LD3986 SERIES

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APPLICATION INFORMATION

CURRENT LIMIT

The device includes short-circuit protection. It

includes a current limiter that controls the pass

transistor’s gate voltage to limit the output current

to about 600mA.

THERMAL OVERLOAD PROTECTION

The Thermal over load protection limits total

power dissipation in the device. When the junction

sensor sends a signal to the shutdown logic,

turning off the pass transistors and allowing the

device to cool. The pass transistors turns on again

after the device’s junction temperature typically

cools by 20°C, resulting in a pulsed output during

continuous thermal overload conditions.

POWER DISSIPATION

Maximum

power

dissipation

of

the

device

depends on the thermal resistance of the case

and circuit board, the temperature difference

between the die junction and ambient air, and the

rate of air flow. The power dissipated by the

device is:

The maximum power dissipation is:

Where:

The device’s pins perform the dual function of

providing an electrical connection as well as

channeling heat away from the die. Use wide

circuit-board

traces and large, solid

copper

polygons to improve power dissipation. Using

multiple vias to buried ground planes further

enhances thermal conductivity.

INPUT CAPACITOR

An input capacitance of

≈ 1µF is required between

the LD3986 input pin and ground (the amount of

the capacitance may be increased without limit).

This capacitor must be located a distance of not

more than 1cm from the input pin and returned to

a clean analog ground. Any good quality ceramic,

tantalum, or film capacitor may be used at the

input.

Important:

Tantalum

capacitors

can

suffer

catastrophic failures due to surge current when

connected to a low impedance source of power

(like a battery or a very large capacitor). If a

tantalum capacitor is used at the input, it must be

guaranteed by the manufacturer to have a surge

current rating sufficient for the application.

There are no requirements for the ESR on the

input capacitor, but tolerance and temperature

coefficient must be considered when selecting the

capacitor to ensure

the capacitance will be

≈ 1µF over the entire

operating temperature range.

OUTPUT CAPACITOR

The LD3986 is designed specifically to work with

very small ceramic output capacitors, any ceramic

capacitor (temperature characteristics X7R, X5R,

Z5U or Y5V) in 1 to 22 µF range with 5m. to 500m.

ESR range is suitable in the LD3986 application

circuit. it may also be possible to use tantalum or

film capacitors at the output, but these are not as

attractive for reasons of size and cost.

The output capacitor must meet the requirement

for minimum amount of capacitance and also have

an ESR (Equivalent Series Resistance) value

which is within a stable range.

NOISE BYPASS CAPACITOR

pin and ground significantly reduces noise on both

regulator outputs.

This cap is connected directly to a high impedance

node in the band gap reference circuit. Any

significant loading on this node will cause a

change on the regulated output voltage.

For this reason, DC leakage current through this

pin must be kept as low as possible for best output

voltage accuracy. The use of this 0.01µF bypass

capacitor is strongly recommended to prevent

overshoot on the output during start up.

The types of capacitors best suited for the noise

bypass

capacitor

are

ceramic

and

film.

High-quality ceramic capacitors with either NPO

or COG dielectric typically have very low leakage.

Polypropylene and polycarbonate film capacitors

are available in small surface-mount packages

and typically have extremely low leakage current.

Unlike many other LDO’s, addition of a noise

reduction capacitor does not effect the transient

response of the device.

TURN-ON/OFF INPUT OPERATION

Each LD3986 output is turned off by pulling the

relevant EN pin low, and turned on by pulling it

high. To assure proper operation, the signal