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RT9177-32CB

Ultra-Low-Noise 200mA LDO Regulator

Richtek Technology 

RT9177

Preliminary

T

CH1

1>

CB = 470pF

2.2nF 10nF

CH2

2>

TT VIN = 3.6V, CIN = 1µF, COUT = 4.7µF

500µS/Div

CH1: 2V/div; CH2 = VOUT, 500mV/div

Fig. 2

Load-Transient Considerations

The RT9177 load-transient response graphs (see

Typical Operating Characteristics) 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 1mA to 100mA is

tens mV, depending on the ESR of the output

capacitor. Increasing the output capacitor’s value

and decreasing the ESR attenuates the overshoot.

Shutdown Input Operation

The RT9177 is shutdown by pulling the SHDN input

low, and turned on by driving the input high. If this

feature is not to be used, the SHDN input should be

tied to VIN to keep the regulator on at all times (the

SHDN input must not be left floating).

To ensure proper operation, the signal source used to

drive the SHDN input must be able to swing above and

below the specified turn-on/turn-off voltage thresholds

which guarantee an ON or OFF state (see Electrical

Characteristics). The ON/OFF signal may come from

either CMOS output, or an open-collector output with

pull-up resistor to the RT9177 input voltage or another

logic supply. The high-level voltage may exceed the

RT9177 input voltage, but must remain within the

absolute maximum ratings for the SHDN pin.

Internal P-Channel Pass Transistor

The RT9177 features a typical 1.1Ω P-channel

MOSFET pass transistor. It provides several

advantages over similar designs using PNP pass

transistors, including longer battery life. The P-channel

MOSFET requires no base drive, which reduces

quiescent current considerably. PNP-based regulators

waste considerable current in dropout when the pass

transistor saturates. They also use high base-drive

currents under large loads. The RT9177 does not

suffer from these problems and consume only 80µA of

quiescent current whether in dropout, light-load, or

heavy-load applications.

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 RT9177 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 RT9177 has an

inherent diode connected between the regulator

input and output (see Fig.3). 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

should be used in any applications where

input/output voltage conditions can cause the

internal diode to be turned on (see Fig.4). 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. < 100 mA to prevent damage to the part.

www.richtek-ic.com.tw

8

DS9177-00 Feb. 2001

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