Building a DC-DC power supply

VINMAX	VINMIN	VOUT	IOUT	Frequency
5	5	3.3	0.5	500kHz

First calculate R_TOFF.

VPMOS	VNMOS	tOFF	RTOFF
45mV	35mV	673ns	66.3kΩ

Choose the value for LIR, then calculate L and I_PEAK.

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. If a calculated capacitor value is not available, choose the next larger standard size.

The inductor is selected primarily based on inductance value, DC resistance (DCR), 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 surface-mount types offer better performance, particularly at high switching frequencies. In our example we have a close match with a Coiltronics? Thin-Pac TP1-150, a 15μH inductor with 0.73A saturation current.

The input capacitor reduces both peak current drawn from the input supply, and 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 that 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 respond violently to large instantaneous voltage steps and high current surges so do not use tantalum for input bypassing where input power will be connected through a mechanical switch. Our circuit input is from a regulated supply, so we need not worry about that limitation. 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 and rated for 10V and 742mA ripple current. The input-capacitor ripple current in a step-down converter can be approximated as:

I_{RIPPLE_CIN (RMS)}= [I_OUT/V_IN][V_OUT(V_IN- V_OUT)]^-1/2

As a starting point, choose a capacitor that meets the recommended minimum value of 22μF and verify that it meets the required voltage rating. 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 t_OFFresistor have no special requirements; select them from the closest available standard values. To complete component selection, choose the remaining values from a typical application circuit or the evaluation (EV) kit schematic.

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 inTable 2, further corrections are not necessary because our R_TOFFand inductor choices have a minimal impact on the circuit's operating point.

Table 2. Checking actual component values

Calculate tOFFand frequency using selected RTOFFvalue.
RTOFF	tOFF	Frequency
68kΩ	688ns	488.8kHz
Calculate LIR and IPEAKusing selected L value.
L	LIR	IPEAK
15μH	30.3%	576mA