LMZ35003 / Maximum soft start time

S.Satoshi

Other Parts Discussed in Thread: LMZ35003, LMZ31710

Hi,

Please let me ask about maximum soft start time. I believe that there is no limitation for maximum soft start time, but I'd like to ask if you have any concern at too slow start-up setting. Actually I plan to set soft start time to 100msec.

Best Regards,
Sonoki / Japan Disty

over 9 years ago

0 J Arrigo over 9 years ago

TI__Mastermind 28020 points

100 msec SS time is OK.

If the SS cap is too big, it will take longer to discharge the SS cap during any shutdown condition. SS cap must be fully discharged before the device can be powered up again and start switching.

0 S.Satoshi over 9 years ago in reply to J Arrigo

Guru 21685 points

Hi Arrigo,

Can you also let me know the equation to calculate SS time from Css? I checked some datasheet of LMZ3 series but I couldn't find it.

Best Regards,
Sonoki

0 S.Satoshi over 9 years ago in reply to S.Satoshi

Guru 21685 points

Hi Arrigo,

I found the equation for LMZ31710 in following thread but it looks not be applied for LMZ35003. So please let me know it for LMZ35003.

Best Regards,
Sonoki

0 J Arrigo over 9 years ago in reply to S.Satoshi

TI__Mastermind 28020 points

Hi Sonoki,

The equation for the slow start time for the LMZ35003 is Css(nF) = ((Tss(ms) X 2)/0.8). There is a 10nF slow start capacitor internal to the device.

0 S.Satoshi over 9 years ago in reply to J Arrigo

Guru 21685 points

Hi Arrigo,

I tried to verify the Css in Table 6 in datasheet by using your equation, but the result of internal capacitor looks be away from10nF. For example, at the case of 17 msec in Table 6, The calculated Css is 42.5 nF and the listed Css is 22nF, so the internal capacitor should be 20.5nF.

Can you explain the reson of this difference?

Best Regards,

Sonoki

Power management

Power management forum

LMZ35003 / Maximum soft start time