How to improve ripple

Keiichi Kajino

Expert 2365 points

Other Parts Discussed in Thread: LM3409HV

Hi, Folks

Following wavefrom is for LM3409HV.

Red : CSP-CSN

Magenta : FET Drain

Cyan: Inductor current

Although no ripple on FET Drain, the "CSP-CSN" have large ripple and some board shows unexpected off as below.

We have already review board pattern, but did not find any fatal problem.

Could you give us how to improve this ripple in order to get fine PWM?

over 9 years ago

0 Clinton Jensen over 9 years ago

TI__Mastermind 25500 points

This is a waveform we have seen many times. It's always caused by noise due to layout. Switchers can be very picky about component placement and routing, particularly for sensitive current sensing nodes. If you would like to attach the layout I'd be happy to look it over and let you know if I have any concerns.

0 Keiichi Kajino over 9 years ago in reply to Clinton Jensen

Expert 2365 points

Jensen,

Thank you for your reply. I already got gerber data.

Since this is customer's data, could you please mailto : keiichi_kajino@ktl-corp.co.jp and then I will reply.

0 Keiichi Kajino over 9 years ago

Expert 2365 points

Following figure is after board layout modification (2nd version). Observed slightly improvement, but in low temp, the device will be dis-continuous mode again.

Could you please tell me deglitch time for Isense comparator in order to know how should we reduce glitches?

0 Clinton Jensen over 9 years ago in reply to Keiichi Kajino

TI__Mastermind 25500 points

The blankout time is specified in the datasheet as the minimum on time for the switch. It sounds like it may be layout related so I will send you an e-mail if you would like to send me layout pics. But first double check component selection. You mention it gets worse at cold. Make sure all ceramic capacitors are rated for X5R/X7R or better. Y5V and those like it can lose a lot more capacitance over temperature. This will also be the case if you happen to have any aluminum electrolytic capacitors in the design.

0 Keiichi Kajino over 9 years ago in reply to Clinton Jensen

Expert 2365 points

I got answers by diret mails. Many thanks!

Power management

Power management forum

How to improve ripple