nPM1300 evaluation kit LDO mode questions

[Poki Huang]

Hi

We have nPM1300 evaluation kit, we use SW1 VOUT1 1.8V as VINLDO1 input, we remove jumper JP7(for LS_LDO_OUT1 connect to R8) on P15 connector, and provide 1kohm loading on LS_LDO_OUT1

Several questions as below

1. At LDO mode, can we set VOUTLDO1 the same 1.8V as VINLDO1?

2. If 1 is yes, is it still LDO config? Or it equivalent to load switch config?

3. At LDO mode, we set  VOUTLDO1 1.8V and see the voltage dropout is about 0.08V , is it normal & acceptable?

4. At LDO mode, we set VOUTLDO1 1.2V, we can see frequency peak about 6KHz as below pic.

    a, The ~6KHz peak is because SW1 VOUT1 in PFM mode? 

    b. The VOUTLDO1 also have ~6KHz peak, since it's LDO mode, I think the VOUTLDO1 should be stable and not affected by VINLDO1, or it is a normal situation for nPM1300 LDO?

Thank you

Poki

[Poki Huang]

Hi

Add question 5

At LDO mode, we set  VOUTLDO1 1.8V

When we turn on LDO1, there is a inrush current as below pic.

The max current value is ~222mA and the duration time is ~200us

The 222mA is over LDO spec. 50mA, so will it damage nPM1300?

Or it is a normal situation for nPM1300 LDO?

Thank you

Poki

[Tomi Koskela]

Hello,

1. 2. 3. In LDO mode the minimum input voltage is 2.6V

4. LDO operates in sampled mode when buck is in PFM mode and no charging enabled. This sampling can cause some ripple on the output voltage. You could check if this ripple you see goes away if you start charging for example. Then LDOs are in continuous mode and you should not see the LDO reference refresh caused ripple anymore.

This ripple can be also caused by the buck converter operating in PFM mode depending how you are powering LDOs etc. and what is the load of the buck. Care should be taken also how to ground the oscilloscope probe to avoid GND bounce affecting the measurement. You can experiment forcing buck to PWM mode to see how it effects the ripple. 

5. Yes there can be high current pulse on the startup to charge the output capacitors. No damage to the nPM1300 will be caused by it, the behavior is normal.