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RT9181

150mA LDO Regulator with POG

Richtek Technology 

Preliminary

RT9181

power dissipation across the device is

P = IOUT (VIN – VOUT)

The GND pin of the RT9181 performs the dual

function of providing an electrical connection to

ground and channeling heat away, Connect the GND

pin to ground using a large pad or ground plane.

Capacitor Selection and Regulator Stability

Like any low-dropout regulator, the external

capacitors used with the RT9181 must be carefully

selected for regulator stability and performance.

Using a capacitor whose value is ≥ 1µF on the

RT9181 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. The RT9181 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

RT9181 output ensures stability. The RT9181 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 RT9181 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, 2.2µ 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

DS9181-00 February 2002

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 RT9181 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 and ESL of the output

capacitor. Increasing the output capacitor’s value and

decreasing the ESR and ESL attenuates the

overshoot.

Reverse Current Path

The power transistor used in the RT9181 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 damage

to the part.

Fig. 1

VIN

VOUT

Fig. 2

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