a switch-on/off function. When the programmed over-
source from the load. When IN decreases below the
load to the power source.
MAX6495鈥?/div>
MAX6499
GND
Overvoltage Limiter
(MAX6495/MAX6496/MAX6499)
When operating in overvoltage-limiter mode, the
MAX6495/MAX6496/MAX6499 feedback path (Figure 2)
consists of OUTFB, OVSET鈥檚 internal comparator, the
internal gate charge pump, and the external n-channel
MOSFET, resulting in the external MOSFET operating
as a voltage regulator.
During normal operation, GATE is enhanced 10V above
OUTFB. The external MOSFET source voltage is moni-
tored through a resistive divider between OUTFB and
OVSET. When OUTFB rises above the adjusted over-
voltage threshold, an internal comparator sinks the
charge-pump current, discharging the external GATE,
regulating OUTFB at the OVSET overvoltage threshold.
OUTFB remains active during the overvoltage transients
and the MOSFET continues to conduct during the over-
voltage event, operating in switched-linear mode.
As the transient begins decreasing, OUTFB fall time will
depend on the MOSFET鈥檚 GATE charge, the internal
charge-pump current, the output load, and the tank
capacitor at OUTFB.
For fast-rising transients and very large-sized MOSFETs,
add an additional bypass capacitor from GATE to GND to
reduce the effect of the fast-rising voltages at IN. The
external capacitor acts as a voltage-divider working
against the MOSFET鈥檚 drain-to-gate capacitance. For a
6000pF gate-to-source capacitance, a 0.1碌F capacitor at
GATE will reduce the impact of the fast-rising V
IN
input.
Caution must be exercised when operating the
MAX6495/MAX6496/MAX6499 in voltage-limiting mode
for long durations. If the V
IN
is a DC voltage greater than
the MOSFET鈥檚 maximum gate voltage, the MOSFET dis-
sipates power continuously. To prevent damage to the
external MOSFET, proper heatsinking should be imple-
mented.
Figure 1. Overvoltage Threshold (MAX6495鈥揗AX6499)
V
IN
C
OUT
GATE
IN
OUTFB
V
OUT
MAX6495
MAX6496
MAX6499
OVSET
GND
R1
R2
Figure 2. Overvoltage-Limiter Protection Switch Configuration
GATE Voltage
The MAX6495鈥揗AX6499 use a high-efficiency charge
pump to generate the GATE voltage. Upon V
IN
exceed-
ing the 5V (typ) UVLO threshold, GATE enhances 10V
above V
IN
(for V
IN
鈮?/div>
14V) with a 100碌A pullup current.
An overvoltage condition occurs when the voltage at
OVSET goes above its V
TH+
threshold. When the
threshold is crossed, GATE falls to OUTFB within 0.5碌s
with a 100mA pulldown current. The MAX6495鈥揗AX6499
include an internal clamp to OUTFB that ensures GATE
is limited to 18V (max) above OUTFB to prevent gate-
to-source damage of the external MOSFET.
7
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