Single/Dual/Quad, Low-Cost, SOT23,

Micropower, Rail-to-Rail I/O Op Amps

12

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The MAX4040–MAX4044 contain special circuitry to

boost internal drive currents to the amplifier output

stage. This maximizes the output voltage range over

which the amplifiers are linear. In an open-loop com-

parator application, the excursion of the output voltage

is so close to the supply rails that the output stage tran-

sistors will saturate, causing the quiescent current to

increase from the normal 10µA. Typical quiescent cur-

Using the MAX4040–MAX4044

as Ultra-Low-Power Current Monitors

The MAX4040–MAX4044 are ideal for applications pow-

ered from a battery stack. Figure 9 shows an application

circuit in which the MAX4040 is used for monitoring the

current of a 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. Scale

R1 to give a voltage drop large enough in comparison

The output voltage of the application can be calculated

using the following equation:

For a 1V output and a current load of 50mA, the choice

of resistors can be R1 = 2

Ω, R2 = 100kΩ, R3 = 1MΩ.

The circuit consumes less power (but is more suscepti-

ble to noise) with higher values of R1, R2, and R3.

R2

R1

OUTPUT

INPUT

MAX4040–

MAX4044

HYSTERESIS

Figure 8. Hysteresis Comparator Circuit

R1

R2

R3

Q1

MAX4040

Figure 9. Current Monitor for a Battery Stack

10,000

10

020

30

10

100

100

1000

40

50 60

70 80

90

Figure 7. Propagation Delay vs. Input Overdrive