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Original question:

BQ25700A: How to design the parameters for the COMP1 and COMP2

BQ25700A: How to design the parameters for the COMP1 and COMP2

Volker Placke
Prodigy 140 points
Part Number: BQ25700A

Hello support-forum,

with the BQ25700A we try to reduce the power dissipation of the input shunt (Rac) by reducing the value to 4 milliohms. Now the input current regulation is unstable.

Is it possible to reduce the input shunt to such a low value or will the compensation not be sufficient? How can the compensation components be calculated for the low shunt value?

It the datasheet the shunt is just given in two values (10 milliohms and 20 milliohms), as I understand this is because of the parameter range in the RSNS_RAC register.

For reduced power dissipation we increased the vlaue of the inductor to 6.8 millihenry. The ripple-current and therefore the power dissipation in the inductor will be reduced.

The datasheet recommends values for the inductor in the range 2.2 micro henry to 3.3 microhenry for a switching frequency of 800 kilohertz. Is it possible to increase the inductor value to 6.8 microhenry and how must the compensation be adjusted?

The resistor at the pin IADPT is used to "read the inductor value through the ADPT pin" (datasheet 8.3.4.1). Is it correct, to set the resistance to 240 kiloohm to set the inductance to 6.8 microhenry (extrapolation of the values in table 3)?

Regards,

Volker

  • 0
    TI__Genius 10240 points
    Volker,

    The Rac sensed current is part of the control signal. When you reduce the Rac, the signal is smaller and could cause stability issue. You should be able to find inductor with lower inductance and lower DCR. Are you using 6.8uH to reduce the switching loss? I would think the benefit would be insignificant. Since the EVM design has been extensively verified, we would like to recommend the same design as the stability could not be guaranteed for different operation conditions with new design parameters.
  • 0 in reply to ezhao
    Prodigy 140 points

    Hello ezhao,

    thank you for your answer.

    I would like to get some information, how the compensation has to be designed, to improve the stability of the circuit.

    Can you give information about the gain and the bandwidth of the internal error amplifiers?

    Which compensation pin (COMP1, COMP2) is for which regulation loop?

    Thank you for information in advance!

    Regards,

    Volker

  • 0 in reply to Volker Placke
    TI__Genius 10240 points
    Volker,

    Mihir will discuss with you for this topic further.