SLVSI23 Data sheet

SLVSI23A September 2025 – December 2025 DRV81646

PRODUCTION DATA

6.3.4.1.1 Effect of Load Resistance on Power Dissipation Before TSD

The resistance of the load affects how long the channel operates in the linear region before hitting thermal shutdown. The resistance functions similarly to a linear drop-out regulator (LDO), where a higher voltage drop requires the device to dissipate more power.

For example, take a 24V system with a 1A I_LIM setting for a 5Ω load versus an 11Ω load. Without current limiting these draw 4.8A and 2.2A respectively, but with the I_LIM feature, these regulate to 1A. Use Equation 3 to calculate the linear region resistance of the FET to achieve this 1A current limit:

Equation 3.

I = \frac{V}{R}

Equation 4.

I_{L I M} = \frac{V_{V M}}{[R_{L O A D} + R_{D S (O N)}]}

Rearrange Equation 4 to solve for R_DS(ON), then plug in the system values for loads 5Ω and 11Ω:

Equation 5.

R_{D S (O N)} = [\frac{V_{V M}}{I_{L I M}}] - R_{L O A D}

Equation 6.

R_{D S (O N)_5 Ω} = (\frac{24 V}{1 A}) - 5 Ω \to R_{D S (O N)_5 Ω} = 19 Ω

Equation 7.

R_{D S (O N)_11 Ω} = (\frac{24 V}{1 A}) - 11 Ω \to R_{D S (O N)_11 Ω} = 13 Ω

Use this resistance to calculate the power dissipated inside the DRV81646 FET:

Equation 8.

P_{F E T_5 Ω} = I^{2} \times R = {1 A}^{2} \times 19 Ω = 19 W

Equation 9.

P_{F E T_11 Ω} = I^{2} \times R = {1 A}^{2} \times 13 Ω = 13 W

As in Equation 8 and Equation 9, even though both loads are limited to 1A, the DRV81646 has to dissipate more power for a 5Ω load than an 11Ω load. This power dissipation directly correlates with the temperature rise of the FET over time. More power dissipated means the channel hits thermal shutdown faster.