LTC4222 Linear Technology, LTC4222 Datasheet - Page 20

• Manufacturer Part Number: LTC4222
• Description: Dual Hot Swap Controller
• Manufacturer: Linear Technology

www.DataSheet4U.com
LTC4222
APPLICATIONS INFORMATION
Selection of the sense resistor, R
threshold of 50mV:
The MOSFET is sized to handle the power dissipation dur-
ing inrush when output capacitor C
A method to determine power dissipation during inrush
is based on the principle that:
This uses:
or 0.024 joules. Calculate the time it takes to charge up
C
The power dissipated in the MOSFET:
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:
20
OUT
Energy in C
R
Energy in C
P
C
C
t
STARTUP
DISS
1=
1
S
:
=
=
C
0 33
50
=
I
I
L
.
INRUSH
MAX
Energy in C
mV
•
I
mF
GATE
=
t
STARTUP
1
L
L
C
A
=
= Energy in Q1
•
L
=
0 01
12
•
. Ω
2
1
I
V I
INRUSH
μA
DD INRUSH
CV
L
or
2
=
=
6
C
2
1
W
1 3 9
=
( .
S
0 33
=
, is set by the overcurrent
.
0 33
nF
OUT
.
mF
mF
)( )
is being charged.
1
A
12
•
12
2
V
=
4 4 ms
The inrush dI/dt is set to 10A/ms using C
For a start-up time of 4ms with a 2x safety margin we
choose:
Note the minimum value of C
The UV and OV resistor string values can be solved in the
following method. First pick R3 based on I
1.235V/R3 at the edge of the OV rising threshold. Then
solve the following equations:
In our case we choose R3 to be 3.4k to give a resistor
string current below 100μA.
Then solving the equations results in R2 = 1.16 kΩ and
R1 = 34.6kΩ.
The FB divider is solved by picking R8 and solving for R7,
choosing 3.57k for R8 we get:
Resulting in R7 = 30k
A 0.1μF capacitor, C
supply glitches from turning off the GATE via UV or OV.
C
C
C
R
R
R
=
SS
2
1
7
TIMER
TIMER
10
=
=
=
10000 0 01
=
μA
V
V
V
V
UV ON
dl/dt
OV OFF
PWRGD UP
UV ON
FB
=
=
UV
• .
(
0 0 0 429 1
(
(
12 2 .3ms/μF
I
2
TH RISING
• .
12.3ms/μF
SS
TH RISING
•
⎛
⎝ ⎜
)
(
)
t
(
8
)
•(
A
(
s
STARTUP
ms
•
⎞
⎠ ⎟
R
R
Ω
)
F
3
•
• .
3
, is placed on the UV pins to prevent
•
0 0429
+
R
•
)
=
)
R
OV
8
UV
+
2
4 3
–
+
)
.
TH FALL
4 7
TH RISING
R
–
C
.
nF choose 4.7nF
(
8
R
•
SS
(
TIMER
nF
R
3
SENSE
•
–
•
2
I I NG
1
R
2 0 68
is 10nF .
2
=
)
)
– 3
.
R
SS
μ
:
F
STRING
being
4222f