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TPS566235: Some questions for Figure 19 in the datasheet

Oba
Genius 17555 points
Part Number: TPS566235


Hello


I have some questions about TPS566235.

 
1.
Could you tell me the purpose of R2(49.9Ohm) in Figure 19?
Is this absolutely required? How much tolerance is required for this 49.9ohm?

2.
Is C10 in the same Figure 19 C_FF in Table 3?
Is 100pF reasonable selection for 3.3V out?

3.
In the same figure 19, could you tell me the purpose of R1 and R5?

4.
Can PG be pulled-up to VOUT though 100K ohm?

5.
The datasheet recommend to use 20Kohm:90Kohm for R_LOWER:R_UPPER for 3.3V out in Table 3.
Is there any minimum requirement for these resistance value?
For example, is it OK to change to 15K:68K or 1.5K:6.8K?

Regards,
Oba

  • 0
    TI__Expert 5470 points

    Hi Oba,

    1. R2 is for bode plot test. It represents a place for resistor to test bode plot. The resistance choose is determined by the loop test equipment. For normal operation, you can use 0ohm resistor.

    2. Yes. C10 in Figure 19 is feedforward capacitor, same as CFF in Table 3. 100pF CFF is reasonable for 3.3V Vout.

    3. R1 is the Rboot connected with Cboot (C4) to control the charging rate of bootstrap capacitor (C4). With larger R1, it can help to slow the slew rate of SW switching. In some special cases, for example, if the PCB layout isn't good enough and the input voltage (Vin) in your application is close to the abs Vin, you find that the maximum voltage of SW ring is close to the abs SW value on our datasheet in test. You can choose to change the R1 to several ohm, then the maximum SW ring voltage will reduce since the slew rate is reduced. Normally, you can just use the 0 ohm to have the fastest slew rate and smallest switching loss.

    R5 is the Rff connected with Cff. Cff generates a zero and a pole in the loop. Rff can further adjust the location of the zero and the pole. Normally you can just use 0ohm Rff and use Cff according to our recommended components value.

    4.Yes. PG is open drain circuit and you can make it pulled up to source other than VCC.

    5. I recommend you to use value which is closed to the recommended value for three reasons: first, if you use Cff in the circuit, the change of feedback divider resistance will affect the location of added zero and pole. You can refer to the application note: 

    Second too small FB resistor will increase some loss and decrease efficiency especially at light load in eco mode. Third, there's internal circuit inside IC. Too small or too large FB resistor might cause some impedance mismatching issue. But from my experience, changing 20kohm to 15kohm is OK and can be accepted.

    Regards,

    Andrew