www.analog.com POWER CAPACITORS 15 www.power-mag.com Issue 2 2025 Power Electronics Europe 470 F POSCAPs on the output side. With a 19A load step (75% of full load) and a slew rate of 19A/ s, 136mVp-p of the transient response was as shown in Figure 3. On AVP implementation, an AVP compensation circuit is applied on COMP as shown in Figure 4, but the RC compensation is not required. At the half load (12.5A), purposefully set the output voltage to a nominal value (1V) by finetuning the R2. On the load transient response, 95mVp-p of the VOUT was obtained as shown in Figure 5. The transient performance has been improved. With the setting output voltage 1V at 25A (full load), the load power is 25W. By decreasing the output reference voltage (VREF) and VOUT feedback are connected to the positive and negative inputs of the error amplifier, respectively. VHI (or INTVCC) connected with RHI supplies the appropriate DC voltage to the amplifier output (ITH or COMP) that keeps the output from going into saturation. RLO (feedback resistor) is placed from the output (ITH) to the negative input (or FB). Therefore, RLO dominates the gm amplifier gain. RHI and RLO values should be much higher than R1 and R2. The load line Equation 1 is shown as: Ki is the current sense gain, and RSENSE is the current sense resistor value (or inductor DCR value for DCR sensing). Compared with the AVP shunt compensation circuit1, the advantage of the series compensation circuit is that the load line is dependent on R1/RLO gain and almost independent of the tolerance of the error amplifier transconductance (gm). IC processes and designs are vast. Unfortunately, some ICs’ gm values have a part-to-part variation as large as ±30%, plus the shunt compensation circuit AVP has its load line directly proportional to 1/gm gain. As a result, the shunt AVP suffers a poor load line. AVP Solution on the LTM4650-2 Regulator On the LTM4650-2 (current-mode synchronous buck regulator), a nominal 1V output capable of delivering 25A load with about ±8% (a 160mVp-p) transient window. On this conventional regulator (non-AVP), an external RC filtering circuit is required to achieve fast Type II control loop compensation. There is a bank of 5 100 F ceramic capacitors + 2 RIGHT: Figure 3. Load transient waveform of non-AVP circuit, 136 voltage transient, COUT1 = 5 100 F ceramic, and COUT2 = 2 470 F POSCAPs. RIGHT: Figure 4. Circuit with AVP (series compensation circuit). RIGHT: Figure 5. Load transient waveform of Figure 4 circuit with AVP, 95mVp-p output voltage transient. COUT1 = 5 100 F ceramic and COUT2 = 2 470 F POSCAPs. voltage to 0.945V at 25A load, the load power is now 23.6W, and the new savings is 1.4W for a single output. For the two outputs, the total net savings is 2.8W. With the AVP implementation, the two POSCAPs can be replaced by two ceramic capacitors, so a total of 7 100 F ceramic capacitors are used on the COUT1. The benefit of using a ceramic capacitor is that it has lower equivalent series resistance (ESR), equivalent series inductance (ESL), cost, smaller size, and more reliable performance. The transient performance has been improved, and the measured result was 104mVp-p of the VOUT as shown in Figure 6.
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