2 x 22.4 mV x 33 PH

0.29: x (35V ± 16.8V)

0.50

+ (2 x 60 ns)

= 1.25 MHz

2 x 22.4 mV x 33 PH

0.29: x (18V ± 16.8V)

0.96

+ (2 x 60 ns)

= 219 kHz

227 mA

2

690 mA +

= 804 mA

(35V ± 11V) x 2 x 60 ns

2 x 22.4 mV

0.29:

33 PH

+

= 227 mA

0.60

1 MHz

-(2 x 60 ns) x [0.29 x (24V - 13.8V)]

2 x 33 PH

= 22.4 mV

L =

0.60

1 MHz

-(2 x 60 ns) x [0.29 x (24V - 13.8V)]

2 x 25 mV

= 29.6 PH

R2 =

25 mV x 5

20 PA

= 6.25 k:

LM3401

www.ti.com

SNVS516C – AUGUST 2007 – REVISED MAY 2013

Since this is a relatively high switching frequency, a low starting point of 25 mV is selected for the comparator

hysteresis to maintain good line regulation. This will allow a larger inductor at the same operating frequency and

is well below the calculated maximum. Set a preliminary hysteresis value with R2:

(27)

For 1 MHz switching frequency and 25 mV hysteresis, inductance can be calculated. Because frequency varies

with input voltage and LED forward voltage, for this calculation, assume typical values of 24V and 13.6V

respectively, and a PFET delay time of 15 ns.

(28)

Select a value of 33 µH and the hysteresis can be adjusted downward by re-arranging the same frequency

equation:

(29)

must be verified that the peak LED current is within the maximum allowed.

For now, the design is created without using a ripple reduction capacitor. Therefore, LED ripple current is equal

to inductor ripple current. Maximum LED ripple current is calculated as:

(30)

Note that the maximum input voltage and minimum anode voltage were used for this worst case calculation.

Now peak LED current can be determined:

(31)

This confirms that the component selections will keep LED peak current below the maximum LED rating. Notice if

a ripple reduction capacitor is chosen, the peak inductor current is still 804mA, but the LED peak current is

reduced. Therefore, the inductor must be rated for a DC current greater than 804 mA. Now that the inductor

value has been selected and verified, the operating frequency range can be determined. Lowest operating

frequency occurs at minimum input voltage and maximum anode voltage. For this example the values are 18V

minimum input and 16.8V maximum anode voltage (200 mV SNS voltage plus the maximum LED forward

voltages) and calculate:

(32)

At duty cycles close to 100% (96% in this case) the frequency equation becomes less accurate. Actual switching

frequency will typically be lower than the calculated value.

(33)

Using the equation in the Switching Frequency section, it can be verified that this maximum frequency is within

the minimum on-time limited frequency (and below the maximum operating frequency).

The maximum frequency calculation is only an estimate, the actual maximum should be verified on the bench.

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