LMR33640: CFF calculation

Jeferson Pehls

Part Number: LMR33640

Hello.

About LMR33640, at page 21/44 of the datasheet is shown this equation.

My question is about the value of Cout. For example, if I am going to use 3x 22uF (22uF nominal) ceramic capacitors at output. Then at the equation should I input 66uF? or should I input the effective capacitance of them (summed) at the DC bias (at the calculated output voltage)?

Regards.

11 months ago

0 Colin Chen 11 months ago

TI__Genius 10941 points

Hi Sir,

I think you can input 66uF under the premise of enough DC rating cap, thanks.

Colin

0 Jeferson Pehls 11 months ago in reply to Colin Chen

Expert 1125 points

Hello, thanks!

Could you help me with this? Are the calculations correct?

------------------------------------
FEEDBACK RESISTORS FOR 8V OUTPUT
------------------------------------
Vout = 8V
Vfb = 1V
Ratio = (8-1)/1 = 7
Resistors = 820K and 115K (100K + 15K)
Vout = 1+1*(820/115) = 8.13V
8.13V / (935K) = 8.7uA

FEEDFORWARD CAPACITOR
CFF < (Vout * Cout) / (120 * Rfbt * sqrt(Vref/Vout))
CFF < (8.13 * Cout) / (120 * 230000 * sqrt(1/8.13))

Output capacitors = 3x the capacitor with this description:
Capacitor ceramic SMD, 22uF, +-10%, 25V, 1206, X5R, -55°C ~ 85°C, CL31A226KAHNNNE (Samsung)
LINK: product.samsungsem.com/.../CL31A226KAHNNN.do
Effective capacitance @ 8.1V ~= 8.8uF (~ -60%)

Calculations of CFF with 22uF and 8.8uF:
22uF:
CFF < ( (8.13 * (22*10^-6)) / (120 * 935000 * sqrt(1/8.13)) ) *10^12
CFF < 4.54pF
8.8uF:
CFF < ( (8.13 * (8.8*10^-6)) / (120 * 935000 * sqrt(1/8.13)) ) *10^12
CFF < 1.81pF (consider this)

------------------------------------
FEEDBACK RESISTORS FOR 3.3V OUTPUT
------------------------------------
Vout = 3.3V
Vfb = 1V
Ratio = (3.3-1)/1 = 2.3
Resistors = 230K (220K + 10K) and 100K
Vout = 1+1*(230/100) = 3.3V
3.3V / (330K) = 10uA

FEEDFORWARD CAPACITOR
CFF < (Vout * Cout) / (120 * Rfbt * sqrt(Vref/Vout))
CFF < (3.3 * Cout) / (120 * 230000 * sqrt(1/3.3))

Calculations of CFF with 22uF and 17.6uF:
22uF:
CFF < ( (3.3 * (22*10^-6)) / (120 * 230000 * sqrt(1/3.3)) ) *10^12
CFF < 4.77pF
17.6uF:
CFF < ( (3.3 * (17.6*10^-6)) / (120 * 230000 * sqrt(1/3.3)) ) *10^12
CFF < 3.82pF (consider this)

0 Colin Chen 11 months ago in reply to Jeferson Pehls

TI__Genius 10941 points

Hi Sir,

As you said: "Output capacitors = 3x the capacitor with this description", the Cout should input 3x22uF=66uF.

Thanks

Colin

0 Jeferson Pehls 11 months ago in reply to Colin Chen

Expert 1125 points

Hello Colin.

Actually, what I wanted to ask is, does those results of the calculations for the CFFs of my previous post, with few pF values, does these values make sense for you?

Thanks.

0 Colin Chen 11 months ago in reply to Jeferson Pehls

TI__Genius 10941 points

Hi Sir,

Generally, CFF are often range from tens of pF to hundreds of pF. From what Cout you choose, the CFF should be:

CFF < ( (3.3 * (22*10^-6*3)) / (120 * 230000 * sqrt(1/3.3)) ) *10^12
CFF < 14.3pF
17.6uF:
CFF < ( (3.3 * (17.6*10^-683)) / (120 * 230000 * sqrt(1/3.3)) ) *10^12
CFF < 11.46pF (consider this)

B R

Colin

0 Jeferson Pehls 11 months ago in reply to Colin Chen

Expert 1125 points

I understood my mistake now. (multiplying by 3 capacitors).

Thank you so much.

0 Colin Chen 11 months ago in reply to Jeferson Pehls

TI__Genius 10941 points

You are very welcomed.

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LMR33640: CFF calculation