hacker

21-03-06, 06:18 AM

I'm looking at doing this new PSU of mine properly, and instead of picking random values for components, I'm going to try and do it properly. So, I've got the LM1086 datasheet out (I'm using those for the built-in pre-regulation) and am looking at the calculations necessary to get the best performance from the regs.

Basic facts:

- 25-0-25 c/t trafo -> dual schottky -> 10000uF kendeil (referred to as Cin)

- Input to lm1086 is therefore 36-37v

- R1 = 100R

- R2 = 2k49

- Adj pin tant = 10uf (referred to as Cadj)

- Output decoupling = 200uF (2x 100uF) Rubycon ZA (referred to as Cout)

The datasheet gives this formula for determining if the value of Cadj is suitable:

1/(2pi * ripple_frequency * Cadj) <= R1

For me, that gives:

1/(6.28 * 100 * 0.00001) = 159

Therefore, Cadj is not big enough. If I was to use a 22uF tant for Cadj, we get:

1/(6.28 * 100 * 0.000022) = 72.3

That's better...I think. Is my math ok, or have I done a brain fart somewhere? The other thing that worries me now is the low-Z of the Rubycon ZA caps on the output of the LM1086... From the datasheet:

Stability consideration primarily concern the phase response of the feedback loop. In order for stable operation, the loop must maintain negative feedback. The LM1086 requires a certain amount series resistance with capacitive loads. This series resistance introduces a zero within the loop to increase phase margin and thus increase stability. The equivalent series resistance (ESR) of solid tantalum or aluminum electrolytic capacitors is used to provide the appropriate zero (approximately 500 kHz).

It sounds from that paragraph like low-Z isn't wanted, but from here:

It is desirable to have large output capacitance for applications that entail large changes in load current (microprocessors for example). The higher the capacitance, the larger the available charge per demand. It is also desirable to provide low ESR to reduce the change in output voltage:

ΔV = ΔI x ESR

It is common practice to use several tantalum and ceramic capacitors in parallel to reduce this change in the output voltage by reducing the overall ESR. Output capacitance can be increased indefinitely to improve

transient response and stability.

So, what's the right answer??? :confused:

Carl

Basic facts:

- 25-0-25 c/t trafo -> dual schottky -> 10000uF kendeil (referred to as Cin)

- Input to lm1086 is therefore 36-37v

- R1 = 100R

- R2 = 2k49

- Adj pin tant = 10uf (referred to as Cadj)

- Output decoupling = 200uF (2x 100uF) Rubycon ZA (referred to as Cout)

The datasheet gives this formula for determining if the value of Cadj is suitable:

1/(2pi * ripple_frequency * Cadj) <= R1

For me, that gives:

1/(6.28 * 100 * 0.00001) = 159

Therefore, Cadj is not big enough. If I was to use a 22uF tant for Cadj, we get:

1/(6.28 * 100 * 0.000022) = 72.3

That's better...I think. Is my math ok, or have I done a brain fart somewhere? The other thing that worries me now is the low-Z of the Rubycon ZA caps on the output of the LM1086... From the datasheet:

Stability consideration primarily concern the phase response of the feedback loop. In order for stable operation, the loop must maintain negative feedback. The LM1086 requires a certain amount series resistance with capacitive loads. This series resistance introduces a zero within the loop to increase phase margin and thus increase stability. The equivalent series resistance (ESR) of solid tantalum or aluminum electrolytic capacitors is used to provide the appropriate zero (approximately 500 kHz).

It sounds from that paragraph like low-Z isn't wanted, but from here:

It is desirable to have large output capacitance for applications that entail large changes in load current (microprocessors for example). The higher the capacitance, the larger the available charge per demand. It is also desirable to provide low ESR to reduce the change in output voltage:

ΔV = ΔI x ESR

It is common practice to use several tantalum and ceramic capacitors in parallel to reduce this change in the output voltage by reducing the overall ESR. Output capacitance can be increased indefinitely to improve

transient response and stability.

So, what's the right answer??? :confused:

Carl