23
LTC4244/LTC4244-1
42441f
the PC board. For 1 ounce copper foil plating, a general rule
is 1 ampere of DC current per via making sure the via is
properly dimensioned so that solder completely fills the
void. For other plating thicknesses, check with your PCB
fabrication facility.
Design Example
As a design example, consider a CPCI Hot Swap applica-
tion with the following power supply requirements:
Table 4. Design Example Power Supply Requirements
VOLTAGE MAXIMUM DC LOAD
SUPPLY SUPPLY CURRENT CAPACITANCE
12V 450mA 100µF
5V 5A 2200µF
3.3V 7A 2200µF
–12V 100mA 100µF
The first step is to select the appropriate values of R
SENSE
for the 5V and 3.3V supplies. Calculating the value of
R
SENSE
is based on I
LOAD(MAX)
and the lower limit for the
circuit breaker threshold voltage (47mV for both the 5V
and 3.3V circuit breakers). If a 1% tolerance is assumed
for the sense resistors, then 5mΩ and 7mΩ resistor
values yield the following minimum and maximum I
TRIP
values:
Table 5. I
TRIP
vs R
SENSE
R
SENSE
(1% RTOL) I
TRIP(MIN)
I
TRIP(MAX)
5mΩ 9.3A 11.5A
7mΩ 6.6A 8.2A
So sense resistor values of 7mΩ and 5mΩ should suffice
for the 5V and 3.3V supplies, respectively.
The second step is to select MOSFETs for the 5V and 3.3V
supplies. The IRF7457’s on resistance is less than 10.5mΩ
for V
GS
> 4.5V and a junction temperature of 25°C. Since
the maximum load current requirement for the 3.3V sup-
ply is 7A, the steady-state power the device may be
required to dissipate is 514mW. The IRF7457 has a
junction-to-ambient thermal resistance of 50°C/Watt. If a
maximum ambient temperature of 50°C is assumed, this
yields a junction temperature of 75.7°C. According to the
IRF7457’s Normalized On-Resistance vs Junction Tem-
perature curve, the device’s on-resistance can be expected
to increase by about 20% over its room temperature value.
Recalculation of the steady-state values of R
ON
and junc-
tion temperature yields approximately 12.6mΩ and 81°C,
respectively. The I • R drop across the 3.3V sense resistor
and series MOSFET at maximum load current under these
conditions will be less than 124mV.
The next step is to select appropriate values for C1
and
C
TIMER
. Assuming that the total current for the 5V supply
is constrained to less than 6A during power-up (6 × 5V
medium length connector pins at 1A per pin), then the
inrush current shouldn’t exceed:
I
INRUSH
< 6A – I
LOAD(5VOUT)
= 6A – 5A = 1A (12)
This yields:
C
IF
I
C
AF
A
nF
GATE MAX
INRUSH MAX
1
2200
1
100 2200
1
220
>
µ
⇒>
µµ
=
()
()
•
•
(13)
Hence a C1 value of 330nF ±10% should suffice. The value
of C
TIMER
for this design example will be constrained by
the duration of the 12V supply inrush current, which
according to Equation 2 is:
t
CV
II
t
FV
mA mA
ms
ON VOUT
LOAD
LIMIT MIN LOAD MAX
ON VOUT
()
() ( )
()
••
–
••
–
12
12
212
2 100 12
550 450
24
<
⇒<
µ
=
(14)
In order to guarantee that the LTC4244’s TIMER fault
inhibit period is greater than 24ms, the value of C
TIMER
should be:
C
ms I
VV
C
ms A
VV
nF
TIMER
TIMER MAX
TIMER MAX
TIMER
>
⇒>
µ
=
24
12
24 26
12 1 9
61 8
•
–
•
–.
.
()
()
(15)
So a value of 82nF (±10%) should suffice.
APPLICATIO S I FOR ATIO
WUUU
