Datasheet

MAX5924/MAX5925/MAX5926
1V to 13.2V, n-Channel Hot-Swap Controllers
Require No Sense Resistor
18 ______________________________________________________________________________________
Optional Sense Resistor
Select the sense resistor in conjunction with R
CB
to set
the slow and fast circuit-breaker thresholds (see the
Selecting a Circuit-Breaker Threshold section). The
sense-resistor power dissipation depends on the device
configuration. If latched mode is selected, P
RSENSE
=
(I
OVERLOAD
)
2
x R
SENSE
; if autoretry is selected, then
P
RSENSE
= (I
OVERLOAD
)
2
x R
SENSE
x (t
ON
/t
RETRY
).
Choose a sense-resistor power rating of twice the
P
RSENSE
for long-term reliable operation. In addition,
ensure that the sense resistor has an adequate I
2
T rating
to survive instantaneous short-circuit conditions.
No-Load Operation
The internal circuitry is capable of sourcing a current at
the OUT terminal of up to 120µA from a voltage V
IN
+
V
GS
. If there is no load on the circuit, the output capacitor
will charge to a voltage above V
IN
until the external MOS-
FET’s body diode conducts to clamp the capacitor volt-
age at VIN plus the body-diode V
F
. When testing or
operating with no load, it is therefore recommended that
the output capacitor be paralleled with a resistor of value:
R = V
X
/ 120µA
where V
X
is the maximum acceptable output voltage
prior to hot-swap completion.
Design Procedure
Given:
• V
CC
= V
S
= 5V
• C
L
= 150µF
• Full-Load Current = 5A
• No R
SENSE
• I
INRUSH
= 500mA
Procedures:
1) Calculate the required slew rate and corresponding
C
SLEW
:
2) Select a MOSFET and determine the worst-case
power dissipation.
3) Minimize power dissipation at full load current and
at high temperature by selecting a MOSFET with an
appropriate R
DS(ON)
. Assume a 20°C temperature
difference between the MAX5924/MAX5925/
MAX5926 and the MOSFET.
For example, at room temperature the IRF7822’s
R
DS(ON)
= 6.5mΩ. The temperature coefficient for
this device is 4000ppm/°C. The maximum R
DS(ON)
for the MOSFET at T
J(MOSFET)
= +105°C is:
The power dissipation in the MOSFET at full load is:
4) Select R
CB
.
Since the MOSFET’s temperature coefficient is
4000ppm/°C, which is greater than TC
ICB
(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).
I
TRIPSLOW
= I
FULL LOAD
+ 20% = 5A + 20% = 6A
R
DS(ON)105
= 8.58mΩ (max), from step 2
I
CB85
= 58µA x (1 + (3300ppm/°C x (85 - 25)°C)
= 69.5µA (min)
R
CB
= ((6A x 8.58mΩ) + 4.7mV)/69.5µA = 808Ω
R
IxR V
I
CB
TRIPSLOW
DS ON CB OS
CB
=
()
+
() ,105
85
PIR A m mW
D
== × Ω=
22
5 8 58 215() .
RmCC
ppm
C
m
DS ON()
. ( )
.
105
6 5 1 105 25 4000
858
=×+°−°×
°
⎛
⎝
⎜
⎞
⎠
⎟
=Ω
Ω
C
SR
F
SLEW
V
ms
=
×
=
×
=
−−
330 10 330 10
33
01
99
.
. µ
SR
I
C
V
ms
INRUSH
L
=
×
=
1000
33 .
Table 4. Component Manufacturers
COMPONENT MANUFACTURER PHONE WEBSITE
Dale-Vishay 402-564-3131 www.vishay.com
Sense Resistors
IRC 828-264-8861 www.irctt.com
Fairchild 888-522-5372 www.fairchildsemi.com
MOSFETs
International Rectifier 310-233-3331 www.irf.com