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RT9167

Low-Noise, Fixed Output Voltage, 300mA/500mA LDO Regulator

Richtek Technology 

RT9167/A

Load-Transient Considerations

The RT9167/A 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 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.

Shutdown Input Operation

The RT9167/A is shutdown by pulling the EN input low,

and turned on by driving the input high. If this feature is

not to be used, the EN input should be tied to VIN to keep

the regulator on at all times (the EN input must not be left

floating).

To ensure proper operation, the signal source used to

drive the EN 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 RT9167/A input voltage or another logic

supply. The high-level voltage may exceed the

RT9167/A input voltage, but must remain within the

absolute maximum ratings for the EN pin.

Internal P-Channel Pass Transistor

The RT9167/A 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 RT9167/A 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 RT9167/

A 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 RT9167/A has an inherent

diode connected between the regulator input and output

(see Figure 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 Figure4).

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

Figure 3

VIN

VOUT

Figure 4

DS9167/A-26 March 2007

www.richtek.com

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