SLYT857 August 2024 TPS1200-Q1 , TPS1211-Q1

5 Design considerations and test results

Consider this system design example for a 50A load:

Battery voltage (V_BATT) = 12V.
Load capacitance (C_LOAD) = 5mF.
1.5m cable = 8AWG connecting the high-side driver to the ECU, leading to L_cable = 2.25µH.
Charging time (T_charge) = 10ms
Freewheeling diode drop (V_D1) = 0.7V.

The design involves selecting the I_SCP and T_AUTO-RETRYparameters. For a 50A load design, the I_SCP threshold is usually set at 20% above the maximum load current, so in this example, that would be 50A × 1.2 = 60A.

Now, to compute T_AUTO-RETRY, see Figure 6 and use the current-voltage relationship of the capacitor at the midpoint of T_charge/2 to get Equation 6:

Equation 6.

\frac{({I_{s t a r t} + I}_{m i d})}{3} \times \frac{T_{c h a r g e}}{2} = C_{L O A D} \times \frac{V_{B A T T}}{2}

where:

Equation 7.

I_{s t a r t} = \frac{I_{S C P} \times (T_{O N 1} + T_{O F F 1})}{2 \times (T_{O N 1} + T_{A U T O - R E T R Y})}

and

Equation 8.

I_{m i d} = \frac{I_{S C P} \times 2 \times T_{O N_m i d}}{2 \times (T_{O N_m i d} + T_{A U T O - R E T R Y})}

The time intervals TON1, TOFF1 and TON_mid can be calculated using Equation 9 to Equation 11:

Equation 9.

T_{O N 1} = \frac{L_{c a b l e} \times I_{S C P}}{V_{B A T T}}

Equation 10.

T_{O F F 1} = \frac{L_{c a b l e} \times I_{S C P}}{V_{D 1}}

Equation 11.

T_{O N_m i d} = \frac{L_{c a b l e} \times I_{S C P}}{(\frac{V_{B A T T}}{2})}

Substituting the known parameters V_BATT, L_cable, I_SCP, V_D1 and C_LOAD and solving for T_AUTO-RETRY gives a retry delay of <200µs to achieve a charging time of 10ms.

Figure 8 and Figure 9 show the application schematic and test setup to charge a 5mF load capacitance using the TPS1211-Q1 high-side driver. T_AUTO-RETRY is 180µs, which results in a charging time of 7ms, as shown in Figure 10.

Figure 8 Typical application schematic for driving a capacitive load.

Figure 9 Test setup using the TPS1211-Q1 evaluation module with a 1.5m cable harness.

Figure 10 Startup with a 5mF load capacitance using the TPS1211-Q1 in switching mode.