As shown in Table 7-1, the maximum value calculated for the current-sensing resistor with these requirements

is 2.5 mΩ. 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-mΩ, current-sensing resistor value offers a tradeoff for reducing the

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

Table 7-1. Calculating the Current-Sensing Resistor, RSENSE

PARAMETER

EQUATION

VALUE

UNIT

Maximum measurement error

5%

IMAX

Maximum current

10

A

PRSENSE

Maximum allowable RSENSE power

dissipation

RSENSE × IMAX 2

250

mW

Initial error

RLIMIT + ILIMIT tolerances

1.5%

RSENSE_MAX

Maximum sensing resistor value

PRSENSE / IMAX 2

2.5

mΩ

VSENSE_MAX

Input sense voltage

RSENSE_MAX × IMAX

25

mV

VOS Error

Offset voltage error

(VOS / VSENSE_MAX) × 100

2%

Error_Available

Maximum allowable offset error

Maximum Error – Initial Error

3.5%

VSENSE_MIN

Minimum input sense voltage

VOS / (Error_Available / 100)

14.3

mV

RSENSE_MIN

Minimum sensing resistor value

VSENSE_MIN / IMAX

1.43

mΩ

PRSENSE_MIN

Minimum power dissipation

RSENSE_MIN × IMAX 2

143

mW

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 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 7-2, 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 adjusts 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 Noise Adjustment Factor (NAF) for

more details on the noise adjustment factor.

In Table 7-2, the process for calculating the required value for RLIMIT 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 7-2. 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

(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.

TI recommends using NAF in calculating the value for VLIMIT and RLIMIT at the 10-µs delay setting. Removing

NAF from the VLIMIT and RLIMIT calculation at the 10-µs delay setting lowers the trigger point of the alert output.

INA300

SBOS613C – FEBRUARY 2014 – REVISED JUNE 2021

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