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RT9167-15CB

Low-Noise, Fixed Output Voltage, 200mA/500mA LDO Regulator

Richtek Technology 

RT9167/A

Applications Guides

Capacitor Selection and Regulator Stability

Like any low-dropout regulator, the external capacitors

used with the RT9167/A must be carefully selected for

regulator stability and performance.

Using a capacitor whose value is > 1µF on the

RT9167/A input and the amount of capacitance can be

increased without limit. The input capacitor must be

located a distance of 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 LDOs

application. The RT9167/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 1uF with

ESR is > 5mΩ on the RT9167/A output ensures

stability. The RT9167/A still works well with output

capacitor of other types due to the wide stable ESR

range. Fig.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 RT9167/A and returned

to a clean analog ground.

www.richtek-ic.com.tw

8

Region of Stable COUT ESR vs. Load Current

100

COUT = 4.7µF

10

COUT = 1µF

1

0.1

0.01

0.001

0

40

80

120

160

200

Load Current (mA)

Fig. 1

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.

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.

Use a 10nF bypass capacitor at BP for low output

voltage noise. The capacitor, in conjunction with an

internal 200KΩ resistor, which connects bypass pin

and the band-gap reference, creates an 80Hz low-

pass filter for noise reduction. Increasing the

capacitance will slightly decrease the output noise,

but increase the start-up time. The capacitor

connected to the bypass pin for noise reduction must

have very low leakage. This capacitor leakage

current causes the output voltage to decline by a

proportional amount to the current due to the voltage

drop on the internal 200KΩ resistor. Fig. 2 shows the

power on response.

DS9167/A-10 July 2001

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