Datasheet
MAX4291/MAX4292/MAX4294
When the output of the comparator is low, the supply
current increases. The output stage has biasing circuit-
ry to monitor the output current. When the amplifier is
used as a comparator, the output stage is overdriven
and the current through the biasing circuitry increases
to maximum. For the MAX4291, typical supply currents
increase to 1.5mA with V
CC
= 1.8V and to 9mA when
V
CC
= 5.0V (Figure 10).
Using the MAX4291/MAX4292/MAX4294
as Low-Power Current Monitors
The MAX4291/MAX4292/MAX4294 are ideal for appli-
cations powered from a two-cell battery stack. Figure
11 shows an application circuit in which the MAX4291
is used for monitoring the current of a two-cell battery
stack. In this circuit, a current load is applied, and the
voltage drop at the battery terminal is sensed.
The voltage on the load side of the battery stack is
equal to the voltage at the emitter of Q1 due to the
feedback loop containing the op amp. As the load cur-
rent increases, the voltage drop across R1 and R2
increases. Thus, R2 provides a fraction of the load cur-
rent (set by the ratio of R1 and R2) that flows into the
emitter of the PNP transistor. Neglecting PNP base cur-
rent, this current flows into R3, producing a ground-ref-
erenced voltage proportional to the load current. To
minimize errors, scale R1 to give a voltage drop that is
large enough in comparison to the op amp’s V
OS
.
Calculate the output voltage of the application using
the following equation:
VI
R1
R2
R3
OUT LOAD
=×
×
Ultra-Small, 1.8V, µPower,
Rail-to-Rail I/O Op Amps
12 ______________________________________________________________________________________
06090
20 30
10
40 50 8070 100
PROPAGATION DELAY
vs. INPUT OVERDRIVE
V
OD
(mV)
10
100
1000
t
PD
(µs)
t
PD
+
,
V
CC
= 5.5V
t
PD
-
,
V
CC
= 5.5V
t
PD
+
,
V
CC
= 1.8V
t
PD
-
,
V
CC
= 1.8V
Figure 8. Propagation Delay vs. Input Overdrive
R2
R1
V
SIG
OUTPUT
INPUT
V
OH
V
OL
V
EE
= GND
V
CC
V
OUT
R
HYST
V
EE
= GND
MAX4291
MAX4292
MAX4294
HYSTERESIS
V
LO
V
HI
V
REF
Figure 9. Hysteresis Comparator Circuit
0
6
2
4
8
10
12
0 2.0 2.5 3.01.00.5 1.5 3.5 4.5 5.04.0 5.5
MAXIMUM SUPPLY CURRENT PER AMPLIFIER
vs. SUPPLY VOLTAGE
SUPPLY VOLTAGE (V)
MAXIMUM SUPPLY CURRENT (mA)
COMPARATOR CONFIGURATION
V
IN
+
= (V
IN
-) - 100mV
Figure 10. Maximum Supply Current per Amplifier vs. Supply
Voltage
V V V
V 1
R1
R2
R1
R
V
V V
R1
R
V
HYST HI LO
HI
HYST
REF
LO HI
HYST
CC
=−
=+ +
=−