e004060.pdf

APPLICATION NOTE

The content of this note is based on information received from manufacturers in the electrical and electronics industries or

their representatives and does not imply practical experience by Elektor Electronics or its consultants.

Simple high-side

By G. Kleine

Maxim has recently introduced a new integrated current meter IC called

the MAX4173. It converts a current measured in the positive supply lead

into a proportional voltage to earth. This can in turn be input into the A/D

converter of a system.

SENSE

diagram for the MAX4173 current mea-

surement IC. The most important exter-

nal element is the sense resistor R sense ,

which is located in the current path.

Resistor R L represents the connected

load, which can be another circuit or a

rechargeable battery, for example. The

output voltage V out must have a very

high input impedance load. This voltage is

proportional to the magnitude of the cur-

rent through R sense . Later on we will see

how to calculate the value of R sense .

SENSE

0...+28V

V CC

+3...+28V

RS+

RS–

V OUT

IC1

(to A/D converter)

VCC

OUT

≈ 450µA

MAX4173

GND

100n

Internal design

of the current sensor IC

The MAX4173 consists of an operational

ampliﬁer, a transistor and a current mir-

ror, as shown in Figure 2. If we assume

that the operational ampliﬁer is an ideal

amplifier, then no current flows in the

inverting input. The voltage at this input

is thus equal to that at RS–. This means

that the voltage across R R1 is equal to

that across R sense , since the transistor

closes the feedback loop around the oper-

ational amplifier. The result is that the

current in resistor R G1 is lower than the

load current I L by a factor of R sense / R G1 :

000008 - 11

Figure 1. Basic application circuit for the MAX4173.

The MAX4173 allows a control microprocessor to

measure and monitor the current consumption of

electronic components. The main advantage of this

new IC is that the voltage V in on the supply lead to be

measured may be higher (as well as lower) than the

supply voltage V CC of the IC itself. This means for

example that it is no problem to use a MAX4173 to

measure the current on a +12-V line, even if the IC

itself is connected to the +5-V or +3.3-V supply line

of the processor. It is not necessary for both supply

voltages to be the same, as with previous designs.

The IC can work with a supply voltage

between +3 V and +28 V, and it draws

only around 450 µA of current. The volt-

age V in in the measurement arm may lie

between 0 and +28 V, independent of the

operating voltage of the IC. This makes

this IC an outstanding choice for battery

charging control systems, among others.

However, the measurement error

increases greatly if V in is less than 1 V.

Figure 1 shows the basic connection

I RG1 = I L ⋅ R sense / R G1

This current reaches the current mirror,

which converts it into another current

Elektor Electronics

4/2000

current meter

APPLICATION NOTE

x =

+20V/V

+50V/V

+100V/V

MAX4173xESA

MAX4173xEUT-T

VCC

RS+

NC 1

OUT

GND

RS–

VCC

RS+

GND

RS–

SO23-6

OUT

SO8

000008 - 13

Figure 2. Internal schematic diagram of the MAX4173.

Figure 3. Pin assignments for both package types.

I RGD with an ampliﬁcation factor B:

The conversion gain G of the current sen-

sor can be deﬁned as:

V out = G I L R sense

I RGD = B I RG1

G = V out / V sense = B ⋅ R GD /R G1

A prerequisite for the accurate operation of the IC

is that the output voltage V out must not be con-

nected to a low-impedance load. To minimise the

error, the load resistance of the following circuit

stage (A/D converter) should be greater than

100 kΩ. For best results, a high-impedance voltage

follower should be used as an output buffer.

The internal 12-kΩ resistor R GD converts

I RGD into the output voltage V out , which

can be picked up by a high-impedance

circuit.

The transfer function of the IC from the

load current I L to the output voltage V out

is thus:

Circuit board tracks as sense resistors

Current-carrying capacities of printed

circuit tracks, for dimensioning a

printed circuit track used as a sense

resistor.

Track width

b sense

l sense

A se nse

35 µ m

I L

Allowed current

I sense

b sense

b sen se

000008 - 14

R S+

R S-

0.1 mm

0.5 A

0.2 mm

0.7 A

The sense resistance can also be produced using a section of a thin copper circuit board

track. The speciﬁc resistance of copper is

0.3 mm

1.0 A

0.5 mm

1.5 A

ρ Cu = 0.0175

Ω

mm 2 / m.

