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Original question:

TPS54531: Request a Design review.

TPS54531: About some equations and a figure

Liu Yang
Expert 8685 points
Part Number: TPS54531
Other Parts Discussed in Thread: TPS54335A

Hi Team,

Please help me answer the follow questions, thank you so much.

question1:Equation of Output Capacitance

The value of Iripple in the Equation (13) described in the data sheet of TPS54531 is 1.9A from the calculation result of 14uF. If the condition of Fsw = 570 KHz, Voripple = 30 mV, Co = 14 uF is applied to the Equation (13), the value of I ripple will be 1.9 A. Where does this 1.9A come from? The peak inductor current is assumed to be 5.96A, but is it about 0.2-0.5 (general) of that value?

question2:Slope Compensation

Please tell me about the equation for resistor R3 which determines the slope compensation value of TPS54531. If the following value is put in the Equation (17), the value of Gpwrstg becomes -26.62. (We are calculating back). Where does this value come from? gmEA = 92 uA, Vref = 0.8, Vout =5, R = 37.4 KΩ

question3:The Crossover Frequency 

In the datasheet of TPS54531, there is the Equation (18) for selecting the capacity of capacitor C6. Among them, the crossover frequency 20KHz refers to the graph in Figure 11. Does the graph change when you change the coil capacity (4.7 uH) and the capacitor capacity (47 uF) on the user side? If the coil is in the range of 1 uH to 47 uH and the capacitor is in the range of 47 uF to 150 uF, refer to this graph and the crossover frequency should be 20 KHz?

Best Regards,

Tom.Liu

  • 0
    TI__Genius 10070 points

    Hi Tom,

    question1:Equation of Output Capacitance

    Selecting Kind=0.4, so the inductor current ripple is 0.4*5A=2A.

    question2:Slope Compensation

    Gpwrstg is 5.1dB, gmEA=92uA/V, Vout=5V, Vref=0.8V, you can get R3=37.7Kohm by equation (17).

    question3:The Crossover Frequency 

    Figure 11 is the bode plot of power stage and is related to gmps, Vout, Cout, Iout, and ESR of output capacitors. You can refer to page 18 of TPS54335A datasheet. 20kHz  is the intended crossover frequency. It is up to you.

    Shawn