TPS37 datasheet(21/48 Pages) TI

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8.3.4 User-Programmable Reset Time Delay

TPS37 has adjustable reset release time delay with external capacitors. Channel timing are independent of each

other.

• A capacitor in CTR1 / MR program the reset time delay of Output 1.

• A capacitor in CTR2 / MR program the reset time delay of Output 2.

• No capacitor on these pins gives the fastest reset delay time indicated in the Section 7.6.

8.3.4.1 Reset Time Delay Configuration

The time delay (tCTR) can be programmed by connecting a capacitor between CTR1 pin and GND, CTR2 for

channel 2. In this section CTRx represent either channel 1 or channel 2.

The relationship between external capacitor CCTRx_EXT (typ) and the time delay tCTRx (typ) is given by Equation 1.

tCTRx (typ) = -ln (0.28) x RCTRx (typ) x CCTRx_EXT (typ) + tCTRx (no cap)

(1)

RCTRx (typ) = is in kilo ohms (kOhms)

CCTRx_EXT (typ) = is given in microfarads (μF)

tCTRx (typ) = is the reset time delay (ms)

The reset delay varies according to three variables: the external capacitor (CCTRx_EXT), CTR pin internal

resistance (RCTRx) provided in Section 7.5, and a constant. The minimum and maximum variance due to the

constant is show in Equation 2 and Equation 3:

tCTRx (min) = -ln (0.31) x RCTRx (min) x CCTRx_EXT (min) + tCTRx (no cap (min))

(2)

tCTRx (max) = -ln (0.25) x RCTRx (max) x CCTRx_EXT (max) + tCTRx (no cap (max))

(3)

The recommended maximum reset delay capacitor for the TPS37 is limited to 10 μF as this ensures enough

time for the capacitor to fully discharge when a voltage fault occurs. Also, having a too large of a capacitor value

can cause very slow charge up (rise times) and system noise can cause the the internal circuit to trip earlier or

later near the threshold. This leads to variation in time delay where it can make the delay accuracy worse in the

presence of system noise.

When a voltage fault occurs, the previously charged up capacitor discharges and if the monitored voltage

returns from the fault condition before the delay capacitor discharges completely, the delay will be shorter than

expected. The capacitor will begin charging from a voltage above zero and resulting in shorter than expected

time delay. A larger delay capacitor can be used so long as the capacitor has enough time to fully discharge

during the duration of the voltage fault. To ensure the capacitor is fully discharged, the time period or duration of

the voltage fault needs to be greater than 5% of the programmed reset time delay.

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TPS37

SNVSBJ1D – OCTOBER 2020 – REVISED DECEMBER 2021

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