LT3476 datasheet(20/36 Pages) AD

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LT3966

Rev. 0

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CH1

CH2

CH3

CH4

CH1

CH2

CH3

CH4

IN PHASE DIMMING (INPH = 1)

OUT OF PHASE DIMMING (INPH = 0)

1/4 DIMMING

CYCLE

3966 F16

Figure 16.

APPLICATIONS INFORMATION

IN PHASE/OUT OF PHASE DIMMING

To reduce input ripple current, each channel’s dimming

operation can be independently programmed in one of

the two modes: in-phase mode and out-of-phase mode

(see Figure 16). When operating in in-phase mode, the

start of CH2/CH3/CH4’s dimming cycle is aligned with

CH1’s. When operating in out-of-phase mode, the start

of CH2/CH3/CH4’s dimming cycle is behind of CH1’s by a

quarter/a half/three quarters of CH2/CH3/CH4’s dimming

cycle. To enable the in-phase/out-of-phase operation, set

the INPH register bit to 1/0. Please note that after the

INPH bit change, an I2C broadcast write (see Figure 11) is

required to reset all dimming generators for this function

to work properly.

POWER COMPONENT SELECTION

Since the LT3966 is an internally-compensated converter,

the external power components are selected to ensure

system stability. By following a few simple guidelines, this

process can be made simple and streamlined.

Inductor Selection

Inductor selection consists of two parameters: saturation

current rating, and inductance value. Higher switching

frequency allows the use of smaller inductance value at

the expense of increased switching loss.

The saturation current rating of the inductor should be

selected appropriately for the 2.4A current limit of the

LT3966. An approximation for maximum inductor cur-

rent (efficiency = 100%) is based on the maximum LED

current and the input-output ratio, Equation 10.

IL =

VOUT

VIN

• ILED

(10)

The desired inductance is determined by the steady-state

current ripple. A typical rule of thumb is to set the inductor

current ripple to a maximum of 20% of the maximum induc-

tor current, Equation 11, Equation 12 and Equation 13.

Boost:

LBOOST ≥

VIN(MIN) •(VOUT(MAX) – VIN(MIN) )

VOUT(MAX) • 0.45A • fSW

⎛

⎝

⎜

⎞

⎠

⎟

(11)

Buck:

LBUCK ≥

VOUT(MAX) •(VIN(MIN) – VOUT(MAX))

VIN(MIN) • 0.45A • fSW

⎛

⎝

⎜

⎞

⎠

⎟

(12)

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