LM7301 datasheet(17/28 Pages) TI1

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OUT

10 k

OUT

5 V

= D

OUT

SENSE

CHARGE _ MAX

CHARGE _ MIN

30 mA

SENSE

CHARGE_MIN

LM7301

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SNOS879I – AUGUST 1999 – REVISED MAY 2016

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Typical Applications (continued)

8.2.1 Design Requirements

The output port is designed for easy interface; it is ground-referenced and it produces 0 V with 0 A of load

current. A typical stage that follows this stage, an ADC which samples the load current for example, is easily

connected to the Q1 collector with no level shifting or additional biasing required.

Apart from a wide supply voltage capability, the operational amplifier used in Figure 27 must have an input

voltage range that includes the V+ rail voltage to allow high side current sensing. Furthermore, it should be unity

gain stable and have an output voltage range which is less than one Vbe from V+. The LM7301 has all these

requirements.

8.2.2 Detailed Design Procedure

8.2.2.1 Selecting RSENSE

Pick the value of RSENSE low enough to minimize its heat / voltage loss while observing Equation 1 for minimum

detectable load current, ICHARGE_MIN , and device offset voltage, VOS:

(1)

With the schematic values shown and LM7301's VOS limit of 6 mV:

(2)

If the system has the ability to be initialized and corrected for initial readings, it may be possible to lower the

value of RSENSE.

8.2.2.2 Selecting R1, and R3 Values

Pick the R3 / R1 ratio to get the proper full-scale VOUT when the maximum load current, ICHARGE_MAX, flows:

(3)

For example, to get 3-V output with 3 A of load current when RSENSE = 0.2 Ω results in:

(4)

Ensure that the resulting transfer function also satisfies the application’s need when the minimum load current,

ICHARGE_MIN is being sensed. In this example, the minimum output voltage will be 30 mV (when ICHARGE_MIN = 30

mA).

With the R3/R1 ratio determined, pick the value of R3 for Q1 collector current less than 1 mA at the maximum

VOUT, and determine R1 from that.

8.2.2.3 R1, R2 Selection

Normally, R2 is set equal to R1 to cancel out the error term due to the input bias current, IB (approximately 200

nA for the LM7301).

8.2.2.4 Error Terms Expressions

Here are the expressions for the output change caused by various parameter shifts, evaluated for Figure 27

values with ICHARGE_MAX= 3 A:

Offset Voltage, ΔVOS:

(5)

Offset current, IOS:

(6)

LM7301 Datasheet(PDF) 17 Page - Texas Instruments

LM7301 Datasheet(HTML) 17 Page - Texas Instruments

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