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RT9202CS

Single Synchronous Buck PWM DC-DC Controller

Richtek Technology 

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

RT9202

Q

VI

L

VL

D

C

R VO

C.C.M.

TS

TON TOFF

VI - VO

VL

- VO

iL

µQ

µIL

IL = IO

iQ

IQ

iD

ID

Table 1

VOUT

Inductor

3.3V

2µH

5µH

2.5V

2µH

5µH

1000µF (ESR=53mΩ) 100mV 40mV 110mV 44mV

1500µF (ESR=33mΩ) 62mV 25mV 68mV 28mV

3000µF (ESR=21mΩ) 40mV 16mV 43mV 18mV

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

The suggested L and C are as follows:

2µH with ≥ 1500µF COUT

5µH with ≥ 1000µF COUT

Fig.5

1.5V

2µH

5µH

93mV 37mV

58mV 23mV

37mV 15mV

DS9202-02 August 2002

www.richtek.com

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