24
LTC4244/LTC4244-1
42441f
OUTPUT VOLTAGE (V)
0
15
20
25
4
4244 F16
10
5
0
1
2
3
5
5V MOSFET
5V R
SENSE
= 0.007&
3.3V R
SENSE
= 0.005&
3.3V MOSFET
Figure 16. Worst-Case 5V and 3.3V MOSFET
Dissipated Power vs Output Voltage
The next step is to verify that the thermal ratings of the
external 5V and 3.3V MOSFETs arent 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 LTC4244s 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:
P
V
I
I
OUT INRUSHMIN LOAD VOUT
OFF-ON
=
+
(
)
n
n
"
( )
(
)
2
(16)
where
I
C
C MAX
I
INRUSHMIN
LOAD
GATE MIN
( )
( )
(
)
"
=
1
(17)
The duration of the power-pulse is given by Equation 11
as:
t
C
V
I
INRUSH
LOAD
OUT
INRUSHMIN
<
"
( )
n
(18)
Solving these equations for the 5V and 3.3V supplies
yields:
Table 6
P
OFF-ON
t
INRUSH(MAX)
5V MOSFET
12.8W
90ms
3.3V MOSFET
11.8W
60ms
Under these conditions, the IRF7457 datasheets Thermal
Response vs Pulse Duration curve indicates that the
junction-to-ambient temperature will increase by 60癈 for
the 5V MOSFET and 46癈 for the 3.3V MOSFET.
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
LTC4244s foldback current limit. Figure 16 shows the
worst-case power dissipated in the 5V and 3.3V external
FETs vs V
5VOUT
and V
3.3VOUT
, respectively. In the case of
the 3.3V external MOSFET, the maximum dissipated power
is 24 Watts (V
3.3VOUT
= 0.9V). For the 5V external MOSFET,
the maximum dissipated power is 22 Watts (V
5VOUT
=
1.75V). The maximum duration of the short-circuit power-
pulse is given by Equation 19 as:
t
C
V V
I
t
nF
nF
V
V
A
t
ms
PULSE
TIMER MAX
TIMER MIN
TIMER MIN
PULSE
PULSE
<
?/DIV>
<
+
(
) (
)
?/DIV>
?/DIV>
<
(
)
( )
( )
"
.
"
.
.
12
82
8 2
12 1 3
16
60 3
(19)
APPLICATIO S I FOR ATIO
U
U
U
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