LT3474 Просмотр технического описания (PDF) - Linear Technology

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LT3474

Step-Down 1A LED Driver

Linear Technology 

LT3474/LT3474-1

APPLICATIONS INFORMATION

Setting the Switching Frequency

The LT3474 uses a constant frequency architecture that

can be programmed over a 200kHz to 2MHz range with a

single external timing resistor from the RT pin to ground.

The current that flows into the timing resistor is used

to charge an internal oscillator capacitor. A graph for

selecting the value of RT for a given operating frequency

is shown in the Typical Performance Characteristics

section. Table 1 shows suggested RT selections for a

variety of switching frequencies.

Table 1. Switching Frequencies

SWITCHING FREQUENCY (MHz)

2

1.5

1

0.7

0.5

0.3

0.2

RT (kΩ)

10

18.7

33.2

52.3

80.6

147

232

Operating Frequency Selection

The choice of operating frequency is determined by sev-

eral factors. There is a tradeoff between efficiency and

component size. 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 LT3474 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

LT3474’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 200ns and f is the switching

frequency.

Example: f = 500kHz, VOUT = 4V

DCMAX = 1− 200ns • 500kHz = 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 160ns and f is the switching

frequency.

Example: f = 500kHz, VOUT = 2.5V

DCMIN = 160ns • 500kHz = 0.08

VIN(MAX)

=

2.5V + 0.4V

0.08

–

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

Rating.

3474fd

10

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