0.8 mm

2.5 A

1.0 mm

3.5 A

With standard printed circuit boards, the thickness of the copper lamination is 35 µm. The

value of R sense can be calculated as

1.5 mm

5.0 A

2 mm

7.0 A

R sense =

ρ Cu l sense / A sense

3 mm

8.5 A

5 mm

12 A

where

10 mm

20 A

A sense = 35

m b sense

and the track width is 0.2 mm. If we rearrange the formula for R sense to solve for

l sense and insert these values, we ﬁnd that the value of l sense is 4 mm. We also have to take into account that the copper track must be able

to handle the power that is dissipated. The values in the table can be used as a guideline for selecting the width of the copper track for

R sense . These are based on an assumed temperature rise of 30 °C.

The last point to consider is that copper has a relatively high thermal coefficient of resistance. The resistance changes by 0.4% per °C,

which amounts to no less than 16% over an ambient temperature range of +5 to +45 °C.

Ω

4/2000

Elektor Electronics

For example, suppose that we need an R sense of 100 m

APPLICATION NOTE

MAX4173 versions

The MAX4173 is available with three different gain

classes and two different package types. In addi-

tion to the usual SO-8 package, there is also an

especially small SOT-23 package with six leads (see

Figure 3). The letter at the end of the part number

indicates the gain from V sense to V out , as shown in

Table 1.

Table 2 shows the recommended component values

for various load currents. The value of R sense is cho-

sen such that it drops 100 mV at the maximum

level of the load current I L . The maximum allowed

voltage difference between the sensor inputs RS+

and RS– is 300 mV.

One thing that you have to watch out for is that

the operating voltage V CC of the MAX4173 (but not

the voltage V in ) must be at least 1.2 V higher than

the maximum level of the output voltage V out,max .

Otherwise, the output voltage V out will not remain

proportional to the measured current at high cur-

rent levels.

The output voltage is generated by an internal

current source that feeds current to a 12-kΩ resistor

connected to earth (see Figure 2). This is why a

low-impedance load should not be connected to

the output. The measurement error due to the load

impedance R out can be calculated using the for-

mula

surement error increases as this voltage

drops below 2 V. At 0.1 V it is typically

9%.

Consequently, if the A/D converter fol-

lowing the current sensor needs to be dri-

ven by a low-impedance source, an oper-

ational ampliﬁer with a high-impedance

input must be used as a buffer between

the sensor IC and the A/D converter. This

amplifier must allow input and output

voltages going down to earth level with a

single supply voltage.

Table 1. MAX4173 gain options.

Typ

G = V out / V sense

MAX 4173T

+20 V/V

MAX 4173F

+50 V/V

MAX 4173H

+100 V/V

value or a combination of two or three

standard values to make up the sense

resistor. Don’t forget that the resistor you

have chosen (or the combination of resis-

tors connected in parallel) will dissipate

an amount of power given by

Calculating R SENSE

The value of the sense resistor R sense can

be calculated using the formula

R sense = V sense / I Lmax

P v,max = R sense I Lmax 2

which it or they must be able to convert

into heat.

The nominal value of V sense is taken to be

100 mV at the maximum load current. For

linear operation, the voltage between

RS+ and RS– must be less than 300 mV.

After calculating the value of R sense , you

can choose a suitable standard resistance

(000008-1)

Literature:

MAX4173 data sheet, available from

http://www.maxim-ic.com.

Table 2. Recommended values for R sense .

Error,% = 100 % ×

[(Rout / (12 kΩ + R out ))-1]

This means that even with a load resistance of

100 kΩ, the error is 10%, and it is still 2% at 500 kΩ.

The speciﬁed measurement error of the MAX4173

is ±0.5% under normal operating conditions, with

the voltage V in at RS+ greater than 2 V. The mea-

I Lmax R sense

P v,max

V out,max @ G = 20 V/V

(MAX 4173T)

V out,max @ G = 50 V/V

(MAX 4173F)

V out,max @ G = 100 V/V

(MAX 4173H)

0.1 A

1 W

100 mW

2.0 V

5.0 V

10.0 V

1 A

0.1 W

100 mW

2.0 V

5.0 V

10.0 V

5 A 0.02 W

100 mW

2.0 V

5.0 V

10.0 V

10 A 0.01 W

100 mW

2.0 V

5.0 V

10.0 V

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4/2000

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