LMR14030-Q1: Webench Design

Hi Mark,

Do you have waveform? the spike is on Vin or Vout?

B R

Andy

Hi Andy, attached is the waveform.

Hi Mark

The Vin has very fast rising slew rate, it may cause output overshoot at startup.

1. Place resistor divider on EN to program the startup voltage at about 15V

2. or add RC delay circuit on EN to startup at Vin ramp up to steady.

B R
Andy

Hi Andy, 

According to my client, they  gave the voltage divider a try (picked R_ENT = 110k and R_ENB = 10k using the formulas on page 12 of the datasheet), with no reduction in the initial spike: Then  added an RC delay circuit to that, and while I can definitely see V_EN rising slowly on the scope when disconnected from the circuit, once I connect it to the regulator's EN pin and start things up it still has an immediate initial spike (visible both on V_EN and on V_OUT). They think it must have something to do with the DC adapter jack I'm using. If they instead connect power via a jumper cable on a breadboard, the RC delay circuit correctly delays the regulator EN. But if they plug in the jack to provide power, there's that initial spike.

CH1 = V_OUT, CH2 = V_EN

Thank you.

Hi Mark,

Can you capture the waveform of VIN, EN,SW and VOUT at same time? set scope BW=20MHz

Pls use 110K and 10K divider on EN pin, and parallel 0.1uF on low-side 10K.

B R

Andy

Hi Andy, 

Attached are the waveforms. /cfs-file/__key/communityserver-discussions-components-files/196/sn_5F00_customerservice_5F00_case_5F00_fd24144847aa0190d4f52e61e36d43a9_5F00_attachments.zip

Unfortunately they don't have a 4 channel scope, so they could only capture two at a time. But here they are, with a 110Kohm and 10Kohm voltage divider on the EN pin, with a 0.1uF cap in parallel with the low side 10Kohm.

CH1 = V_IN, CH2 = V_EN And then, separately recorded, with an external trigger on V_IN,

CH1 = V_SW, CH2 = V_OUT, And finally, just in case,  recorded that last one again with 10x probes: BW limit enabled for all of those.

Thank you.

Hi Mark,

Why not mark the channel information the scope waveform?

On SDS0001, is CH1=VIN and CH2=EN? is it measured with C1=10uF or 0.1uF? EN waveform is not correct as its voltage has fixed dividing ratio to VIN

On the schematic, is CIN rating 50V or higher? the CFF 470pF should be used.

B R
Andy

Sorry for the typo, the CFF 470pF should NOT be used.

B R
Andy

Hi Andy,

CH1=VIN and CH2=EN for SDS0001. It's a "104" labeled cap, so yes, 0.1uF.
Not sure how to get a better reading on EN -- that should be the correct waveform. Even when probing it as a single channel that's what I see, so I don't think it's cross-probe interference.

For CIN I have www.mouser.com/.../CL10B104KB8NNNC (10uF) in parallel with www.mouser.com/.../CL10B104KB8NNNC (0.1uF),

both of which are 50 VDC rated. And I've already removed CFF for all these waveforms. 

Hi Mark,

SDS0001 waveform was tested with R_ENT = 110k and R_ENB = 10k, 0.1uF is parallel on RENB, right?

Vin=19V, EN=1.6V is correct at steady sate, but the EN has spike up 4.6V at Vin ramp up, there is RC low pass filter on EN pin, this is not correct unless Vin has spike up to 55V, can you check why this happen?

B R

Andy