LFC789D25

DUAL LINEAR FET CONTROLLER

SLLS565B − MARCH 2003 − REVISED SEPTEMBER 2004

5

POST OFFICE BOX 655303

APPLICATION INFORMATION

FUNCTIONAL DESCRIPTION

A linear voltage regulator can be broken down into four essential building blocks: a pass transistor, a voltage

reference, a feedback network, and a control circuit to drive the pass element, based on the comparison

between the output voltage (as sampled by the feedback network) and the voltage reference. With the exception

of the pass transistor, the -ADJ provides the other three building blocks needed. Thus, with minimal external

components and low overall solution cost, a designer can create two independent, tightly regulated output

voltages capable of delivering high currents in excess of 3 A (as limited by the external pass transistor). One

output is fixed at 2.5 V. The other output can be adjusted to any desired voltage via an externally applied signal

easy to implement a tracking regulator. See the typical application circuit (Figure 1).

internal reference

The fixed 2.5-V output controller uses an internal temperature-compensated bandgap reference centered at

1.2 V. Its tolerance is designed to be

<±2% over the specified temperature range, which, when coupled with the

low offset of the driver circuit, allows the 2.5-V output to have a tolerance of 2% over the specified temperature

range and full load.

regulate an output voltage that is some fraction of the external voltage source. And, because any changes in

the external voltage source are sensed by the voltage divider, the regulated output tracks those changes.

pin. This signal can be provided by a simple device such as the TL431 adjustable shunt regulator.

divider or the shunt regulator. The accuracy of the adjustable output depends on the accuracy of the signal

accuracy.

feedback network (SENSE pins)

that essentially halves the sensed output voltage and feeds it back to the controller for comparison to the internal

bandgap reference.

controller/driver

Both drivers essentially are error amplifiers that can output a worst-case minimum of 9 V (10.5 V at 25

°C) when

the LFC789D25 is powered by 12 V. This allows the controllers to regulate a large range of output

voltages, as limited by the threshold voltages of the external NMOS. Both drivers sample the output voltage via

a SEN pin. For the adjustable version, this SEN pin typically sources a current of 20 nA and, thus, has minimal

loading on the output voltage. For the 2.5-V version, this SEN pin sinks a current of approximately 125

µA

(including the currents through the internal resistor divider); this results in minimal loading on the output voltage.

Although not tested, both of these controllers are designed with very low offset (typically less than 4 mV),

resulting in very accurate control of the drive signals.