Design Note 198: Optimizing a DC/DC Converter's Output Capacitors with the LTC1435A
All DC/DC power supplies comprise closed-loop systems. In any closed-loop system, control theory dictates the need for adequate gain and phase margin for overall system stability. Trade-offs have to be made between phase and gain margin and transient response by adjusting the feedback gain for a given power stage.
Some power IC controller manufacturers have designed their products with internal loop compensation. Hence, the user is forced to select power stage components (mainly COUT) to meet stability criteria. The LTC1435A step-down DC/DC controller, on the other hand, allows the power stage component values to be chosen based simply on power requirements and allows feedback gain to be set independently, thus allowing minimization of expensive bulk output capacitors. This important design freedom results from the OPTI-LOOP® architecture that makes the ITH compensation point available in all Linear Technology DC/DC controllers.
Figures 1 through 6 show the output transient response of an LTC1435A circuit (application schematic in Figure 7) using various kinds of output bulk capacitors. The amplitude of the transient response is less than ±100mV. In each case, the number of output bulk capacitors used was chosen to produce 50mV or less of output voltage ripple at ILOAD = 3.0A. Figure 8 shows the phase and gain margin for the application circuit in Figure 7 for COUT = 47μF ×2, 6.3V OS-CON capacitors. It can be observed from Figure 8 that the loop crosses 0dB at 21.8kHz and has a phase margin of 47.3°, which is more than enough for the loop to be unconditionally stable.
Figure 1. Transient Response of LTC1435A. COUT = 2 w 1500μF Sanyo VGX, CC1 = 100pF, CC2 = 100pF, RC = 33k, IL = 0.5A TO 3A, VIN = 12V, VOUT = 3.3V.
Figure 2. Transient Response of LTC1435A. COUT = 2 w 47μF OS-CON, CC1 = 470pF, CC2 = 100pF, RC = 22k, IL = 0.5A TO 1.2A, VIN = 12V, VOUT = 3.3V.
Figure 3. Transient Response of LTC1435A. COUT = 1 w 47μF OS-CON, CC1 = 1000pF, CC2 = 100pF, RC = 15k, IL = 0.5A TO 1.2A, VIN = 12V, VOUT = 3.3V.
Figure 4. Transient Response of LTC1435A. COUT = 2 w 47μF POSCAP, CC1 = 1000pF, CC2 = 100pF, RC = 22k, IL = 0.5A TO 1.8A, VIN = 12V, VOUT = 3.3V.
Figure 5. Transient Response of LTC1435A. COUT = 1 w 47μF Panasonic, SP, CC1 = 1000pF, CC2 = 100pF, RC = 15k, IL = 0.5A TO 1.2A, VIN = 12V, VOUT = 3.3V.
Figure 6. Transient Response of LTC1435A. COUT = 2 w 100μF NEOCAP, CC1 = 180pF, CC2 = 100pF, RC = 47k, IL = 0.5A TO 2A, VIN = 12V, VOUT = 3.3V.
Figure 7. LTC1435A Constant Frequency, High Efficiency Converter.
Figure 8. Loop Gain and Phase vs Frequency.
For each output capacitor type, the feedback loop compensation was adjusted for similar phase margin and dynamic performance. Note that the compensating resistor and capacitor values are not the same for each output capacitor configuration. Clearly, a fixed internal loop-compensation scheme does not allow optimization for all applications. For any general purpose power supply controller, the ability to tailor the feedback loop to the power path offers a significant advantage to the designer.
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