LT3475IFE-PBF データシートの表示（PDF） - Linear Technology

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LT3475IFE-PBF

Dual Step-Down 1.5A LED Driver

Linear Technology 

LT3475/LT3475-1

APPLICATIONS INFORMATION

Table 1 shows suggested RT selections for a variety of

switching frequencies.

Operating Frequency Selection

The choice of operating frequency is determined by

several factors. There is a tradeoff between efficiency and

component size. A higher switching frequency allows the

use of smaller inductors at the cost of increased switching

losses and decreased efficiency.

Another consideration is the maximum duty cycle. In certain

applications, the converter needs to operate at a high duty

cycle in order to work at the lowest input voltage possible.

The LT3475 has a fixed oscillator off time and a variable

on time. As a result, the maximum duty cycle increases

as the switching frequency is decreased.

Input Voltage Range

The minimum operating voltage is determined either by the

LT3475’s undervoltage lockout of 4V, or by its maximum

duty cycle. The duty cycle is the fraction of time that the

internal switch is on and is determined by the input and

output voltages:

DC

=

(VOUT +

(VIN – VSW

VF

+

)

VF

)

where VF is the forward voltage drop of the catch diode

(~0.4V) and VSW is the voltage drop of the internal switch

(~0.4V at maximum load). This leads to a minimum input

voltage of:

VIN(MIN)

=

VOUT + VF

DCMAX

–

VF

+ VSW

with DCMAX = 1–tOFF(MIN) • f

where t0FF(MIN) is equal to 167ns and f is the switching

frequency.

Example: f = 600kHz, VOUT = 4V

DCMAX = 1− 167ns • 600kHz = 0.90

VIN(MIN)

=

4V

+ 0.4V

0.9

–

0.4V

+

0.4V

=

4.9V

The maximum operating voltage is determined by the

absolute maximum ratings of the VIN and BOOST pins,

and by the minimum duty cycle.

VIN(MAX )

=

VOUT + VF

DCMIN

–

VF

+

VSW

with DCMIN = tON(MIN) • f

where tON(MIN) is equal to 140ns and f is the switching

frequency.

Example: f = 750kHz, VOUT = 3.4V

DCMIN = 140ns • 750kHz = 0.105

VIN(MAX

)

=

3.4V + 0.4V

0.105

–

0.4V

+

0.4V

=

36V

The minimum duty cycle depends on the switching fre-

quency. Running at a lower switching frequency might

allow a higher maximum operating voltage. Note that

this is a restriction on the operating input voltage; the

circuit will tolerate transient inputs up to the Absolute

Maximum Ratings of the VIN and BOOST pins. The input

voltage should be limited to the VIN operating range (36V)

during overload conditions (short circuit or start up).

Minimum On Time

The LT3475 will regulate the output current at input volt-

ages greater than VIN(MAX). For example, an application

with an output voltage of 3V and switching frequency of

1.2MHz has a VIN(MAX) of 20V, as shown in Figure 3. Figure

4 shows operation at 35V. Output ripple and peak inductor

VOUT

500mV/DIV

(AC COUPLED)

IL

1A/DIV

VSW

20V/DIV

3475 F03

Figure 3. Operation at VIN(MAX) = 20V.

VOUT = 3V and fSW = 1.2MHHz

3475fb

9

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