LM3445MMXSLASHNOPB datasheet(24/46 Pages) TI2

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IL-PK(UNDIM) =

750 mV

IAVE(UNDIM) = IL2-PK(UNDIM) - '

IL2-PK = IAVE + '

tOFF

tON

IL2 (t)

IAVE

IL2-PK

'iL

IL2-MIN

L2 =

fSW x 'i

VLED

VBUCK

VLED

tOFF =

VBUCK

fSW

L2 # tOFF x

VLED

'i # tOFF x

VLED

LM3445

SNVS570M – JANUARY 2009 – REVISED NOVEMBER 2015

www.ti.com

Application Information (continued)

(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.

(20)

Where:

(21)

Finally:

(22)

See Typical Application to better understand the design process.

8.1.5 Setting the LED Current

The LM3445 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 25. Inductor Current Waveform in CCM

Knowing the desired average LED current, IAVE and the nominal inductor current ripple, ΔiL, the peak current for

an application running in continuous conduction mode (CCM) is defined in Equation 23.

(23)

Or, the maximum, or undimmed, LED current would then be,

(24)

This is important to calculate because this peak current multiplied by the sense resistor R3 will determine when

the internal comparator is tripped. The internal comparator turns the control MOSFET off once the peak sensed

voltage reaches 750 mV, as shown in Equation 25.

(25)

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