LM3405A

SNVS508C – OCTOBER 2007 – REVISED MAY 2013

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FEED-FORWARD CAPACITOR (C4)

The feed-forward capacitor (designated as C4) connected in parallel with the LED string is required to provide

multiple benefits to the LED driver design. It greatly improves the large signal transient response and suppresses

LED current overshoot that may otherwise occur during PWM dimming; it also helps to shape the rise and fall

times of the LED current pulse during PWM dimming thus reducing EMI emission; it reduces LED current ripple

by bypassing some of inductor ripple from flowing through the LED. For most applications, a 1µF ceramic

capacitor is sufficient. In fact, the combination of a 1µF feed-forward ceramic capacitor and a 1µF output ceramic

capacitor leads to less than 1% current ripple flowing through the LED. Lower and higher C4 values can be used,

but bench validation is required to ensure the performance meets the application requirement.

Figure 24 shows a typical LED current waveform during PWM dimming without feed-forward capacitor. At the

beginning of each PWM cycle, overshoot can be seen in the LED current. Adding a 1µF feed-forward capacitor

can totally remove the overshoot as shown in Figure 25.

Figure 24. PWM Dimming without Feed-Forward

Figure 25. PWM Dimming with a 1µF Feed-Forward

Capacitor

Capacitor

CATCH DIODE (D1)

The catch diode (D1) conducts during the switch off-time. A Schottky diode is required for its fast switching time

and low forward voltage drop. The catch diode should be chosen such that its current rating is greater than:

(20)

The reverse breakdown rating of the diode must be at least the maximum input voltage plus appropriate margin.

To improve efficiency, choose a Schottky diode with a low forward voltage drop.

BOOST DIODE (D2)

3.3V, a small-signal Schottky diode is recommended for better efficiency. A good choice is the BAT54 small

signal diode.

BOOST CAPACITOR (C3)

A 0.01µF ceramic capacitor with a voltage rating of at least 6.3V is sufficient. The X7R and X5R MLCCs provide

the best performance.

POWER LOSS ESTIMATION

The main power loss in LM3405A includes three basic types of loss in the internal power switch: conduction loss,

switching loss, and gate charge loss. In addition, there is loss associated with the power required for the internal

circuitry of IC.

The conduction loss is calculated as:

(21)

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