Detailed Description

The MAX4069–MAX4072 bidirectional, high-side, cur-

rent-sense amplifiers are ideal for portable equipment.

The wide 1.35V to 24V input common-mode voltage

range is independent of the supply voltage, ensuring

that the current-sense feedback remains accurate even

when connected to a battery pack in deep discharge.

The MAX4069/MAX4071/MAX4072 operate from a 2.7V

to 24V single supply. Because the MAX4070 has a 2.5V

internal reference, it operates from 3.6V to 24V. All

devices have a low 100µA supply current that reduces

to only 10µA (typ) in shutdown mode. To achieve maxi-

mum flexibility, an external current-sense resistor is

used along with a gain select pin (GSEL) to choose

either 50V/V or 100V/V. Drive GSEL low to select gain of

50V/V or drive GSEL high to choose gain of 100V/V

(see

Functional Diagram). The MAX4069 has an

adjustable reference voltage set by two external resis-

tors between REFOUT and ADJ, and GND pins. The

MAX4070 contains an internal 2.5V reference. The

MAX4071 is similar to the MAX4070 but with a fixed

internal reference voltage of 1.5V. The MAX4072 has a

reference input pin to allow use of external references.

Charging current is represented by an output voltage

rent is given from the reference voltage to GND. The

The input stage of the MAX4069–MAX4072 is shown in

Figure 1. Its unique topology allows for monitoring bidi-

for instance, current flows from RS+ to RS-, the

MAX4069–MAX4072 match for the voltage drop over the

rent through the internal Q1 and RG1. At the same time,

the current through Q2 and RG2 decreases, however,

internal circuitry not shown in Figure 1 prevents Q2 from

turning off completely. Likewise, if current flows from RS-

to RS+, the current through Q2 and RG2 increases and

the current through Q1 decreases. In this way, the volt-

ages at the input terminals of the internal amplifier A1 are

kept constant and an accurate measurement of the sense

voltage is achieved. In the following amplifier stages of

the MAX4069–MAX4072, the output signal of amplifier A2

resulting in a voltage at the output pin (OUT) that swings

Applications Information

Bidirectional, Current-Sense Amplifier

Systems such as laptop computers and other devices

that have internal charge circuitry require a precise bidi-

rectional, current-sense amplifier to accurately monitor

the battery’s current regardless of polarity. Figure 2

shows the MAX4069–MAX4072 used as a bidirectional

assigned to charge and discharge, assuming charge is

greater than discharge. The practical choice of direction

is based on dynamic range at OUT, given that the range

ground. This is useful for implementing either smart bat-

tery packs or fuel gauges.

High-Current Measurement

The MAX4069–MAX4072 can achieve high-current

measurements by using low-value sense resistors,

which can be paralleled to further increase the current-

sense limit.

The MAX4069 has an output reference voltage that can

be set to a desired voltage by a two-resistor divider

between REFOUT, ADJ, and GND. If REFOUT and ADJ

are connected together, the minimum output reference

voltage is obtained, 1.23V (typ). The maximum voltage

≥ 5.1V). The rec-

ommended range for the external resistors is: R1 + R2

should be > 20k

Ω and < 500kΩ.

Bidirectional, High-Side, Current-Sense

Amplifiers with Reference

10

______________________________________________________________________________________

MAX4069–MAX4072

RS-

RS+

RG2

RG1

A1

A2

Q1

Q2

OUT

Figure 1. Detailed Input Stage