ILED_Acc% = RSNS%

2 + V

SNS%

2

Irms = ILED x

VANODE

VIN

x 1 -

VANODE

VIN

to

SNS

L1

R1

DIM signal

LM3401

SNVS516C – AUGUST 2007 – REVISED MAY 2013

www.ti.com

Figure 21. Parallel FET Dimming

When the FET switches on, inductor current flows through the FET and the regulated average inductor current is

unchanged. Using this method, inductor current rise time does not limit the dimming frequency. A ripple reduction

capacitor should not be used with the parallel FET dimming method since it significantly slows the LED current

rise time. However, a small noise filter capacitor can be used.

INPUT CAPACITOR SELECTION

An input bypass capacitor is required between VIN and ground. The input capacitor prevents large voltage

transients at the input and provides the instantaneous current when the PFET turns on. The important

parameters for the input capacitor are the voltage rating and the RMS current rating. Follow the manufacturer’s

recommended voltage de-rating. RMS current can be calculated with the equation below. The highest RMS

current will occur around 50% duty cycle.

(18)

A ceramic input capacitor must be placed close to the drain of the PFET. This minimizes the trace inductance

between VIN and the PFET, which is a source of switching noise. If the input capacitor is not properly located,

switching noise can cause current limit and stability problems.

CATCH DIODE SELECTION

The catch diode provides the current path to the LED string during the PFET off-time and must be rated higher

than the average current through the diode, which can be calculated as shown:

IDIODE = ILED x (1-D)

(19)

The peak reverse voltage across the catch diode is approximately equal to the input voltage. Therefore, the

diode’s peak reverse voltage rating should be larger than the maximum input voltage, plus some safety margin.

A Schottky diode is recommended because its low forward voltage maximizes efficiency. For high temperature

applications, diode leakage current may become significant and require a higher reverse voltage rating or a low

leakage diode to achieve acceptable performance.

LED CURRENT ACCURACY

The total accuracy of average LED current is affected by several factors, both internal and external to the

LM3401. Total static accuracy is the part-to-part variation and can be calculated from the equation below:

(20)

Where the worst case VSNS% is ±6%, and RSNS% is the sense resistor accuracy. Because these factors are not

correlated, the RSS (root-sum-square) method of calculation is used.

The LED current will also show some variation with input voltage. This is primarily due to propagation delay and

the dynamic resistance of the LED. In longer on-time operation, the error due to dynamic resistance tends to

dominate, while at shorter on-time, the propagation delay will dominate. These two effects counteract each other,

resulting in typical regulation curves similar to those shown in Figure 22. A larger inductor will reduce the error

due to propagation delay and will result in better overall line regulation.

14

Submit Documentation Feedback

Copyright © 2007–2013, Texas Instruments Incorporated

Product Folder Links: LM3401