t

tOFF

tON

IL2 (t)

IAVE

IL2-PK

'iL

IL2-MIN

L2 =

fSW x 'i

VLED

VLED

VBUCK

1

K

1

u

VLED

tOFF =

VBUCK

fSW

1

K

1

u

L2

# tOFF x

VLED

'i

'i

# tOFF x

VLED

L2

VL(OFF-TIME) = VLED = L x

'i

't

VL(OFF-TIME) = VLED = L x

(I(MAX) - I(MIN))

't

LM3444

www.ti.com

SNVS682D – NOVEMBER 2010 – REVISED DECEMBER 2015

Application Information (continued)

The value of VL(OFF-TIME) is relatively constant, because the LED stack voltage remains constant. If we rewrite the

equation for an inductor inserting what we know about the circuit during the off-time, Equation 18 shows that we

get:

(18)

Rearranging this gives Equation 19.

(19)

From this, we can see that the ripple current (

Δi) is proportional to off-time (tOFF) multiplied by a voltage, which is

dominated by VLED divided by a constant (L2).

These equations can be rearranged to calculate the desired value for inductor L2, as shown in Equation 20.

(20)

The off time can be calculated using Equation 21:

(21)

Substituting toff in Equation 21 results in Equation 22:

(22)

See Typical Application to better understand the design process.

8.1.5 Setting the LED Current

The LM3444 constant off-time control loop regulates the peak inductor current (IL2). The average inductor current

equals the average LED current (IAVE). Therefore the average LED current is regulated by regulating the peak

inductor current.

Figure 16. Inductor Current Waveform in CCM

Copyright © 2010–2015, Texas Instruments Incorporated

Submit Documentation Feedback

17

Product Folder Links: LM3444