Keywords: comparator, low battery, LED, low cost, low power consumption, comparators
DC = tON/(tON + tOFF)On-time is derived from the equation for time-varying voltage across a charging capacitor:
V(t) = V(1 - e-t/RC)Off-time is derived from the equation for time-varying voltage across a discharging capacitor:
tON = -R5Cln(1 - VTRIPHI/VOUT)
V(t) = Ve-t/RCThe comparator's high and low trip levels are found using Kirchoff's current laws:
tOFF = -R4Cln(VTRIPLO/VOUT)
VTRIPHI = VOUT[R3(R1 + R2)]/[R3(R1 + R2) + R1R2]The typical VOUT for a comparator is VDD. Assuming a 2.5% duty cycle and assuming that LBO trips the comparator on when battery voltage equals 3V, then the resulting trip levels are 1V for low and 2V for high. The standard component values corresponding to this performance are:
VTRIPLO = VOUT[R3R2]/[R3(R1 + R2) + R1R2]
C1 = 0.1µFThe Figure 1 circuit accommodates any type of LBO output (active high or active low). Supply current drawn by the MAX9030 is typically 35µA when active and only 0.05µA while in shutdown. The circuit's small size, low power consumption, and low cost are ideal for battery-powered systems.
R1 = R2 = R3 = 1MΩ
R4 = 3.6MΩ
R5 = 91kΩ
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APPLICATION NOTE 1872, AN1872, AN 1872, APP1872, Appnote1872, Appnote 1872
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