RT9184ACH View Datasheet(PDF) - Richtek Technology

Part Name

Description

Manufacturer

RT9184ACH

500mA Dual LDO Regulator

Richtek Technology 

RT9184

Preliminary

Applications Information

Capacitor Selection and Regulator Stability

Like any low-dropout regulator, the external

capacitors used with the RT9184 must be carefully

selected for regulator stability and performance.

Using a capacitor whose value is greater than 1µF on

the RT9184 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 with 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 RT9184 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 on the RT9184

output ensures the stability. The RT9184 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 RT9184 and returned with 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.

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 RT9184 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.

Input-Output (Dropout) Voltage

A regulator’s minimum input-output voltage

differential (or dropout voltage) determines the lowest

usable supply voltage. In battery-powered systems,

this will determine the useful end-of-life battery

voltage. Because the RT9184 uses a P-channel

MOSFET pass transistor, the dropout voltage is a

function of drain-to-source on-resistance [RDS(ON)]

multiplied by the load current.

Reverse Current Path

The power transistor used in the RT9184 has an

inherent diode connected between each regulator

input and output (see Fig.1). If the output is forced

above the input by more than a diode-drop, this diode

will become forward biased and current will flow from

the VOUT terminal to VIN. This diode will also be

turned on by abruptly stepping the input voltage to a

value below the output voltage. To prevent regulator

mis-operation, a Schottky diode could be used in the

applications where input/output voltage conditions

can cause the internal diode to be turned on (see

Fig.2). As shown, the Schottky diode is connected in

parallel with the internal parasitic diode and prevents

it from being turned on by limiting the voltage drop

across it to about 0.3V < 100mA to prevent damaging

the part.

VIN

VOU T

Fig. 1 VOUT Structure of RT9184

www.richtek-ic.com.tw

6

DS9184-00 March 2002

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