LTC4150 datasheet(12/28 Pages) AD

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LT6119-1/LT6119-2

611912f

For more information www.linear.com/LT6119-1

applicaTions inForMaTion

The LT6119 high side current sense amplifier provides

accurate monitoring of currents through an external sense

resistor. The input sense voltage is level-shifted from

the sensed power supply to a ground referenced output

and is amplified by a user-selected gain to the output.

The output voltage is directly proportional to the current

flowing through the sense resistor.

The LT6119 comparators have a threshold set with a

built-in 400mV precision reference and have 10mV of

hysteresis. The open-drain outputs can be easily used to

level shift to digital supplies.

Amplifier Theory of Operation

An internal sense amplifier loop forces SENSEHI to have

the same potential as SENSELO, as shown in Figure 3.

Connecting an external resistor, RIN, between SENSEHI

and VSUPPLY forces a potential, VSENSE, across RIN. A

corresponding current, IOUTA, equal to VSENSE/RIN, will

flow through RIN. The high impedance inputs of the sense

amplifier do not load this current, so it will flow through

an internal MOSFET to the output pin, OUTA.

The output current can be transformed back into a voltage

by adding a resistor from OUTA to V–(typically ground).

The output voltage is then:

VOUT = V– + IOUTA • ROUT

where ROUT = R1 + R2 + R3, as shown in Figure 3.

Table 1. Example Gain Configurations

GAIN

RIN

ROUT VSENSE FOR VOUT = 5V IOUTA AT VOUT = 5V

499Ω

10k

250mV

500µA

200Ω

10k

100mV

500µA

100

100Ω

10k

50mV

500µA

Useful Equations

Input Voltage: VSENSE = ISENSE • RSENSE

Voltage Gain:

VOUT

VSENSE

ROUT

RIN

Current Gain:

IOUTA

ISENSE

RSENSE

RIN

Note that VSENSE(MAX) can be exceeded without damaging

the amplifier, however, output accuracy will degrade as

VSENSEexceedsVSENSE(MAX),resultinginincreasedoutput

current, IOUTA.

Selection of External Current Sense Resistor

Theexternalsenseresistor,RSENSE,hasasignificanteffect

on the function of a current sensing system and must be

chosen with care.

First, the power dissipation in the resistor should be

considered. The measured load current will cause power

dissipation as well as a voltage drop in RSENSE. As a

result, the sense resistor should be as small as possible

while still providing the input dynamic range required by

the measurement. Note that the input dynamic range is

the difference between the maximum input signal and the

minimum accurately reproduced signal, and is limited pri-

marily by input DC offset of the internal sense amplifier of

the LT6119. To ensure the specified performance, RSENSE

should be small enough that VSENSE does not exceed

VSENSE(MAX) under peak load conditions. As an example,

an application may require the maximum sense voltage

be 100mV. If this application is expected to draw 2A at

peak load, RSENSE should be set to 50mΩ.

Once the maximum RSENSE value is determined, the mini-

mum sense resistor value will be set by the resolution or

dynamic range required. The minimum signal that can be

accurately represented by this sense amplifier is limited by

theinputoffset.Asanexample,theLT6119hasamaximum

input offset of 200µV. If the minimum current is 20mA, a

sense resistor of 10mΩ will set VSENSE to 200µV. This is

the same value as the input offset. A larger sense resis-

tor will reduce the error due to offset by increasing the

sense voltage for a given load current. Choosing a 50mΩ

RSENSE will maximize the dynamic range and provide a

system that has 100mV across the sense resistor at peak

load (2A), while input offset causes an error equivalent to

only 4mA of load current.

In the previous example, the peak dissipation in RSENSE

is 200mW. If a 5mΩ sense resistor is employed, then

the effective current error is 40mA, while the peak sense

voltage is reduced to 10mV at 2A, dissipating only 20mW.

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