MAX6499 Overvoltage and Undervoltage Protection

The MAX6499 comprises undervoltage and overvoltage comparators for window detection. WHen the monitored voltage is within selected “window”, GATE is enhances and the n-channel MOSFET is on. GATE falls to OUTFB when the monitored voltage falls below the lower limit (V triplow) or exceeds the upper limit (Vtriphigh) of the window, turning off the MOSFET. The MAX6499 enabling the DC-DC converter when the monitored voltage is in the selected window is shown on the diagram below:

We can calculate the value of R1,R2,R3 as follows:

V triplow = (Vth-) )(Rtotal/(R2+R3))
V triphigh = (Vth+) (Rtotal/R3)
where Rtotal = R1+R2+R3

The values of R1, R2, and R3 can be found with this following steps:

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 5MOhm.
2. Calculate R3 based on RTOTAL and the desired upper trip point:
   • R3 = (Vth+ x Rtotal)/Vtriphigh
3. Calculate R2 based on RTOTAL, R3, and the desired lower trip point:
   • R2 = [((Vth-) x Rtotal)/Vtriplow]-R3
4. Calculate R1 based on RTOTAL, R2, and R3:
   • R1 = RTOTAL – R2 – R3

Keep R3 <= 1kOhm  to improve ESD protection. The value of Vth- and Vth+ can be obtained from the data sheet [Source: maxim-ic.com]