建设电源工程

VINMAX	VINMIN	VOUT	IOUT	Frequency
5	5	3.3	0.5	50kHz
First Calculate RTOFF
VPMOS	VNMOS	tOFF	RTOFF
45mV	35mV	673ns	66.3kΩ
Choose the value for LIR, then calculate L and IPEAK
LIR_INIT	L	IPEAK
30%	14.8μH	575mA
Next the output capacitor parameters are calculated
IRIPPLE	ESR_MIN	COUT_MIN
529.6mA	0.22Ω	6.73μF
Soft-start Calculations
tSS	CSS
100ms	0.22μF

Component Selection

Using values from Table 1, select the DC-DC converter's peripheral components. Check the data sheet recommendations to ensure each component's suitability for the task. If the calculated inductor value is not available, choose the next smaller standard value available. If a calculated capacitor value is not available, choose the next larger standard size available.

The inductor is selected primarily by inductance value and the peak-current requirement. Also make sure that the inductor is designed to operate at the desired switching frequency. If that data is not provided, choose another inductor for which the data is available. Inductors are available in surface-mount and through-hole versions, but in general the surface-mount types offer better performance. In our example, we have a very close match with a Coiltronics Thin-Pac TP1-150, which is a 15μH inductor with 0.73A saturation current.

The input capacitor reduces peak current drawn from the input supply, and reduces radiated noise to other elements of the system. Most data sheets either suggest specific values or give equations for calculating the input capacitor value. Make sure the capacitor is specified with a ripple-current rating near the selected switching frequency. For our 500kHz example, the capacitor could be an organic electrolyte, organic polymer, ceramic, or tantalum type.

Tantalum capacitors can be sensitive to instantaneous voltage changes, so don't use tantalum if the input capacitor is connected to a mechanical switch. Our circuit with its fixed-5V input comes from a regulated supply, so we don't have to worry about that problem. We therefore select a capacitor that meets the ripple-current rating and voltage requirements, such as an AVX TPS series 100μF capacitor in the C size case, rated for 10V and 742mA ripple current.

For loop stability, some DC-DC converters (not the one in this example) require the output capacitor to specify a minimum equivalent series resistance (ESR). If that specification is interpreted as a maximum instead of a minimum, you may lose many hours before finding the error.

The example circuit is designed to operate with a regulated +5V input, so in this case a tantalum capacitor is the best choice. Choose (as a starting point) a capacitor that meets the recommended minimum value of 0.22μF, and verify that it meets the required voltage and capacitance values. For our example, the AVX TPS series 33μF capacitor in the C size case is rated for 10V operation. Its maximum ESR is 0.375Ω, which is close to the target.

The soft-start capacitor and the tOFF resistor have no special requirements, and are selected from the closest available standard values. To complete the component-selection process, choose the remaining values from a typical application circuit.

Spreadsheet Revisited

Because capacitors and inductors come in a finite number of standard values, the closest available value can differ from the calculated value by more than 20%. In that case, the spreadsheet should be recalculated with actual values to verify that the circuit still meets its design objectives. As shown in Table 2, further corrections are not necessary because our RTOFF and inductor choices have a minimal impact on the circuit's operating point.

Table 2. Checking actual component values