7 Design Example and Experimental Validation with TPS62933F

Figure 7-1 is a proposed design flow for PCM converter with second stage filter. All the inductance and capacitance used in calculation are effective value considering degrading.

To clarify, f_cross≤f_SW/10 for (C_o+C₂) calculation in Figure 7-1 is a conservative restriction to simplify the design flow. The bandwidth can be set higher according to actual conditions.

A design example is given with operating condition as: Vin=24V, Vout=1.2V, f_sw=500kHz, I_out=3A. Targeted output ripple pk-pk amplitude V_{o2-ripple-target} is smaller than 1mV.

Based on the Equation 16 in TPS62933F data sheet, L=2.2uH is selected with K=0.345 (K is the ripple ratio of the inductor current: ΔI_L / I_{OUT_MAX}).

With Equation 26 for TPS62933F, we can get the lower limitation of (C_o+C₂) as 105.8uF by setting f_cross=50kHz, which equals to f_SW/10. This set bandwidth is also much larger than f_Z-EA of TPS62933F 10.6kHz^[5].

Pre-select C_o=69uF (47uF+22uF) and C₂=47uF. The estimated f_cross is 45.6kHz with Equation 26.

Calculate L₂ lower limit with equation 7 in Part I SLVAFD4: L₂≥8.2nH.

Calculate L₂ upper limit with Equation 24 in this application note: L₂<109nH.

Here we select two ferrite beads for verification of both limits: BLE18PS080SN1 with 15.3nH@1MHz close to lower limit and BLM18SN220TN1 with 103.4nH@1MHz close to upper limit.

For the feedback network, firstly select R₁=5kΩ and R₂=10kΩ following equation (1) in TPS62933F data sheet. Using the tool in appendix A for making f_Zff > f_cross, we can get C_ff=620pF corresponding to f_Zff=48.3kHz for the case with L₂=15.3nH, and C_ff=470pF corresponding to f_Zff=47.4kHz for the case with L₂=103.4nH.

Figure 7-2 to Figure 7-4 show the experimental verification results of the power design with BLE18PS080SN1. The ripple amplitude is within 1mVpp in Figure 7-3 and can satisfy the requirement. Figure 7-4 is the load transient waveform when changing output current from 0.75A (25% of 3A) to 2.25A (75% of 3A) with 2.5A/us slew rate, which shows the good loop response.

Figure 7-5 to Figure 7-7 show the experimental verification results of the power design with BLM18SN220TN1. With larger L₂ value, the ripple amplitude is further reduced to within 1mVpp in Figure 7-6. Figure 7-7 is the load transient waveform when changing output current from 0.75A (25% of 3A) to 2.25A (75% of 3A) with 2.5A/us slew rate, which shows the good loop response. The details can be found in TI reference design 3.8V to 30V Input, 3A, 1.2V Low-Output-Ripple Power Supply With Second Stage Filter Reference Design.