L1913

Preliminary

CMOS IC

UNISONICTECHNOLOGIESCO.,LTD

6 of 9

www.unisonic.com.tw

QW-R502-439.a

APPLICATION INFORMATION

The UTC L1913 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 0.12 V. If the voltage of SET pin is smaller than 0.12 V, 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

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.

(1)

Alternatively, the relationship between R1 and R2 is expressed in the following:

(2)

In order to reduce power dissipation and loop stability, it is recommended that R2 is 100KΩ. For UTC L1913

In Figure 2, the SET pin voltage will be larger than 120mV 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.