LTC4244-1 Linear Technology, LTC4244-1 Datasheet - Page 24

• Manufacturer Part Number: LTC4244-1
• Description: CompactPCI Bus Hot Swap Controllers
• Manufacturer: Linear Technology

w w w . D a t a S h e e t 4 U . c o m
APPLICATIO S I FOR ATIO
LTC4244/LTC4244-1
The next step is to verify that the thermal ratings of the
external 5V and 3.3V MOSFETs aren’t being exceeded
during power-up cycles into the designed loads or into a
short circuit.
The amount of heating in the 5V and 3.3V MOSFETs during
a normal power cycle depends on the LTC4244’s GATE pin
current (refer to Gate Current vs Temperature plot in the
Typical Performance Characteristics section). The magni-
tude of the off-on power pulse that results in maximum
heating of the MOSFETs is given by Equation 10 as:
where
The duration of the power-pulse is given by Equation 11
as:
Solving these equations for the 5V and 3.3V supplies
yields:
Table 6
Under these conditions, the IRF7457 datasheet’s Thermal
Response vs Pulse Duration curve indicates that the
junction-to-ambient temperature will increase by 60 C for
the 5V MOSFET and 46 C for the 3.3V MOSFET.
24
P
I
t
INRUSH MIN
INRUSH
OFF-ON
3.3V MOSFET
5V MOSFET
(
n
C
)
I
LOAD
V
INRUSH MIN
OUT
C MAX
U
1
C
•
(
•
LOAD
n
(
I
INRUSH MIN
V
OUT
P
U
12.8W
11.8W
OFF-ON
)
)
•
I
GATE MIN
(
2
)
(
W
I
LOAD VOUT
)
t
INRUSH(MAX)
(
n
90ms
60ms
U
)
(16)
(17)
(18)
The duration and magnitude of the power pulse that
results during a short-circuit condition on either the 5V or
3.3V outputs are a function of the TIMER capacitor and the
LTC4244’s foldback current limit. Figure 16 shows the
worst-case power dissipated in the 5V and 3.3V external
FETs vs V
the 3.3V external MOSFET, the maximum dissipated power
is 24 Watts (V
the maximum dissipated power is 22 Watts (V
1.75V). The maximum duration of the short-circuit power-
pulse is given by Equation 19 as:
t
t
t
PULSE
PULSE
PULSE
Figure 16. Worst-Case 5V and 3.3V MOSFET
Dissipated Power vs Output Voltage
5VOUT
25
20
15
10
5
0
0
3.3VOUT
C
60 3
and V
82
TIMER MAX
.
1
nF
3.3V MOSFET
ms
= 0.9V). For the 5V external MOSFET,
OUTPUT VOLTAGE (V)
3.3VOUT
(
8 2
2
.
)
nF
16
5V R
3.3V R
•
, respectively. In the case of
12
3
SENSE
•
5V MOSFET
A
SENSE
V V
12
I
TIMER MIN
–
= 0.007
= 0.005
V
4
TIMER MIN
– .
4244 F16
(
1 3
5
V
(
)
)
5VOUT
(19)
42441f
=