**Part Number:**LM5168

**Other Parts Discussed in Thread:**LM5017, , LM5169

Hello,

I am designing a Flybuck converter with three outputs, spec as follows :

1. Input: 20 – 60Vdc

2. Outputs : (a) Non-isolated(main) 5Vdc, 50mA with LDO to get 3.3V

(b) Isolated(1) 6.5Vdc, 150mA with LDO to get 5V

(c) Isolated(2) 6.5Vdc, 50mA with LDO to get 5V

(For ease of calculations, b & c can be considered as single output of 6.5V, 200mA)

3. Operating Freq : 680KHz

We are considering the LM5017/5018 as I have used the same earlier for a lower power design or the LM5168/51699 as they are the newer/suggested devices. Exact part to be finalized based on current requirement and availability.

I have designed the transformer for LM5168 using the datasheet design example iand the calculator spreadsheet - attached.

The design arrived at is - Primary L of 58uH for Fsw of 680KHz.

Using larger Tr.primary L of 84uH would lead to a lower Iripple of 70mA(28%) and hence lower Ipk of 320mA - so LM5168 may be adequate, since 5169 is out of stock.

Other values - Cout 33uF, Ca 2200pF, Cb 1000pf, Ra 100k giving injected ripple of 27mV which should be adequate.

Hence required transformer specs would be L = 84uH, primary saturation current ~400mA, N1-N2/3 ratio of 1:1.3.

But there are multiple aspects which need clarification.

1. The 516x is suggested as newer replacement for the 501x and hence the design process would be same / similar.

However, when I tried to cross verify the design using different guides - both datasheets examples, AN2292(snva674c) and 501x EVK as examples there is wide variation in the results achieved, specifically the value of 'K' and delta-'I' calculated or to be assumed.

While Rt calculation for both families is different and clear, please advise which document is best suited to design for respective family.

2. Most ref. designs/examples - 5168 datasheet & EVK, 5017 example etc. arrive at/use 33uH primary inductance and are for Fsw of 750KHz.

All BoMs specify 33uH transformer from Coilcraft, Wurth etc for the same. But the transformer datasheets specify L values at 100KHz - i.e. selected device is 33uH @ 100KHz.

Will not using the same device at 750KHz effectively reduce the L 'seen' by 516x/501x i.e. make the 33uH, 33/7.5 = 4.4uH ?

Is this scaling built into the design math? i.e. required L is scaled to 100KHz in the math?

We will be using a custom transformer - so what specs should I be giving to the manufacturer - 84uH @ 680KHz or 84uH @100KHz?

Using a basic overview (I may be totally wrong about this so please advise), impedance of L of 84uH @ 100KHz calculated at 680KHz comes to 358 ohms.

It would seem that for min i/p of 20V and o/p of 5V, such imedance value will not allow 250mA of primary load current. Is this assumption accurate?

3. Based on the above design is 5168 adequate or is 5169 required? When would it be availe in at least smaller qty - ~200nos.

Please advise at the earliest possible.

Thanks.

4454.LM5169_calc_revD.xlsm