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LM3445

SNVS570L – JANUARY 2009 – REVISED MAY 2013

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Figure 22. Inductor Current Waveform in CCM

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

TI offers an LM3445 demonstration PCB for customer evaluation through our website. The following description

and theory uses reference designators that follow our evaluation PCB. The LM3445 Master/Slave schematics are

illustrated below (Figure 23 through Figure 25) 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 requires use of the Master/Slave circuit

for any number of slaves.

MASTER BOARD MODIFICATIONS

•

Remove R10 and replace with a BAS40 diode

•

•

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

SLAVE BOARD(S) MODIFICATIONS

•

Remove R11 (disconnects BLDR)

•

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 threshold. 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 converter.

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