MAX5924 MAXIM [Maxim Integrated Products], MAX5924 Datasheet - Page 17

• Manufacturer Part Number: MAX5924
• Description: 1V to 13.2V, n-Channel Hot-Swap Controllers Require No Sense Resistor
• Manufacturer: MAXIM [Maxim Integrated Products]

Given:
Procedures:
1) Calculate the required slew rate and corresponding
2) Select a MOSFET and determine the worst-case
3) Minimize power dissipation at full load current and
4) Select R
R
DS ON
• V
• C
• Full-Load Current = 5A
• No R
• I
I
C
power dissipation.
at high temperature by selecting a MOSFET with an
appropriate R
difference between the MAX5924/MAX5925/
MAX5926 and the MOSFET.
For example, at room temperature the IRF7822’s
R
this device is 4000ppm/°C. The maximum R
for the MOSFET at T
The power dissipation in the MOSFET at full load is:
Since the MOSFET’s temperature coefficient is
4000ppm/°C, which is greater than TC
(3300ppm/°C), calculate the circuit-breaker thresh-
old at high temperature so the circuit breaker is
guaranteed not to trip at lower temperature during
normal operation (Figure 15).
TRIPSLOW
C
(
I
DS(ON)
SLEW
INRUSH
CB85
CC
SLEW
L
R
)
= 150µF
1V to 13.2V, n-Channel Hot-Swap Controllers
105
P
DS(ON)105
= V
SENSE
D
:
= 58µA x (1 + (3300ppm/°C x (85 - 25)°C)
= 69.5µA (min)
CB
=
=
=
=
= 6.5mΩ. The temperature coefficient for
S
= 500mA
6 5
8 58
I R
SR
= I
2
.
= 5V
330
.
.
m
FULL LOAD
=
=
DS(ON)
m
Ω
______________________________________________________________________________________
SR
(
1000
×
Ω
= 8.58mΩ (max), from step 2
5
×
A
I
INRUSH
10
)



2
J(MOSFET)
1 105
−
. Assume a 20°C temperature
+
×
9
×
(
=
+ 20% = 5A + 20% = 6A
C
8 58
L
.
330
° −
C
=
Design Procedure
m
3 3
3 3 .
= +105°C is:
.
Ω =
×
25
ms
10
V
°
ms
C
V
215
)
−
9
×
mW
=
4000
0 1
. µ
DS(ON)
ppm
F
°
C
Require No Sense Resistor
ICB



Keep all traces as short as possible and maximize the
high-current trace dimensions to reduce the effect of
undesirable parasitic inductance. Place the MAX5924/
MAX5925/MAX5926 close to the card’s connector. Use
a ground plane to minimize impedance and induc-
tance. Minimize the current-sense resistor trace length
(<10mm), and ensure accurate current sensing with
Kelvin connections.
When the output is short circuited, the voltage drop
across the external MOSFET becomes large. Hence, the
power dissipation across the switch increases, as does
the die temperature. An efficient way to achieve good
power dissipation on a surface-mount package is to lay
out two copper pads directly under the MOSFET pack-
age on both sides of the board. Connect the two pads
to the ground plane through vias, and use enlarged
copper mounting pads on the top side of the board.
It is important to maximize the thermal coupling between
the MOSFET and the MAX5925/MAX5926 to balance the
device junction temperatures. When the temperatures of
the two devices are equal, the circuit-breaker trip
threshold is most accurate. Keep the MOSFET and the
MAX5925/MAX5926 as close to each other as possible
to facilitate thermal coupling.
GND
V
V
CC
S
R
BACKPLANE
SEE FIGURE 2 FOR A DETAILED TYPICAL OPERATING CIRCUIT WITH R SENSE .
CB
R
CB
= ((6A x 8.58mΩ) + 4.7mV)/69.5µA = 808Ω
=
Typical Operating Circuits
(
I
TRIPSLOW
REMOVABLE CARD
TYPICAL OPERATION WITH R
1V TO V
2.25V TO 13.2V
CC
R
CB
V
GND
Layout Considerations
x R
CC
CB
DS ON
I
CB
R
SENSE
(
85
MAX5924
MAX5926
)
SENSE GATE
105
(continued)
SENSE
)
+
N
V
CB OS
,
OUT
V
OUT
17