INA300 datasheet(12/41 Pages) TI1

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INA300

SBOS613B – FEBRUARY 2014 – REVISED APRIL 2016

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Feature Description (continued)

As shown in Table 1, the maximum value calculated for the current-sensing resistor with these requirements is

2.5 m

Ω. Although this value satisfies the maximum power dissipation requirement of 250 mW, headroom is

available from the 5% maximum total error to reduce the value of the current-sensing resistor and reduce the

power dissipation further. Selecting a 1.5-m

Ω, current-sensing resistor value offers a good tradeoff for reducing

the power dissipation in this scenario by approximately 40% while still remaining within the defined accuracy

region.

Table 1. Calculating the Current-Sensing Resistor, RSENSE

PARAMETER

EQUATION

VALUE

UNIT

Maximum measurement error

IMAX

Maximum current

PRSENSE

Maximum allowable RSENSE power

dissipation

RSENSE × IMAX

250

Initial error

RLIMIT + ILIMIT tolerances

1.5%

RSENSE_MAX

Maximum sensing resistor value

PRSENSE / IMAX

2.5

VSENSE_MAX

Input sense voltage

RSENSE_MAX × IMAX

VOS Error

Offset voltage error

(VOS / VSENSE_MAX) × 100

Error_Available

Maximum allowable offset error

Maximum Error – Initial Error

3.5%

VSENSE_MIN

Minimum input sense voltage

VOS / (Error_Available / 100)

14.3

RSENSE_MIN

Minimum sensing resistor value

VSENSE_MIN / IMAX

1.43

PRSENSE_MIN

Minimum power dissipation

RSENSE_MIN × IMAX

143

(1)

NAF is used with the 10-µs delay setting. NAF can be omitted in the RLIMIT calculation for the 50-µs and 100-µs delay settings.

7.3.2 Setting The Current-Limit Threshold

The device determines if an overcurrent event is present by comparing the measured differential voltage

developed across the current-sensing resistor to the corresponding signal programmed at the LIMIT terminal.

The threshold voltage for the LIMIT terminal can be set using a resistor or an external voltage source.

7.3.2.1 Resistor-Controlled Current Limit

The typical approach for setting the limit threshold voltage is to connect a resistor from the LIMIT terminal to

ground. The value of this resistor, RLIMIT, is chosen in order to create a corresponding voltage at the LIMIT

terminal equivalent to the voltage, VTRIP, developed by the load current flowing through the current-sensing

resistor. An internal 20-µA current source is present at the LIMIT terminal that creates the corresponding voltage

depending on the value of RLIMIT. In the equations from Table 2, VTRIP represents the overcurrent threshold the

device is programmed to monitor for and VLIMIT is the programmed signal set to detect the VTRIP level. The term

noise adjustment factor (NAF) is included in the VLIMIT equation for the 10-µs delay setting. This value is equal to

500 µV and is used to adjust the operating point for the internal noise in this delay setting. The 50-µs and 100-µs

delay settings do not use the NAF term in calculating the VLIMIT threshold. See the Noise Adjustment Factor

(NAF) section for more details on the noise adjustment factor.

In Table 2, the process for calculating the required value for RLIMIT in order to set the appropriate threshold

voltage, VLIMIT, is shown. This calculation is based on the 10-µs delay setting so the NAF term is included in the

calculation. For a delay setting of 50 µs or 100 µs, the NAF term is omitted.

Table 2. Calculating the Limit Threshold Setting Resistor, RLIMIT

PARAMETER

EQUATION

VTRIP

Desired current trip value

ILOAD × RSENSE

VLIMIT

Programmed threshold limit voltage

VLIMIT = VTRIP

VLIMIT

(1)

Threshold voltage

(ILIMIT × RLIMIT) – NAF

RLIMIT

(1)

Threshold limit setting resistor

(VLIMIT + NAF) / ILIMIT

RLIMIT

(1)

Limit setting resistor

(VLIMIT + 500 µV) / 20 µA

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