1.2MHZ MICROPOWER SYNCHRONOUS STEP-UP CONVERTER

2007-4-19

7/9

FSP3603

the switching frequency, L is the inductance value and D is the duty cycle.

Larger inductors mean less inductor current ripple and usually less output voltage ripple. Larger inductors also

mean more load power can be delivered. But large inductors are also with large profile and costly. The inductor

ripple current is typically set for 20% to 40% of the maximum inductor current. When selecting an inductor, the DC

current rating must be high enough to avoid saturation at peak current. For optimum load transient and efficiency,

the low DCR should be selected. Table 2 lists some typical surface mount inductors that meet target applications

for the FSP3603:

Part

Number

L

(µH)

Max

DCR

(mΩ)

Rated

D.C.

Current

(A)

Size

WxLxH

(mm)

Sumida

CR43

4.7

10

108.7

182

1.15

1.04

4.3x4.8x3.

5

Sumida

CDRH4D

28

4.7

5.6

6.8

10

72

101

109

128

1.32

1.17

1.12

1.00

5.0x5.0x3.

0

Toko

D53LC

4.7

6.8

10

45

68

90

1.87

1.51

1.33

5.0x5.0x3.

0

Output Capacitor Selection

The output capacitor is required to keep the output voltage ripple small and to ensure regulation loop stability. A

2.2µF to 10µF output capacitor is sufficient for most applications. If output capacitor is larger than 10µF, a phase

lead capacitor must be included to maintain enough phase margin. The output capacitor must have low impedance

at the switching frequency. Ceramic capacitors with X5R or X7R dielectrics are recommended due to their low ESR

and high ripple current ratings.

The input capacitor reduces the surge current drawn from the input and switching noise from the device. A minimum

4.7µF input capacitor is needed for most applications. The input capacitor impedance at the switching frequency

should be less than input source impedance to prevent high frequency switching current passing to the input. A low

ESR input capacitor sized for maximum RMS current must be used. Ceramic capacitors with X5R or X7R

dielectrics are highly recommended because of their low ESR and small temperature coefficients.

Output Diode Selection

An Shottky diode should be included when the output voltage is above 4.5V. The Schottky diode is optional for the

output voltage not more than 4.5V, but can improve efficiency by about 2% to 3%.