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9 Application and Implementation

Note

Information in the following applications sections is not part of the TI component specification,

and TI does not warrant its accuracy or completeness. TI’s customers are responsible for

determining suitability of components for their purposes, as well as validating and testing their design

implementation to confirm system functionality.

9.1 Adjustable Voltage Thresholds

Equation 7 illustrates an example of how to adjust the voltage threshold with external resistor dividers. The

resistors can be calculated depending on the desired voltage threshold and device part number. TI recommends

using the 0.8 V voltage threshold device when using an adjustable voltage variant. This variant bypasses the

internal resistor ladder.

For example, consider a 12 V rail being monitored VMON for undervoltage (UV) using channel 1 of the

TPS38A010122DSKRQ1 variant. Using Equation 7 and shown in Figure 9-1, R1 is the top resistor of the

resistor divider that is between VMON and VSENSE2, R2 is the bottom resistor that is between VSENSE2 and GND,

VMON is the voltage rail that is being monitored and VSENSE2 is the input threshold voltage. The monitored UV

threshold, denoted as VMON-, where the device will assert a reset signal occurs when VSENSE2 = VIT-(UV) or, for

this example, VMON- = 10.8V which is 90% from 12 V. Using Equation 7 and assuming R2 = 10kΩ, R1 can be

calculated shown in Equation 8 where IR1 is represented in Equation 9:

VSENSE2 = VMON- × (R2 ÷ (R1 + R2))

(7)

R1 = (VMON- - VSENSE2) ÷ IR1

(8)

IR1 = IR2 = VSENSE2 ÷ R2

(9)

Substituting Equation 9 into Equation 8 and solving for R1 in Equation 7, R1 = 125kΩ. The

TPS38A010122DSKRQ1 is typically meant to monitor a 0.8 V rail with ±2% voltage threshold hysteresis. For the

reset signal to become deasserted, VMON would need to go above VIT- + VHYS. For this example,

VMON = 11.016 V when the reset signal becomes deasserted.

There are inaccuracies that must be taken into consideration while adjusting voltage thresholds. Aside from the

tolerance of the resistor divider, there is an internal resistance of the SENSE pin that may affect the accuracy

of the resistor divider. Although expected to be very high impedance, users are recommended to calculate the

values for the design specifications. The internal SENSE resistance RSENSE can be calculated by the SENSE

voltage VSENSE divided by the SENSE current ISENSE as shown in Equation 11. VSENSE can be calculated using

Equation 7 depending on the resistor divider and monitored voltage. ISENSE can be calculated using Equation 10.

ISENSE = [(VMON - VSENSE) ÷ R1] - (VSENSE ÷ R2)

(10)

RSENSE = VSENSE ÷ ISENSE

(11)

TPS38A-Q1

SENSE1

RESET1

VDD

10 k

VMON

VSENSE2

GND

SENSE2

VMON

VSENSE1

CTS1

CTS2

RESET2

CTR2

CTR1

10 k

VDD

Figure 9-1. Adjustable Voltage Threshold with External Resistor Dividers

TPS38-Q1

SNVSBI8C – AUGUST 2020 – REVISED DECEMBER 2021

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