LM1238 datasheet(8/24 Pages) NSC

8 / 24 page

Pin Descriptions and Application Information

No.

Pin Name

Schematic

Description

V Flyback

Required for OSD synchronization and is also

used for vertical blanking of the video outputs.

The actual switching threshold is about 35% of

CC. For logic level inputs C4 can be a jumper,

but for flyback inputs, an AC coupled

differentiator is recommended, where R

V is large

enough to prevent the voltage at pin 1 from

exceeding V

CC or going below GND. C4 should

be small enough to flatten the vertical rate ramp

at pin 1. C

24 may be needed to reduce noise.

REF Bypass

Provides filtering for the internal voltage which

sets the internal bias current in conjunction with

EXT. A minimum of 0.1 µF is recommended for

proper filtering. This capacitor should be placed

as close to pin 2 and the pin 4 ground return as

possible.

REF Current Set

External resistor, 10k 1%, sets the internal bias

current level for optimum performance of the

LM1238. This resistor should be placed as close

to pin 3 and the pin 4 ground return as possible.

Analog Ground

This is the ground for the analog portions of the

LM1238 internal circuitry.

Blue Video In

Red Video In

Green Video In

These video inputs must be AC coupled with a

.0047 µF cap. Internal DC restoration is done at

these inputs. A series resistor of about 33

Ω and

external ESD protection diodes should also be

used for protection from ESD damage.

PLL Ground

PLL V

The ground pin should be connected to the rest

of the circuit ground by a short but independent

PCB trace to prevent contamination by

extraneous signals. The V

CC pin should be

isolated from the rest of the V

CC line by a ferrite

bead and bypassed to pin 8 with an electrolytic

capacitor and a high frequency ceramic.

LM1238 Datasheet(PDF) 8 Page - National Semiconductor (TI)

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