LM3414 datasheet(14/20 Pages) NSC

14 / 20 page

PULSE-LEVEL-MODULATION (PLM) OPERATION

PRINCIPLES

The Pulse-Level-Modulation is a patented method to ensure

accurate average output current regulation without the need

of direct output current sensing. Figure 3 shows the current

waveforms of a typical buck converter under steady state,

where, I

L1 is the inductor current and ILX is the main switch

current flowing into the LX pin. For a buck converter operating

in steady state, the mid-point of the RAMP section of the main

switch current is equal to the average level of the inductor

current hence the average output current. In short, by regu-

lating the mid-point of the RAMP section of the main switch

current with respect to a precise reference level, PLM

achieves output current regulation by sensing the main switch

current solely.

PWM DIMMING CONTROL

The DIM pin of the LM3414/14HV is an input with internal pull-

up that accepts logic signals for average LED current control.

Applying a logic high (above 1.2V) signal to the DIM pin or

leaving the DIM pin open will enable the device. Applying a

logic low signal (below 0.9V) to the DIM pin will disable the

switching activity of the device but maintain VCC regulator

active. The LM3414/14HV allows the inductor current to slew

up to the preset regulated level at full speed instead of charg-

ing the inductor with multiple restrained switching duty cycles.

This enables the LM3414/14HV to achieve high speed dim-

ming and very fine dimming control as shown in Figure 4 and

Figure 5:

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FIGURE 4. LED Current Slews up with Multiple Switching Cycle

LM3414 Datasheet(PDF) 14 Page - National Semiconductor (TI)