This original circuit (2x47uF) has a 150mV peak-to-peak (pk-pk) voltage deviation. When doubling the output capacitor to 4x47uF, we would expect the voltage deviation to be reduced to half. However, due to the limitation of simple internal compensation, increases in the output capacitance consequently reduce the converter loop bandwidth, thus adversely affecting the converter performance. Therefore, we measured 134mV pk-pk in this case.
So, with simple (limited) internal compensation, doubling the output capacitance only reduces the output load transient response peak to peak slightly to 89%. This 11% reduction figure doesn’t justify the cost and size increase for adding the output capacitor. As a system engineer, you are stuck with the original converter performance. Trying to improve output voltage deviation performance further incurs exponential cost and size increases.
Smart, wide-bandwidth internal compensation example
Figure 6a and 6b and Table 2 shows the schematic, evaluation kit board, and BOM of the buck converter with smart, wide-bandwidth internal compensation.
|C1||2.2μF ±10%, 100V X7R ceramic capacitor (1210)||1|
|C2||2.2μF ±10%, 10V X7R ceramic capacitor (0603)||1|
|C3||5600pF ±10%, 25V X7R ceramic capacitor (0402)||1|
|C4||47μF ±10%, 10V X7R ceramic capacitor (1210)||1|
|C5||0.1μF ±10%, 16V X7R ceramic capacitor (0402)||1|
|C7||47μF, 80V aluminum electrolytic capacitor (D = 10mm)||1|
|L1||6.8μH, 5A inductor. Coilcraft MSS1048-682NL.
Taiyo Yuden NS10165T6R8NNA
|U1||4.5V-60V, 2.5A, high-efficiency, synchronous step-down DC-DC converter with internal compensation||1|
Table 2. BOM for buck converter with wide-bandwidth internal compensation.
This particular converter is also a fixed-frequency peak current mode control device. However, it features smart, wide-bandwidth internal compensation. The switching frequency of its evaluation kit is defaulted to 450kHz. The internal compensation is optimized for: 24V input, 3.3V output, 450kHz switching frequency, and output capacitor of 47uF ceramic. Let’s note that this is only half the amount of the output capacitance on the other board discussed earlier. We’ll see later that this buck converter requires only half of the amount of output capacitance for a comparable output transient response because of its wide loop bandwidth.
To examine the performance of smart, wide-bandwidth internal compensation, let’s observe the converter responses to load step transient at various circuit operating conditions. Figure 7a shows a test result with the original configuration (47uF), while Figure 7b shows the result with twice the amount of output capacitor (2x47uF).
The original circuit (47uF) has a 176mV pk-pk voltage deviation. When doubling the output capacitor to 2x47uF, the voltage deviation reduces to 48% at 84mV pk-pk thanks to its smart, wide-bandwidth internal compensation. The well-behaved recovering wave shape of the output voltage after each deviation also indicates very stable loop operation. Furthermore, this converter requires only half of the amount of output capacitance for a comparable output transient response because of its wide loop bandwidth, when compared to the simple internal compensation solution.
Table 3 shows a side-by-side comparison of the simple internal compensation buck converter with the option featuring smart, wide-bandwidth internal compensation.
|Smart, Wide-Bandwidth Internal Compensation|
|Output Capacitance||Vo Deviation, pk-pk||Output Capacitance||Vo Deviation, pk-pk|
|Doubling Output Capacitance||4x47uF||134mV||2x47uF||84mV|
|Percentage Vo Deviation Reduction||11%||52%|
Table 3. Comparing two types of buck converters.
It should be noted that when measuring output voltage load transient response, a low-noise scope probe must be used. Figure 8 shows what was used to measure these voltages. Equipment used in this experiment includes: power supply-HP6032A, electronic load – Agilent 6060B, and oscilloscope – Tektronix TDS3034B. Load step slew rate: 1A/us, output voltage probe bandwidth, Ch4: 20MHz, output current probe bandwidth, Ch3: 300MHz.
Internal compensation helps reduce external component count and circuit complexity for higher integration power solutions. Simple internal compensation trades off the benefits with lower system performance and also might cause loop instability. Smart, wide-bandwidth internal compensation, which is available in the MAX17503, achieves the best of both worlds: high integration while maintaining optimum loop bandwidth and control loop stability, reducing size and cost by reducing the external output capacitance requirement.