LM3445 datasheet(17/46 Pages) TI2

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tOFF

tON

IL2 (t)

IAVE

IL2-PK

'iL

IL2-MIN

= D =

tON

tON + tOFF

= tON x fSW

VIN

LM3445

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SNVS570M – JANUARY 2009 – REVISED NOVEMBER 2015

Feature Description (continued)

7.3.13 Overview of Constant Off-Time Control

A buck converter’s conversion ratio is defined using Equation 3.

(3)

Constant off-time control architecture operates by simply defining the off-time and allowing the on-time, and

therefore the switching frequency, to vary as either VIN or VO changes. The output voltage is equal to the LED

string voltage (VLED), and should not change significantly for a given application. The input voltage or VBUCK in

this analysis will vary as the input line varies. The length of the on-time is determined by the sensed inductor

current through a resistor to a voltage reference at a comparator. During the on-time, denoted by tON, MOSFET

switch Q2 is on causing the inductor current to increase. During the on-time, current flows from VBUCK, through

the LEDs, through L2, Q2, and finally through R3 to ground. At some point in time, the inductor current reaches a

maximum (IL2-PK) determined by the voltage sensed at R3 and the ISNS pin. This sensed voltage across R3 is

compared against the voltage of dim decoder output, FLTR2, at which point Q2 is turned off by the controller.

Figure 19. Inductor Current Waveform in CCM

During the off-period denoted by tOFF, the current through L2 continues to flow through the LEDs via D10.

7.3.14 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 4.9 V (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.

7.3.15 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 20 through Figure 22) 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.

7.3.16 Master Board Modifications

•

Remove R10 and replace with a BAS40 diode

•

Connect TP18 to TP14 (VCC)

•

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

7.3.17 Slave Board Modifications

•

Remove R11 (disconnects BLDR)

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