Datasheet MAX6846, MAX6847, MAX6848, MAX6849 (Maxim) - 9

Low-Power, Adjustable Battery Monitors with Hysteresis and Integrated µP Reset MAX6846–MAX6849 Manual Reset
Many microprocessor-based products require manual reset capability, allowing the operator, a test technician, VDD VDD LBO* or external logic circuitry to initiate a reset while the monitored supplies remain above their reset thresholds. R1 MAX6846 These devices have a dedicated active-low MR pin. MAX6847 When MR is pulled low, RESET asserts a one-shot low MAX6848 pulse for the MR reset timeout period. The MR input has LTHIN MAX6849 an internal 1.5kΩ pullup resistor to VCC and can be left (LBOH) unconnected if not used. MR can be driven with CMOS- R2 (LBOL) logic levels, open-drain/open-collector outputs, or a momentary pushbutton switch to GND (the MR function HTHIN GND is internally debounced for the tDEB timeout period) to create a manual reset function. If MR is driven from long R3 cables, or if the device is used in a noisy environment, connect a 0.1µF capacitor from MR to GND to provide * FOR THE MAX6846/MAX6847. additional noise immunity (see Figure 4). ( ) FOR THE MAX6848/MAX6849.
Hysteresis
Figure 5. Adjustable Threshold Selection Hysteresis increases the comparator’s noise margin by increasing the upper threshold or decreasing the lower threshold. The hysteresis prevents the output from 2) Calculate R3 based on RTOTAL and the desired oscillating (chattering) when monitor input is near the upper trip point: low-battery threshold. This is especially important for applications where the load on the battery creates sig- mV 615 R R TOTAL 3 = × nificant fluctuations in battery voltages (see Figures 2 VTRIPHIGH and 3). For the MAX6846/MAX6847, hysteresis is set using three 3) Calculate R2 based on RTOTAL, R3, and the desired external resistors (see Figure 5). The MAX6848/MAX6849 lower trip point: have dual, low-battery input levels. Each input level has a 5% (typ) hysteresis.  mV 615 R  R TOTAL 2 = × R3  V  -
Applications Information
TRIPLOW
Resistor-Value Selection (Programming
4) Calculate R1 based on RTOTAL, R3, and R2:
the Adjustable Thresholds)
R1 = R - R2 - R3
MAX6846/MAX6847
TOTAL
MAX6848/MAX6849
V = V = m 615 V LTH HTH VLTH- = VHTH- = 582mV  R1+ R2 + R3 LBOL low-trip level: V = V TRIPLOW LTH ×  R2 + R3   R1+ R2 + R3 V  R1 + R2 + R3 = V TRIPLOW LTH - × V = V TRIPHIGH HTH ×  R2 + R3   R3  R = R1 + R2 + R3 LBOH low-trip level: TOTAL Use the following steps to determine values for R1, R2,  R1+ R2 + R3 V = V - × and R3 of Figure 5. TRIPHIGH HTH  R3  1) Choose a value for RTOTAL, the sum of R1, R2, and R = R1 + R2 + R TOTAL 3 R3. Because the MAX6846/MAX6847 have very high input impedance, RTOTAL can be up to 500kΩ. Use the following steps to determine values for R1, R2, and R3 of Figure 5.
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