SLVAFO4 august   2023 TPS61299

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Overview
  5. 2Load Connected to Input Voltage
    1. 2.1 Body Diode Pass (TPS61288)
    2. 2.2 Force Pass Through (TPS61253)
    3. 2.3 Bypass (TPS61291)
    4. 2.4 Summary
  6. 3Load Disconnected to Input Voltage
    1. 3.1 Synchronous HSD FET With Switchable Body Diode (TPS61299)
    2. 3.2 Extra ISO FET to Cut-Off Leakage Path
  7. 4Summary
  8. 5References

2.1 Body Diode Pass (TPS61288)

When high-side FET is an N-MOS, the MOSFET’s body diode anode is connected to the inductor, as Figure 2-1 shows. When the device is shutdown, the input voltage connects the output load through the inductor and body diode, similar to non-synchronous boost during shutdown.

GUID-20230731-SS0I-5FXH-X3JG-RLXQJVN2GQDJ-low.svg Figure 2-1 Body Diode Pass During Shutdown

The output voltage equals the input voltage minus the voltage drop across the DCR of the inductor and body diode forward conduction voltage. Output voltage follows input voltage.

Equation 1. V O U T = V I N - I O U T × D C R - V D

In this situation, the output voltage is not zero even when boost is shutdown. Also, there is power loss and a voltage drop in the inductor and the body diode.

Equation 2. P l o s s = I O U T 2 × D C R + I O U T × V D

Meanwhile, the feedback resistor has leakage current since the output voltage is not zero which has an effect on the quiescent current. Take the TPS61288 as an example. Figure 2-2 shows the shutdown waveforms of TPS61288. The image shows that there is a distinct voltage drop between the output voltage and the input voltage. With a 50-Ω resistor load, there is still inductor current during shutdown.

GUID-20230821-SS0I-MG1F-JVHV-7ZH6SXRZZWL5-low.svg Figure 2-2 TPS61288 Shutdown Waveforms