Including external bulk capacitor to an LDO

Ujwal Vijaykumar

Other Parts Discussed in Thread: TLV701, TPS62153, TPS62147

We’re using the LDO TLV70133DBVR in one of the Power Tools project and have the following two use cases:

External Capacitor added on 3V3 line:

External capacitor on 5V line:

How is the performance of the LDO affected in each of the above two cases? Kindly throw some light on the above so that we can proceed with the design.

over 6 years ago

0 Gerard Copeland over 6 years ago

TI__Genius 10490 points

Hi,

The TLV701 is stable with any output capacitor greater than 0.47 uF. It is also stable without any input capacitor. In regards to performance, increasing the size of the capacitors in either case will improve the transient response and output noise.

However, there is a potential problem when using a large output capacitor in the first case. The TLV701 has a PMOS pass transistor that leaves the device vulnerable to reverse (OUT to IN) current. For large output capacitors that take longer to discharge, the output voltage may exceed the input voltage by 0.7 V if VIN drops suddenly. As a result, the body diode of the pass PMOS can turn on and discharge the output capacitor very quickly, potentially damaging the device. If there is a possibility of this occurring in your application, Mark's blog post discusses some possible solutions here:

LDO Basics: Preventing reverse current

In most low-dropout regulators (LDOs), current flow is like one-way street – go in the wrong direction and major problems can occur! Reverse current is current that flows from V OUT to V IN instead of…

By Mark Sellers over 7 years ago in Technical articles > Power management

Thanks,

Gerard

0 Ujwal Vijaykumar over 6 years ago in reply to Gerard Copeland

Prodigy 140 points

Thank you for reverting with the critical comments. The document shared was quite helpful too.

Also, I wanted to know what would be the stability of the Switching regulator TPS62153RGTR when the bulk cap is connected on the 5V line (the 2nd case)?

0 Gerard Copeland over 6 years ago in reply to Ujwal Vijaykumar

TI__Genius 10490 points

Hi,

I will be looping in someone more familiar with our switching regulators to help you with your question regarding the TPS62153.

Thanks,
Gerard

0 Chris Glaser over 6 years ago in reply to Ujwal Vijaykumar

TI__Guru**** 150950 points

Hi Ujwal,

Is the 1000uF ceramic or does it have some ESR? ESR will help. This app note details the stability of this family: www.ti.com/.../slva463a.pdf

As well, we have the newer, smaller, more accurate, lower noise TPS62147/8 to use in place of the TPS62153.

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Including external bulk capacitor to an LDO