Power-Good Output
The power-good output,
PGOOD
(PGOOD), is open-
drain and asserts when the external MOSFET is fully
enhanced and V
DS
is less than V
PG
(75% of the circuit
breaker threshold, V
CB
). For versions without the circuit
breaker function (MAX5900N/MAX5901N),
PGOOD
(PGOOD) asserts when the external MOSFET is fully
enhanced.
PGOOD
(PGOOD) deasserts within 2μs when a circuit
breaker event occurs or if the die temperature exceeds
+125
°
C.
PGOOD
(PGOOD) deasserts if |V
EE
| < |V
UVLO
|
for longer than 20ms or ON/
OFF
is held low for longer
than 20ms.
The MAX5900
PGOOD
is active-low and the MAX5901
PGOOD is active-high. Both are open-drain N-channel
MOSFETs with their sources connected to V
EE
, and can
withstand up to 100V.
Selecting a Circuit Breaker Threshold
The MAX5900A/MAX5901A and the MAX5900L/
MAX5901L offer a circuit breaker function to protect the
external MOSFET and the load from the potentially
damaging effects of excessive current. As load current
flows through the external MOSFET, a voltage, V
DS
, is
generated from drain to source due to the MOSFET
’
s
on-resistance, R
DS(ON)
. The MAX5900A/MAX5901A
and MAX5900L/MAX5901L monitor V
DS
when the exter-
nal MOSFET is fully enhanced. If V
DS
exceeds the cir-
cuit breaker threshold, the external MOSFET is turned
off and
PGOOD
(PGOOD) is deasserted.
To accommodate different MOSFETs and different load
currents, the MAX5900/MAX5901 are available with cir-
cuit breaker threshold voltages of 200mV, 300mV, and
400mV.
The circuit breaker function is intended to disconnect
the load if a gross overcurrent or short-circuit condition
occurs. For calculating the circuit breaker threshold use
the MOSFET
’
s R
ON
at the worst possible operating con-
dition, and add a 25% overcurrent margin to the maxi-
mum circuit current. For instance, if a MOSFET has an
R
ON
of 0.06
at T
A
= +25
°
C, and a normalized on-
resistance factor of 1.75 at T
A
= +130
°
C (from the
MOSFET data sheet), the R
ON
used for calculation is
the product of these two numbers, or (0.06
) x (1.75) =
0.105
. Then, if the maximum current is expected to be
2A, using a 25% margin, the current for calculation is
(2A) x (1.25) = 2.5A. The resulting minimum circuit
breaker threshold is then the product of these
two results, or (0.105
) x (2.5A) = 0.263V. The next
highest minimum available threshold is 0.265V of the
MAX590_ _BEUT, which is an ideal choice given these
parameters. Using this method to choose a circuit
breaker threshold allows the circuit to operate under
worst-case conditions without causing a circuit
breaker fault, but the circuit breaker function will still
operate if a short-circuit or gross overcurrent condition
occurs. See Table 1 for MOSFET suggestions. The
MAX5900N/MAX5901N have no circuit breaker func-
tion. For these parts choose an external MOSFET that
meets the load requirements.
Determining Inrush Current
Determining a circuit
’
s inrush current is necessary to
help choose the proper MOSFET. The MAX5900/
MAX5901 regulate the inrush current by means of con-
trolling the load voltage slew rate, but inrush current is
also a function of load capacitance. Determine inrush
current using:
where C is the load capacitance, and SR is the
MAX5900/MAX5901 Load Voltage Slew Rate Magnitude
from the
Electrical Characteristics
table. For example,
assuming a load capacitance of 100μF, and using the
typical value of 10V/ms for the slew rate, the inrush cur-
rent is 1A typical.
If the maximum possible Load Voltage Slew Rate is
used, the maximum inrush current calculates to 1.7A.
Choose a MOSFET with a maximum pulsed current
specification that exceeds the maximum inrush current.
I
CdV
dt
C
SR
=
=
×
M
-100V SOT23 Simple Swapper Hot-Swap
Controllers
8
_______________________________________________________________________________________
M A XIM U M
I
LOA D
( A
SU GG ESTED
EXTER N AL
M O SF ET
SU GG ESTED
M A XIM PA R T
0.25
0.5
1
2
3
4
IRFL110
IRFL4310
IRFR3910
IRF540NS
IRF1310NS
IRF1310NS
MAX590_ _CEUT
MAX590_ _BEUT
MAX590_ _CEUT
MAX590_ _BEUT
MAX590_ _BEUT
MAX590_ _CEUT
VIN= -9V to -90V
Suggested External MOSFETs