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RT9168

200mA/500mA Fixed Output Voltage LDO Regulator

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RT9168/A

Application Information

Capacitor Selection and Regulator Stability

Like any low-dropout regulator, the external capacitors used

with the RT9168/A must be carefully selected for regulator

stability and performance.

Using a capacitor whose value is > 1μF on the

RT9168/A input and the amount of capacitance can be

increased without limit. The input capacitor must be

located not more than 0.5" from the input pin of the IC

and returned to a clean analog ground. Any good quality

ceramic or tantalum can be used for this capacitor. The

capacitor with larger value and lower ESR (equivalent

series resistance) provides better PSRR and line-

transient response.

The output capacitor must meet both requirements for

minimum amount of capacitance and ESR in all LDO

applications. The RT9168/A is designed specifically to

work with low ESR ceramic output capacitor in space-

saving and performance consideration. Using a ceramic

capacitor whose value is at least 1μF with ESR is > 5mΩ

on the RT9168/A output ensures stability. The

RT9168/A still works well with output capacitor of other

types due to the wide stable ESR range. Figure 1 shows

the curves of allowable ESR range as a function of load

current for various output voltages and capacitor values.

Output capacitor of larger capacitance can reduce noise

and improve load-transient response, stability, and PSRR.

The output capacitor should be located not more than

0.5" from the VOUT pin of the RT9168/A and returned to a

clean analog ground.

Note that some ceramic dielectrics exhibit large

capacitance and ESR variation with temperature. It may

be necessary to use 2.2μF or more to ensure stability at

temperatures below -10°C in this case. Also, tantalum

capacitors, 2.2μF or more may be needed to maintain

capacitance and ESR in the stable region for strict

application environment.

Region of Stable Cout ESR v.s Load Current

100

Unstable Region

COUT = 4.7uF

10

Stable Region

COUT = 1uF

1

0.1

0.01

Unstable Region

0.001

0

40

80

120

160

200

Load Current (mA)

Figure 1

Tantalum capacitors maybe suffer failure due to surge

current when it is connected to a low-impedance source

of power (like a battery or very large capacitor). If a

tantalum capacitor is used at the input, it must be

guaranteed to have a surge current rating sufficient for

the application by the manufacture.

Load-Transient Considerations

The RT9168/A load-transient response graphs (see Typical

Operating Characteristics) show two components of the

output response: a DC shift from the output impedance

due to the load current change, and the transient response.

The DC shift is quite small due to the excellent load

regulation of the IC. Typical output voltage transient spike

for a step change in the load current from 0mA to 50mA is

tens mV, depending on the ESR of the output capacitor.

Increasing the output capacitor's value and decreasing the

ESR attenuates the overshoot.

Shutdown Input Operation

The RT9168/A is shutdown by pulling the EN input low,

and turned on by driving the input high. If this feature is

not to be used, the EN input should be tied to VIN to

keep the regulator on at all times (the EN input must not

be left floating).

DS9168/A-15 March 2007

www.richtek.com

7

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