ILED

50 mA /Div

IL

50 mA/Div

VCS

20V/Div

400 ns/ DIV

ILED_RIP = IL_RIP =

(VIN - VANODE) x 2 x delay

2 x SNSHYS

RSNS

L

+

ton =

D

fSW

LM3401

SNVS516C – AUGUST 2007 – REVISED MAY 2013

www.ti.com

If the input voltage falls below the forward voltage of the LED string, the LM3401 will operate at 100% duty cycle.

In this state, the anode voltage will be equal to the input voltage and LED current is determined by the v-i curve

of the LED. At 100% duty cycle, LED current will be continuous with a maximum value equal to the ILED_PK level

set at the HYS pin.

In some applications, maximum operating frequency will be limited by the minimum on-time as shown in the

following equation:

(10)

When the on-time reaches minimum (150 ns typical) due to increasing input voltage, the frequency will be

reduced in order to maintain the proper duty cycle.

INDUCTOR SELECTION

The most important parameters for the inductor are the inductance and the current rating. The LM3401 operates

over a wide frequency range and can use a wide range of inductance values.

Once an inductance value is determined from the frequency equation, the maximum operating current must be

verified.

Although peak-to-peak ripple current is controlled by the hysteresis value, there is some variation due to

propagation delay. This means that the inductance has a direct effect on LED current line regulation.

In general, a larger inductor will result in lower frequency and better line regulation. The actual peak-to-peak

inductor current can be calculated as follows:

(11)

Use the maximum value of Vin to determine the worst case ILED_RIP value. This value should be used to

determine the peak current, ILED_PK, shown in the Output Current Setting section.

Since the LM3401 can operate at 100% duty cycle, the inductor must be rated to handle ILED_PK continuously.

RIPPLE REDUCTION CAPACITOR

The typical application schematic shown on the front page is the simplest application of the LM3401. In this

schematic, inductor current is equal to LED current. Therefore, the equations for ripple current apply to both LED

ripple and inductor ripple.

However, LED ripple current can be reduced without altering the inductor current by using a bypass capacitor

placed in parallel with the LED string (shown as C2 in the Figure 24 schematic).

This allows larger hysteresis values to be used while significantly reducing ripple current in the LED string.

Figure 18 below shows this effect: inductor ripple current is unaffected while LED ripple current is dramatically

reduced.

Figure 18. LED Ripple Current Reduction with a 1 µF Ripple Reduction Capacitor

If a ripple reduction capacitor is used, the equation for SNSHYS_MAX only applies for 100% duty operation.

10

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