LM2641MTC-ADJ Datasheet, PDF (19/31 Pages) Texas Instruments – LM2641 Dual Adjustable Step-Down Switching Power Supply Controller

LM2641
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SNVS040B – JANUARY 2000 – REVISED APRIL 2013
L2 = 6.8 µH
R_L = 0.825Ω
DC gain = 55dB
The values of compensation components will be: C10 = 2200 pF, R11 = 8.2k, and C12 will not be used. Using
this data, the poles and zeros are calculated:
f_p(C10) = 1 / [2π X C10 (R11 + 160k) ] = 430Hz
f_z(R11) = 1 / [2π X R11 (C10 + C12) ] = 8.8kHz
f_p(C_OUT) = 1 / [2π X R_L X C_OUT] = 960Hz
f_z(ESR) = 1 / [2π X ESR X C_OUT] = 27kHz
f_p(HF) ∼ 40kHz
Using these values, the calculated gain plot is shown in Figure 23.
Figure 23. Calculated Gain Plot for 3.3V/4A Application
Looking at the plot, it can be seen that the unity-gain crossover frequency f_c is expected to be about 25kHz.
Using this value, the phase margin at the point is calculated to be about 84°.
To verify the accuracy of these calculations, the circuit was bench tested using a network analyzer. The
measured gain and phase are shown plotted in Figure 24.
Figure 24. Measured Gain/Phase Plot for 3.3V/4A Application
The measured gain plot agrees very closely to the predicted values. The phase margin at 0dB is slightly less
than predicted (71° vs. 84°), which is to be expected due to the negative phase shift contributions of high
frequency poles not included in this simplified analysis.
It should be noted that 70° phase margin with 25kHz bandwidth is excellent, and represents the optimal
compensation for this set of values for V_IN, V_OUT, inductor and R_L.
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