RT9182FPES datasheet(8/9 Pages) RICHTEK

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RT9182

DS9182-17

March 2007

www.richtek.com

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 by the manufacture.

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.

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

RT9182 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 RT9182 has an inherent

diode connected between each regulator input and output

(see Figure2). 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 Figure3). 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.

VIN

VOUT

VIN

VOUT

Figure 2

Figure 3

Figure 1

Region of Stable Cout ESR v.s Load Current

0.001

0.01

0.1

100

120

160

200

Load Current (mA)

COUT = 1uF

COUT = 4.7uF

Instable

Stable

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