RT9202CS datasheet(11/16 Pages) RICHTEK

11 / 16 page

RT9202

DS9202-02 August 2002

www.richtek.com

Applications Information

Inductor Selection

The RT9202 was designed for VIN = 5V, step-down

application mainly. Fig.5 shows the typical topology

and waveforms of step-down converter.

The ripple current of inductor can be calculated as

follows:

ILRIPPLE = (5V - VOUT)/L × TON

Because operation frequency is fixed at 300kHz,

TON = 3.33 × VOUT/5V

The VOUT ripple is

VOUT RIPPLE = ILRIPPLE × ESR

ESR is output capacitor equivalent series resistor

Table 1 shows the ripple voltage of VOUT: VIN = 5V

Table 1

Fig.5

VOUT

3.3V

2.5V

1.5V

Inductor

µH5µH2µH5µH2µH5µH

1000

µF (ESR=53mΩ)

100mV

40mV

110mV

44mV

93mV

37mV

1500

µF (ESR=33mΩ)

62mV

25mV

68mV

28mV

58mV

23mV

3000

µF (ESR=21mΩ)

40mV

16mV

43mV

18mV

37mV

15mV

*Refer to Sanyo low ESR series (CE, DX, PX…)

The suggested L and C are as follows:

µH with ≥ 1500µF COUT

µH with ≥ 1000µF COUT

TON

TOFF

VI - VO

- V

IL = IO

µQ

µI

C.C.M.

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