LT3745EUJ 查看數據表（PDF） - Linear Technology

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LT3745EUJ

16-Channel 50mA LED Driver with Buck Controller

Linear Technology 

LT3745

Applications Information

Globally, the LT3745 converts a higher input voltage to a

single lower LED bus voltage (VOUT) supplying 16 parallel

LED strings with the adaptive-tracking-plus-precharging

technique. Locally, the part regulates and modulates the

current of each string to an independent dot correction and

grayscale PWM dimming setting sent by TTL/CMOS logic

serial data interface. This Application Information section

serves as a guideline of selecting external components

(refer to the Block Diagram) and avoiding common pitfalls

for the typical application.

Programming Maximum VOUT

The adaptive-tracking-plus-precharging technique regu-

lates VOUT to its maximum value during the startup and

precharging phases, and adaptively lowers the voltage to

keep the minimum active LED pin voltage around 0.7V

during the tracking phase. Therefore, the maximum VOUT

should be programmed high enough to keep all the LED

pin voltages higher than 0.8V to maintain LED current

regulation across temperature, current, and manufactur-

ing variation. As a starting point, the maximum LED bus

voltage, VOUT(MAX), can be calculated as:

VOUT(MAX) = 0.8V + n • VF(MAX)

where n is the number of LED per string and VF(MAX) is

the maximum LED forward voltage rated at the highest

operating current and the lowest operating temperature.

The VOUT(MAX) is programmed with a resistor divider

between the output and the FB pin. The resistor values

are calculated as:

RFB2

= RFB1



VOUT(MAX )

1.205V

−

1



Tolerance of the feedback resistors will add additional

errors to the output voltage, so 1% resistor values should

be used. The FB pin output bias current is typically

120nA, so use of extremely high value feedback resistors

could also cause bias current errors. A typical value for

RFB1 is 10k.

VIN Power Input Supply Range

The power input supply for the LT3745 can range from 6V

to 55V, covering a wide variety of industrial power supplies.

Another restriction on the minimum input voltage VIN(MIN)

is the 2.1V minimum dropout voltage between the VIN and

ISN pins, and thus the VIN(MIN) is calculated as:

VIN(MIN) = VOUT(MAX) + 2.1V

Choosing Switching Frequency

Selection of the switching frequency is a trade-off between

efficiency and component size. Low frequency operation

improves efficiency by reducing MOSFET switching losses

and gate charge losses. However, lower frequency opera-

tion requires larger inductor and capacitor values.

Another restriction on the switching frequency comes

from the input and output voltage range caused by the

minimum switch on and switch off-time. The highest

switching frequency fSW(MAX) for a given application can

be calculated as:

fSW (MAX )

=

MIN





DMIN , 1– DMAX

tON(MIN) tOFF(MIN)





where the minimum duty cycle DMIN and the maximum

duty cycle DMAX are determined by:

DMIN =

VOUT(MIN) + VD

VIN(MAX) + VD

and DMAX =

VOUT(MAX) + VD

VIN(MIN) + VD

tON(MIN) is the minimum switch on-time (~200ns), tOFF(MIN)

is the minimum switch off-time (~120ns), VOUT(MIN) is the

minimum adaptive output voltage, VIN(MAX) is the maximum

input voltage, and VD is the catch diode forward voltage

(~0.5V). The calculation of fSW(MAX) simplifies to:

fSW(MAX) =



MIN5 •

VOUT(MIN) + VD , 8.33 •

VIN(MAX) + VD

VIN(MIN) – VOUT(MAX

VIN(MIN) + VD

)

MHz

Obviously, lower frequency operation accommodates both

extremely high and low VOUT to VIN ratios.

3745f

19

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