Datasheet
MAX5924/MAX5925/MAX5926
1V to 13.2V, n-Channel Hot-Swap Controllers
Require No Sense Resistor
14 ______________________________________________________________________________________
Undervoltage Lockout (UVLO)
UVLO circuitry prevents the MAX5924/MAX5925/
MAX5926 from turning on the external MOSFET until V
CC
exceeds the UVLO threshold, V
UVLO
, for t
D,UVLO
. UVLO
protects the external MOSFET from insufficient gate-drive
voltage, and t
D,UVLO
ensures that the board is fully
plugged into the backplane and V
CC
is stable prior to
powering the hot-swapped system. Any input voltage
transient at V
CC
below the UVLO threshold for more than
the UVLO deglitch period, t
DG
, resets the device and ini-
tiates a startup sequence. Device operation is protected
from momentary input-voltage steps extending below the
UVLO threshold for a deglitch period, t
DG.
However, the
power-good output(s) may momentarily deassert if the
magnitude of a negative step in V
CC
exceeds approxi-
mately 0.5V, and V
CC
drops below V
UVLO
. Operation is
unaffected and the power-good output(s) assert(s) within
200µs as shown in Figure 8b. This figure also shows that
if the UVLO condition exceeds t
DG
= 900µs (typ), the
power-good output(s) again deassert(s) and the load is
disconnected.
Determining Inrush Current
Determining a circuit’s inrush current is necessary to
choose a proper MOSFET. The MAX5924/MAX5925/
MAX5926 regulate the inrush current by controlling the
output-voltage slew rate, but inrush current is also a
function of load capacitance. Determine an anticipated
inrush current using the following equation:
where C
L
is the load capacitance in µF and SR is the
selected MAX5924/MAX5925/MAX5926 output slew rate
in V/ms. For example, assuming a load capacitance of
100µF and using the value of SR = 10V/ms, the anticipat-
ed inrush current is 1A. If a 16V/ms output slew rate is
used, the inrush current increases to 1.6A. Choose SR
so the maximum anticipated inrush current does not trip
the fast circuit-breaker comparator during startup.
Slew Rate
The MAX5924/MAX5925/MAX5926 limit the slew rate of
V
OUT
. Connect an external capacitor, C
SLEW
, between
SLEW and GND to adjust the slew-rate limit. Floating
SLEW sets the maximum slew rate to the minimum value.
Calculate C
SLEW
using the following equation:
C
SLEW
= 330 10
-9
/ SR
where, SR is the desired slew rate in V/ms and C
SLEW
is in nF.
This equation is valid for C
SLEW
≥ 100nF. For higher
SR, see the
Typical Operating Characteristics
.
A 2µA (typ) pullup current clamped to 1.4V causes an
initial jump in the gate voltage, V
GATE
, if C
GATE
is small
and the slew rate is slow (Figure 3). Figure 9 illustrates
how the addition of gate capacitance minimizes this ini-
tial jump. C
GATE
should not exceed 25nF.
IAC
dV
dt
CSR
INRUSH L
OUT
L
()
=
×
=×
1000
5V/div
V
GATE
0V
0V
V
S
= V
CC
= 13.2V
C
SLEW
= 1µF
C
L
= 10µF
10ms/div
MOSFET ONLY
MOSFET AND
C
GATE
= 20nF
Figure 9. Impact of C
GATE
on the V
GATE
Waveform
2V/div
1V/div
1V/div
200µs/div
GATE
PGOOD
V
CC
Figure 8b. PGOOD Behavior with Large Negative Input-Voltage
Step when V
S
is Near V
S(MIN)