8.3.2 Quick Output Discharge (QOD)
The TPS22918-Q1 includes a QOD feature. The QOD pin can be configured in one of three valid ways:
- QOD pin shorted to VOUT pin. Using this method, the discharge rate after the switch becomes disabled is controlled with the value of the internal resistance RPD. The value of this resistance is listed in the Electrical Characteristics table.
- QOD pin connected to VOUT pin using an external resistor REXT. After the switch becomes disabled, the discharge rate is controlled by the value of the total resistance of the QOD. To adjust the total QOD resistance, Equation 1 can be used.
Equation 1. R
QOD = R
PD + R
EXT
where
- RQOD is the total output discharge resistance
- RPD is the internal pulldown resistance
- REXT is the external resistance placed between the VOUT and QOD pin.
- QOD pin is unused and left floating. Using this method, there is no quick output discharge functionality, and the output remains floating after the switch is disabled.
The fall times of the device depend on many factors including the total resistance of the QOD, VIN, and the output capacitance. When QOD is shorted to VOUT, the fall time changes over VIN as the internal RPD varies over VIN. To calculate the approximate fall time of VOUT for a given RQOD, use Equation 2 and Table 1.
Equation 2. V
CAP = V
IN × e
-t/τ
where
- VCAP is the voltage across the capacitor (V)
- t is the time since power supply removal (s)
- τ is the time constant equal to RQOD × CL
The fall times' dependency on VIN becomes minimal as the QOD value increases with additional external resistance. See Table 1 for QOD fall times.
Table 1. QOD Fall Times
| VIN (V) |
FALL TIME (μs) 90% - 10%, CIN = 1 μF, IOUT = 0 A , VON = 0 V(1) |
| TA = 25°C |
TA = 85°C |
| CL = 1 μF |
CL = 10 μF |
CL = 100 μF |
CL = 1 μF |
CL = 10 μF |
CL = 100 μF |
| 5.5 |
42 |
190 |
1880 |
40 |
210 |
2150 |
| 5 |
43 |
200 |
1905 |
45 |
220 |
2200 |
| 3.3 |
47 |
230 |
2150 |
50 |
260 |
2515 |
| 2.5 |
58 |
300 |
2790 |
60 |
345 |
3290 |
| 1.8 |
75 |
430 |
4165 |
80 |
490 |
4950 |
| 1.2 |
135 |
955 |
9910 |
135 |
1035 |
10980 |
| 1 |
230 |
1830 |
19625 |
210 |
1800 |
19270 |
(1) Typical values with QOD shorted to VOUT