SC4530 datasheet(10/23 Pages) SEMTECH

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Applications Information (Continued)

SC4530

For a given V

IN and inductance L, the continuous-

conduction switching frequency is:

CESAT

OUT

)

(

)

)(

(

)

)(

(

(6)

The minimum inductance is first found using Equation

(5). Next the switching frequencies are estimated at V

extremes using Equation (6). The inductance is then

adjusted for achieving desired switching frequency. The

resulting switch on-time at the maximum V

IN must exceed

the minimum controllable switch on-time, which can be

as high as 80ns. This prevents the inductor current from

running away when the output is shorted to ground.

Example: Select the inductor for a 3.3V output regulator,

with input voltage ranging from 0V to 26V. The desired

switching frequency is about 600kHz.

The minimum inductance is found using Equation (5):

)

(

MIN

Duty cycles and switching frequencies at the input

voltage extremes can be found using Equations (3) and (6)

respectively. The results for 22mH and 33mH are tabulated

(Table ).

Table 1. Estimated Switching Frequencies for 3.3V Output

Input Voltage

IN (V)

Duty Cycle

D (%)

Switching Frequency

f (kHz)

L = 22µH

L = 33µH

37.7

740

490

4.8

000

670

The 33mH inductance will be chosen, as it gives the desired

switching frequency range.

The resulting switch on-time is checked against the

minimum controllable switch on-time. The switch on-

time can be calculated using Equation (7) below:

OUT

CESAT

(7)

With L = 33µH, the switch on-time at 0V and 26V V

IN are

770ns and 220ns respectively, above the 80ns minimum

controllable on-time.

Table 2 lists some recommended inductor values for

various output voltages.

Table 2. Recommended Inductor Values

Output Voltage

OUT (V)

Inductor Value (µH)

IN = 6 V

IN = 30 V

2.5

3.3

5.0

The saturation current of the inductor should be at least

20%~30% higher than the peak inductor current. Low-

cost inductors with powder iron cores are not suitable for

high-frequency switching due to their high core losses.

Inductors with ferrite cores are recommended.

Input Capacitor Selection

A step-down regulator draws pulse current from the input

power supply. A capacitor placed between the supply and

the converter filters the AC current and keeps the current

drawn from the supply to a DC constant. The input

capacitance should be high enough to filter the pulse

input current. Its equivalent series resistance (ESR) should

be low so that power dissipated in the capacitor does

not result in significant temperature rise and degrade

reliability.

Multi-layer ceramic capacitors, which have very low ESR

(a few mW) and can easily handle high RMS ripple current,

are the ideal choice. A single 4.7µF (X5R or X7R) ceramic

capacitor should be enough for most applications. Using

a larger capacitor (for example, 0µF) will reduce SW

node jitters if the minimum input voltage is less than 0.7V

above the output voltage. For applications with high input

voltage, a small (µF ~ 2.2µF) ceramic capacitor can be

placed in parallel with a low ESR electrolytic capacitor to

satisfy both the ESR and bulk capacitance requirements.

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