L1923 Datasheet, PDF (5/7 Pages) Unisonic Technologies – 500mA HIGH PSRR LDO REGULATORS

L1923
Preliminary
CMOS IC
„ APPLICATION INFORMATION
The UTC L1923 contains an error amplifier, 1.25V bandgap reference, PMOS output transistor, internal feedback
voltage divider, mode comparator, shutdown logic, over current protection circuit, and over temperature protection
circuit as shown in the block diagram.
The process of operation is like this: the mode comparator compares the voltage of SET pin with a pre-set voltage
reference of 0.35V. If the voltage of SET pin is smaller than 0.35V, the internal feedback voltage divider’s central tap
is connected to the error amplifier’s non-inverting input. Then, the error amplifier compares non-inverting input with
the 1.25V bandgap voltage reference. If the feedback voltage is larger than the voltage reference 1.25V, the error
amplifier’s output becomes higher. This makes the PMOS output transistor’s V_GS (gate-to-source voltage) smaller, in
that condition, the current carrying capability of the PMOS output transistor is reduced, as a result the output voltage
decreases until the feedback voltage becomes equal to 1.25V. Similarly, when the feedback voltage is less than
1.25V, the error amplifier causes the output PMOS to conductor more current to pull the feedback voltage up to
1.25V. Thus, through this feedback action, the error amplifier, output PMOS, and the voltage dividers effectively form
a unity-gain amplifier with the feedback voltage force to be the same as the 1.25V bandgap reference.
The output voltage, V_OUT, can be calculated as follows:
V_OUT = 1.25 (1 + R1/R2).
(1)
Alternatively, the relationship between R1 and R2 is expressed in the following:
R1 = R2 (V_OUT /1.25 - 1).
(2)
In order to reduce power dissipation and loop stability, it is recommended that R2 is 100KΩ. For UTC L1923-330,
R1 is 164K, and the pre-set V_OUT is 3.30V.
In Figure 2, the SET pin voltage will be larger than 350mV when external voltage divider functions in the operation.
The non-inverting input of the amplifier will be connected to the external voltage divider. The conditions of equations
1 and 2 are still true since the operation of the feedback loop is the same. Equation 1 still presents the output
voltage.
Over Current Protection
To monitor the output current, the UTC L1923 applies a current mirror. A portion of the PMOS output transistor’s
current is mirrored to a resistor such that the voltage across this resistor is proportional to the output current. Once
the output current exceeds limit threshold, UTC L1923 would be protected with a limited output current. Further more,
when the output is short to ground, the output current would be folded-back to a less limit.
Over Temperature Protection
Over-temperature protection circuit is designed to prevent this device from being damaged due to abnormal
temperature during operation. It is used to turn off the output MOSFET if the temperature is higher than 145°C, the
switch does not turn on until the temperature drops to 120°C.
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.
UNISONIC TECHNOLOGIES CO., LTD
www.unisonic.com.tw
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