MAX6495 データシートの表示（PDF） - Maxim Integrated
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MAX6495
Maxim Integrated
MAX6495 Datasheet PDF : 16 Pages
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MAX6495–MAX6499
72V, Overvoltage-Protection
Switches/Limiter Controllers
with an External MOSFET
The gate cycles during overvoltage-limit and overvoltage-
switch modes are quite similar but have distinct characteristics.
In overvoltage-switch mode, GATE is enhanced to (VIN
V TRIPLOW
=
(VTH− )
R TOTAL
R2 + R3
+ 10V) while the monitored VIN voltage remains below
the overvoltage fault threshold (OVSET < VTH+). When
an overvoltage fault occurs (OVSET ≥ VTH+), GATE is
pulled one diode drop below OUTFB, turning off the external
MOSFET and disconnecting the load from the input.
GATE remains low (MOSFET off) as long as the VIN voltage
is above the overvoltage fault threshold. As VIN falls back
below the overvoltage fault threshold, GATE is again
enhanced to (VIN + 10V).
In overvoltage-limit mode, GATE is enhanced to (VIN
+10V) while the monitored OUTFB voltage remains below
the overvoltage fault threshold (OVSET < VTH+). When
an overvoltage fault occurs (OVSET ≥ VTH+), GATE is
pulled one diode drop below OUTFB until OUTFB drops
5% below the overvoltage fault threshold (MAX6495/
V TRIPHIGH
=
(V TH+
)
R
TOTAL
R3
where RTOTAL = R1 + R2 + R3.
Use the following steps to determine the values for R1,
R2, and R3:
1) Choose a value for RTOTAL, the sum of R1, R2, and
R3. Because the MAX6499 has very high input impedance,
RTOTAL can be up to 5MΩ.
2) Calculate R3 based on RTOTAL and the desired upper
trip point:
R3 = VTH+ × R TOTAL
VTRIPHIGH
MAX6496/MAX6499). GATE is then turned back on until
OUTFB reaches the overvoltage fault threshold and
3) Calculate R2 based on RTOTAL, R3, and the desired
lower trip point:
GATE is again turned off. GATE cycles in a saw-tooth
waveform until OUTFB remains below the overvoltage
fault threshold and GATE remains constantly on (VIN
+10V). The overvoltage limiter’s sawtooth GATE output
= R2
(VTVHT−R) ×IPRLOTOWTAL
− R3
operates the MOSFET in a switched-linear mode while 4) Calculate R1 based on RTOTAL, R2, and R3:
the input voltage remains above the overvoltage fault
threshold. The sawtooth frequency depends on the load
capacitance, load current, and MOSFET turn-on time
R1 = RTOTAL – R2 – R3
To improve ESD protection, keep R3 ≥ 1kΩ.
(GATE charge current and GATE capacitance).
GATE goes high when the following startup conditions are
met: VIN is above the UVLO threshold, SHDN is high, an
overvoltage fault is not present, and the device is not in
VIN
thermal shutdown.
DC-DC
CONVERTER
IN
OUT
GND
Undervoltage Monitoring (MAX6499)
The MAX6499 includes undervoltage and overvoltage
comparators for window detection (see Figure 3 and
Figure 12). GATE is enhanced and the nMOSFET is
on when the monitored voltage is within the selected
“window.” When the monitored voltage falls below
the lower limit (VTRIPLOW) or exceeds the upper limit
(VTRIPHIGH) of the window, GATE falls to OUTFB turning
off the MOSFET. The application in Figure 3 shows
the MAX6499 enabling the DC-DC converter when the
monitored voltage is in the selected window.
IN GATE OUTFB
R1
SHDN
UVSET
R2
MAX6499
OVSET
CLEAR GND
R3
The resistor values R1, R2, and R3 can be calculated
as follows:
Figure 3. MAX6499 Window-Detector Circuit
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