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RT9177-27CBR

Ultra-Low-Noise 200mA LDO Regulator

Richtek Technology 

Preliminary

RT9177

Application Guides

Capacitor Selection and Regulator Stability

Like any low-dropout regulator, the external capacitors

used with the RT9177 must be carefully selected for

regulator stability and performance.

Using a capacitor whose value is >1µF on the RT9177

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 RT9177 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 RT9177 output ensures stability.

The RT9177 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 RT9177 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.

COUT = 4.7µF

COUT = 1µF

Fig. 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.

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.

DS9177-00 Feb. 2001

www.richtek-ic.com.tw

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