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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