
For a start-up time of 4ms with a 2x safety margin we
choose:
C
TIMER
= 2•
t
STARTUP
12.3ms/µF
+ C
SS
•10
C
TIMER
=
8ms
12.3ms/µF
+ 7.5nF • 10  0.68µF
Note the minimum value of C
TIMER
is 10nF, and each 1nF
of soft-start capacitance needs 10nF of TIMER capaci-
tance/time during start-up.
The UV and OV resistor string values can be solved in the
following method. First pick R3 based on I
STRING
being
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