LM3445MM datasheet(14/26 Pages) NSC

14 / 26 page

MASTER/SLAVE OPERATION

Multiple LM3445s can be configured so that large strings of

LEDs can be controlled by a single triac dimmer. By doing so,

smooth consistent dimming for multiple LED circuits is

achieved.

When the FLTR1 pin is tied above 4.9V (typical), preferably

to VCC, the ramp comparator is tri-stated, disabling the dim

decoder. This allows one or more LM3445 devices or PWM

LED driver devices (slaves) to be controlled by a single

LM3445 (master) by connecting their DIM pins together.

MASTER/SLAVE CONFIGURATION

National Semiconductor offers an LM3445 demonstration

PCB for customer evaluation through our website. The fol-

lowing description and theory uses reference designators that

follow our evaluation PCB. The LM3445 Master/Slave

schematics are illustrated below (figures 13 - 15) for clarity.

Each board contains a separate circuit for the Master and

Slave function. Both the Master and Slave boards will need

to be modified from their original stand alone function so that

they can be coupled together. Only the Master LM3445 re-

quires use of the Master/Slave circuit for any number of

slaves.

MASTER BOARD MODIFICATIONS

•

Remove R10 and replace with a BAS40 diode

•

Connect TP18 to TP14 (V

CC)

•

Connect TP17 (gate of Q5) to TP15 (gate of Q2)

SLAVE BOARD(S) MODIFICATIONS

•

Remove R11 (disconnects BLDR)

•

Tie TP14 (FLTR1) to V

MASTER/SLAVE(S) INTERCONNECTION

•

Connect TP19 of Master to TP10 of Slave (Master VCC

Control)

•

Connect TP6 (DIM pin) of Master to TP6 (DIM pin) of Slave

(Master DIM Control)

MASTER/SLAVE THEORY OF OPERATION

By placing two series diodes on the Master VCC circuit one

forces the master VCC UVLO to become the dominant thresh-

old. When Master VCC drops below UVLO, GATE stops

switching and the RC timer (>200 µs) rises above the TL431

threshold (2.5V) which in turn pulls down on the gate of the

Slave pass device (Q1).

The valley-fill circuit could consist of one large circuit to power

all LM3445 series connected, or each LM3445 circuit could

have a separate valley-fill circuit located near the buck con-

verter.

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