L1923_11 Datasheet, PDF (6/7 Pages) Unisonic Technologies – 500mA HIGH PSRR LDO REGULATORS

L1923
Preliminary
CMOS IC
„ APPLICATION INFORMATION (Cont.)
Shutdown Mode
The UTC L1923 enters shutdown mode if the SHDN pin is applied to a logic low voltage. Under the shutdown
state, all the analog circuits are turned off completely, which reduces the current consumption to only the leakage
current. The UTC L1923 output pass transistor would get into high impedance level. There is an internal discharge
path to help to shorten discharge delay time.
Operating Region and Power Dissipation
The UTC L1923 is a linear regulator, so the power dissipation can be expressed by: P = I_OUT (V_IN – V_OUT). And the
maximum power dissipation is calculated by:
P_D(MAX) = (T_J–T_A)/θ_JA,=(150°C -25°C)/240°C/W= 520mW
(Where (T_J–T_A) is the temperature difference the die and the ambient air; θ_JA, is the thermal resistance of the
chosen package to the ambient air.)
For surface mount device, heat sinking is accomplished by using the heat spreading capabilities of the PC board
and its copper traces. The die attachment area of the UTC L1923’s lead frame is connected to pin 2, which is the
GND pin. Therefore, the GND pin of UTC L1923 can carry away the heat of the UTC L1923 die very effectively.
Connecting the GND pin to ground using a large ground plane near the GND pin can improve the power
consumption.
Capacitor Selection and Regulator Stability
A 1µF capacitor on the input and a 4.7µF capacitor on the output are recommended for most applications of the U
TC L1923. To get a better supply-noise rejection and transient response, input capacitors with larger values and
lower ESR are needed. A higher-value input capacitor (10µF) may be necessary if large, fast transients are
anticipated and the device is located several inches from the power source.
Power-Supply Rejection and Operation from Sources Other than Batteries
The UTC L1923 is particularly designed to deliver low dropout voltages and low quiescent currents in battery-
powered systems. Power-supply rejection is 42dB at low frequencies. As the frequency increases above 20kHz, the
output capacitor is the major contributor to the rejection of power-supply noise.
When operating from sources other than batteries, improve supply-noise rejection and transient response by
increasing the values of the input and output capacitors, and using passive filtering techniques.
Input-Output (Dropout) Voltage
The minimum input-output voltage differential (or dropout voltage) of a regulator determines the lowest usable
supply voltage. In battery-powered system applications, this will determine the useful end-of-life battery voltage.
Because the UTC L1923 use a P-channel MOSFET pass transistor, their dropout voltage is a function of R_DS(ON)
multiplied by the load current.
UNISONIC TECHNOLOGIES CO., LTD
www.unisonic.com.tw
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