RT9182ACE datasheet(7/10 Pages) RICHTEK

7 / 10 page

Preliminary

RT9182

DS9182-02 April 2002

www.richtek-ic.com.tw

Applications Information

Capacitor Selection and Regulator Stability

any

low-dropout

regulator,

the

external

capacitors used with the RT9182 must be carefully

selected for regulator stability and performance.

Using a capacitor whose value is > 1

µF on the

RT9182 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

LDO applications (see Fig.1). The RT9182 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 1

µF with ESR is > 5mΩ on the

RT9182 output ensures stability. The RT9182 still

works well with output capacitor of other types due to

the wide stable ESR range. 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 RT9182 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 2.2

µF or more to ensure

stability at temperatures below -10°C in this case.

Also, tantalum capacitors, 2.2

µF 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

the

manufacture.

Use a 10nF bypass capacitor at BP pin for low output

voltage noise. The capacitor, in conjunction with an

internal resistor, which connects bypass pin and the

band-gap reference, creates a low-pass filter for

noise reduction. Increasing the capacitance will

slightly decrease the output noise, and it is almost

independent of 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.

Load-Transient Considerations

The RT9182 load-transient response graphs 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.

Fig. 1

COUT = 4.7

µF

COUT = 1

µF

Region of Stable COUT ESR vs. Load Current

Load Current (mA)

120

160

200

100

0.1

0.01

0.001

RT9182ACE Datasheet(PDF) 7 Page - Richtek Technology Corporation

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