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LM3423 switching frequency

David Park
Expert 6710 points
Other Parts Discussed in Thread: LM3423

Hi,

In LM3423 datasheet, it has off timer and it is composed by RC curcuit on RCT pin.

I calculated switching frequency of LM3423 as below table.

When R=147kOhm, C=1nF

as below table, switching frequency is varying by input voltage.

but in datasheet, it is fixed to 25/RC

Could you explain detail about setting of switching frequency?

FYI, in below table, Toff and switching frequency is calculated as below.

Toff=-R*C*ln(1-Ref/Vin), fsw=(1-D)/Toff

Vin Ref Toff Vout Duty Frequency
7 0.28 6.00083E-06 46.8 0.850427 24925.314
8 0.32 6.00083E-06 46.8 0.82906 28486.073
9 0.36 6.00083E-06 46.8 0.807692 32046.832
10 0.4 6.00083E-06 46.8 0.786325 35607.591
11 0.44 6.00083E-06 46.8 0.764957 39168.35
12 0.48 6.00083E-06 46.8 0.74359 42729.109
13 0.52 6.00083E-06 46.8 0.722222 46289.868
14 0.56 6.00083E-06 46.8 0.700855 49850.627
15 0.6 6.00083E-06 46.8 0.679487 53411.386
16 0.64 6.00083E-06 46.8 0.65812 56972.146
17 0.68 6.00083E-06 46.8 0.636752 60532.905
18 0.72 6.00083E-06 46.8 0.615385 64093.664
19 0.76 6.00083E-06 46.8 0.594017 67654.423
20 0.8 6.00083E-06 46.8 0.57265 71215.182

Thanks.

  • 0
    TI__Mastermind 25500 points

    The equation you are using is incorrect. The RC is connected to the switch node, so the voltage it sees is Vout, not Vin. Toff should be changing with the input voltage and in your table it is constant. Replace Vin with Vout in your equation and the switching frequency should be much more constant although it will change some over such a wide range.