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TPSM84624: Turbo Trans resistor value in table 8

Takahiro Saito
Expert 5465 points
Part Number: TPSM84624


Hi team,

RTT values in the table 8 in the datasheet might be calculated by equation (3). It seems that the RTT value in 0.8V < Vout < 1.8 is almost consistent with the result from the eq (3). However, there are some gaps in 3.3V < Vout < 7.5V between RTT value in table 8 and calculated value by eq (3). I calculated the RTT value as the following table.

Could you check if the RTT values in table 8 is correct ?

VOUT[V] KTT COUT1 Ceramic [uF] COUT2 BULK[uF] RTT[kΩ] RTT Datasheet[kΩ]
0.8 0.65 300 220 3.408 3.4
0.8 0.65 400 330 5.592 5.36
1 1 200 　 2 2
1 0.6 200 220 3.04 3.01
1 1 400 　 6 6.04
1 0.6 400 220 5.44 5.49
1.2 1.12 200 　 3.376 3.4
1.2 0.6 200 220 4.048 4.02
1.2 1.12 400 　 8.752 8.87
1.2 0.6 400 220 6.928 7.68
1.8 1.5 100 　 3.4 3.4
1.8 1.5 300 　 14.2 14.3
1.8 0.6 100 220 4.912 4.87
3.3 2 100 　 11.2 8.66
3.3 0.9 100 220 17.008 31.6
3.3 2 200 　 24.4 19.1
5 2 100 　 18 10
5 1.2 100 220 36.4 31.6
5 2 200 　 38 22.1
7.5 2.25 100 　 31.75 10.5
7.5 1.8 47 220 70.09 28

Regards,

Saito

  • 0
    TI__Guru 50120 points

    Hi Saito-san,

    Thank you for bringing this to my attention. Let me talk to the apps engineer that wrote the datasheet and get back to you on this.

    Regards,
    Jimmy

  • 0 in reply to Jimmy Hua
    TI__Guru 50120 points

    Hi Saito-san,

    Equation 3 requires the calculation of the total effective output capacitance which accounts for DC biasing of the capacitor.

    For the case of VOUT=3.3V; Ktt=2; Cout_ceramic=100uF, let's assume you are using 2 x 47uF ceramic capacitors. You will notice that at 3.3Vout, the output capacitance of a single 47uF is now 40uF. Therefore the total output capacitance is actually 80uF instead of 94uF. 

    Plugging that back into Equation 3 yields the following: Rtt = ((2*3.3V*80uF)/50 - 2) = 8.56kOhm which is closer to Table 8 recommended 8.66kOhm. By using the typical instead of the effective capacitance the error becomes more significant at higher Vouts. 

    Do note that in general ceramics will derate faster than polymer for a given voltage. 

    Hope this answer provides insight on why there appears to be a discrepancy between calculated and recommended.

    Regards,

    Jimmy 

  • 0 in reply to Jimmy Hua
    TI__Expert 5465 points
    Thanks! I understand.