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MAX6495
(Rev.:2005)

72V, Overvoltage-Protection Switches/Limiter Controllers with an External MOSFET

Maxim Integrated 

72V, Overvoltage-Protection Switches/Limiter

Controllers with an External MOSFET

The gate cycles during overvoltage-limit and overvolt-

age-switch modes are quite similar but have distinct

characteristics. In overvoltage-switch mode, GATE is

enhanced to (VIN + 10V) while the monitored VIN volt-

age 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 discon-

necting 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/MAX6496/MAX6499). GATE is then turned

back on until OUTFB reaches the overvoltage fault

threshold and GATE is again turned off. GATE cycles in

a sawtooth waveform until OUTFB remains below the

overvoltage fault threshold and GATE remains con-

stantly on (VIN +10V). The overvoltage limiter’s saw-

tooth GATE output operates the MOSFET in a

switched-linear mode while the input voltage remains

above the overvoltage fault threshold. The sawtooth fre-

quency depends on the load capacitance, load current,

and MOSFET turn-on time (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 thermal shutdown.

Undervoltage Monitoring (MAX6499)

The MAX6499 includes undervoltage and overvoltage

comparators for window detection (see Figures 3 and

12). GATE is enhanced and the n-channel MOSFET 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 turn-

ing 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.

The resistor values R1, R2, and R3 can be calculated

as follows:

( ) VTRIPLOW

=

VTH−





RTOTAL

R2 + R3





( ) VTRIPHIGH

=

VTH+





RTOTAL

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+ × RTOTAL

VTRIPHIGH

3) Calculate R2 based on RTOTAL, R3, and the desired

lower trip point:

( ) R2

=





VTH−

×

RTOTAL





− R3

 VTRIPLOW 

4) Calculate R1 based on RTOTAL, R2, and R3:

R1 = RTOTAL – R2 – R3

VIN

R1

R2

R3

IN GATE OUTFB

SHDN

UVSET

MAX6499

OVSET

CLEAR GND

DC-DC

CONVERTER

IN

OUT

GND

Figure 3. MAX6499 Window-Detector Circuit

8 _______________________________________________________________________________________

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