LM555: Asking for the LM555 schematic reference design

SHH

Part Number: LM555
Other Parts Discussed in Thread: NE555, LMC555, TLC555

Hi Team,
Customer would like to use LM555 to send out PWM duty cuty 15% in 25KHz modulation frequency.
Could you provide the recommended RA and RB and C? is that ok to follow figure 14 circuit?

Thanks,
SHH

over 8 years ago

0 Ron Michallick over 8 years ago

TI__Guru**** 164476 points

SHH,

I'll start with duty cycle formula and solve for Ra
D = Rb / (Ra + 2Rb)
(Ra+2Rb) = Rb / D
Ra = Rb / D - 2Rb
Ra = Rb * (1/D - 2)
Ra = Rb * (1/15% - 2)
Ra = Rb * (1/15% - 2)
Ra = Rb * 14/3

Now I'll move to frequency formula and solve for Rb and C
F = 1.44 / [(Ra + 2Rb) *C]
(Ra + 2Rb)*C = 1.44 / F
(14/3*Rb + 2Rb) * C = 1.44 / F
20/3*Rb*C = 1.44 / F
Rb = 3/20*1.44/(F*C)

Based on chart 1nF to 10nF is good.
I'll pick 1nF
Rb = 3/20*1.44/(25kHz*1nF)
Rb = 8.64k; Ra = Rb*14/3 = 40.32k

However the formulas do not compensate for propagation delay.
For NE555, I substitute F * (1+F/350kHz) for F. LM555 correction will be similar.
New F used is 25kHz * (1+25kHz/350kHz) = 26.786 kHz
Rb = 3/20*1.44/(26.786kHz*1nF)
Rb = 8.064k; Ra = Rb*14/3 = 37.63k

0 SHH over 8 years ago in reply to Ron Michallick

TI__Mastermind 32126 points

Hi Ronald,

Thanks for great support.

Could you help review the schematic?

Thanks,

SHH

0 SHH over 8 years ago in reply to SHH

TI__Mastermind 32126 points

Hi Ronald,
Could you help review the schematic?
Thanks,
SHH

0 Ron Michallick over 8 years ago in reply to SHH

TI__Guru**** 164476 points

SHH,

I suggest a bypass capacitor between pins 8 & 1 (VCC and GND)

I want to test the timing components. I believe I can find time on Thursday to do this.

0 Ron Michallick over 8 years ago in reply to Ron Michallick

TI__Guru**** 164476 points

SHH,

I built the board with the values close to my Rb = 8.064k; Ra = 37.63k prediction but the output frequency was too low due to higher propagation delay than expected.

I picked new resistors scaled by observed frequency error and capacitor tolerance error. Using closest 1% resistor, the new Rb is 7.15k and Ra is 33.25k.

My test resistors were within 0.2% stated value however the capacitor was 8.1% higher than 1nF.

Therefore I expect the capacitor tolerance will be the largest variable for frequency but it should not affect duty cycle. The LM555 will add some frequency variability due to propagation delay variance.

I noticed that the duty cycle is for the low output. The output was low 16% of the time. If a ~15% output high time is needed then the output can be inverted.

Sample	Freq kHz	N DC %	Ideal cap
1	23.203	16.032	25.082
2	23.138	16.084	25.012
3	23.211	16.044	25.091
4	23.185	16.030	25.063
5	23.204	16.022	25.084
6	23.188	16.036	25.066

The samples match well, but they all came from the same bag.

0 Ron Michallick over 8 years ago in reply to Ron Michallick

TI__Guru**** 164476 points

SHH,

If I use the new values of Rb is 7.15k, Ra is 33.25k and C = 1nF using the data sheet formulas I get the following result.

Duty cycle is 15.05% ; remember this is a low time duty cycle.
Frequency is 30.32 kHz ; I had to try for 30kHz to get 25 kHz.
So LM555 at 25kHz has negative duty cycle that is 1% higher than formula predicts.

If accuracy is important then TLC555 or LMC555 would be better choices.