Discharge that output rail?


A power supply’s output discharge is either a feature or a bug, depending on your point of view.  On the one hand, your load may have a requirement on the ramp down rate of its supply voltage.  If this rate is too slow, the microcontroller for example, may possibly have a memory loss or latch up.  Therefore, an active output discharge is one way to ensure that the supply voltage decreases as fast as required by the system when the power supply is disabled.

On the other hand, low power or energy harvesting applications may require lowest output leakage when the power supply is disabled.  For example, consider a solar panel application.  A power supply might take energy from the panel and charge a battery or super cap.  But when a cloud passes by and reduces the light on the solar panel, the power supply will likely shut down from lack of input power.  In this case, it is very frustrating to see that power supply discharge your battery or super cap through an active discharge circuit!

What is the system designer to do?  Does he or she need to add diodes and other circuitry to achieve the desired functionality?  The ideal solution to this discharge dilemma is a configurable device that discharges the output when that is desired and goes high impedance otherwise—without any extra, complicated circuits.  But which devices support this configurability? 

Enter the TPS62120 and TPS62130A family.  The TPS62120 is an ultra-low power (11-µA IQ), wide-Vin (2-15V input) device ideal for energy harvesting applications.  It contains an SGND pin, which is connected directly to the output when output discharge is desired.  If output discharge is not desired, just leave it floating.  No diodes or transistors needed!

The TPS62130A family (TPS62130A, TPS62140A, and TPS62150A) is slightly more complicated.  Beyond the TPS62120, only a single resistor is needed to set the discharge rate.  These devices use the PG (power good) pin output to control the discharge.  When these devices are disabled, the PG pin goes low and sinks current.  With the proper choice of resistor value, the discharge rate can be programmed.  If discharge is not needed, just don’t use a small value resistor or leave the PG pin floating altogether.  Alternatively, the TPS62130, TPS62140, and TPS62150 (non-A versions) do not hold the PG pin low when disabled and are pin to pin compatible.

How has output discharge helped or hurt your designs?

 

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