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LT3745EUJ

16-Channel 50mA LED Driver with Buck Controller

Linear Technology 

LT3745

Applications Information

Besides these common considerations, the specific appli-

cation also plays an important role in switching frequency

choice. In a noise-sensitive system, the switching frequency

is usually chosen to keep the switching noise out of a

sensitive frequency band.

Switching Frequency Setting and Synchronization

The LT3745 uses a constant switching frequency that can

be programmed from 200kHz to 1MHz with a resistor

from the RT pin to ground. Table 2 shows RT values for

common switching frequencies.

Table 2. Switching Frequency fSW vs RT Value

fSW (kHz)

RT * (kΩ)

200

280

300

182

400

133

500

105

600

84.5

700

71.5

800

60.4

900

53.6

1000

46.4

* Recommend 1% Standard Values

Synchronizing the LT3745 oscillator to an external fre-

quency can be achieved using the SYNC pin. The square

wave amplitude, compatible to TTL/CMOS logic, should

have valleys that are below 0.6V and peaks that are above

2.4V. The synchronization frequency also ranges from

200kHz to 1MHz, in which the RT resistor should be cho-

sen to set the internal switching frequency around 20%

below the synchronization frequency. In the case of 200kHz

synchronization frequency, RT = 348k is recommended.

It is also important to note that when the synchroniza-

tion frequency is much higher than the RT programmed

internal frequency, the internal slope compensation will

be significantly reduced, which may trigger sub-harmonic

oscillation at duty cycles greater than 50%.

Inductor Current Sense Resistor RS and Current Limit

The current sense resistor, RS, monitors the inductor

current between the ISP and ISN pins, which are the in-

puts to the internal current sense amplifier. The common

mode input voltage of the current sense amplifier ranges

from 0V to (VIN – 2.1V) or 36V absolute maximum value,

whichever is lower. The current sense amplifier not only

provides current information to form the current-mode

control, but also a 46.5mV threshold. The 46.5mV threshold

across the RS resistor imposes an accurate current limit

to protect both P-channel MOSFET M1 and catch diode

D1, and also to prevent inductor current saturation. Good

Kelvin sensing is required for accurate current limit. The

RS resistor value can be determined by:

I OUT(MAX )

=

I L(MAX )

–

∆ IL

2

where the maximum inductor current IL(MAX) is set by:

I L(MAX )

=

46.5mV

RS

IOUT(MAX) is the maximum output load current, and ∆IL

is the inductor peak-to-peak ripple current. Allowing ad-

equate margin for ripple current and external component

tolerances, RS can be estimated as:

R

S

=

35mV

I OUT(MAX

)

3745f

20

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