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TPS65217DEVM: Transient loading of LDO1

Part Number: TPS65217DEVM

Transient  loading of  LDO1

Transient loading of LDO_3 looks  as  expected but the transient load of LDO_1  looks more like a Buck than a LDO,( See screen dumps) could you explain why ?

  • Hi Lars,

    There are a number of possible reasons to explain difference between the two scope shots.

    First, the current usage is different. Since neither LDO_1 or LDO_3 has external feedback, there will be some voltage drop due to bond wire resistance. This voltage drop for LDO3 with 400 mA is approximately 7 mV, so we would expect voltage drop on LDO_1 to be approximately one quarter of that amount, so approximately 2 mV. Depending on actual packaging this could be even less. This explains the square wave appearance vs. flat line.

    Regarding the transient performance, LDO_3 on the EVM has 10 uF output capacitor while LDO_1 has 2.2 uF. The spike shown in the LDO_1 graph depends on the output capacitance. If it is increased, the spike size should decrease. LDO_3 transient response is hidden by load regulation voltage change.

    Finally, there are some bumps on the LDO_1 scope shot that do look similar to buck operation. How are you taking the measurement? They could be the result of noise injection / ground bounce due to one of the bucks on the board causing interference in your measurement. A small ground loop is necessary to reduce interference (for example, www.ti.com/.../slpa010.pdf shows how much noise can be picked up if using standard clip for measurement).