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 external
MOSFET is fully enhanced. If V
DS
exceeds the circuit-
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
condition, and add a 25% overcurrent margin to the
maximum circuit current. For instance, if a MOSFET has
an R
ON
of 0.06Ω 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 (0.06Ω) x (1.75) =
0.105Ω. 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 (0.105Ω) 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 function. 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 current 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.
MAX5900/MAX5901
-100V, SOT23/TDFN, Simple Swapper
Hot-Swap Controllers
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