E007007.PDF

007

3-volts Car Adapter

A. Grace

This circuit is based on a standard LT1074CT switching regu-

lator IC. For sure, Application Note AN35 published by Linear

Technology describes the design far more elegantly than the

author could in this short article. Interested readers are there-

fore strongly advised to get a copy of AN35.

The schematic shows the LT1074CT used as a positive

step-down or ‘buck’ converter. The ‘switcher’ is used to con-

vert a +12-volt car battery voltage down to +3 volts for use

with the personal hi-ﬁ’s and handheld games for the author’s

two boisterous children on long car journeys. Note at under

ten years of age, children will rarely be hi-ﬁ aﬁcionado’s and

are generally not concerned with any noise generated by the

‘switcher ‘circuit.

The circuit is connected to the car +12-V system via the

cigarette lighter socket — is advisable to use a fused version

of the cigarette lighter plug. The +12-V arrives on the board

via screw- terminal block J2. Diode D2 provides a reverse volt-

age protection, while C3 decouples the input to the switcher IC.

The LT1074CT briskly switches the supply voltage on and off

in response to the signal applied to its F/B input, to the extent

that the average output voltage is at the required level. The

values of potential divider resistors R1-R3 have been chosen

to attenuate the output voltage so that there is 2.5 V at the F/B

pin. The difference between the attenuated output voltage and

the internal 2.5-V reference is used to control the modulation

IC1

270µH

VIN

VSW

1N4001

LT1074CT

GND

1N5818

470µ

000014 - 11

effect of the switcher. Components R2 and C2 provide fre-

quency stabilisation for the feedback loop. Inductor L1 along

with the LT1074CT form the main switching components,

while C1 provides decoupling for the output load. The 3-V out-

put voltage is taken from screw terminal J1.

With this circuit built, boxed up and installed in your car,

you can look forward to possibly your first ‘quiet’ long car

journey.

(000014-1)

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