LT3486 Просмотр технического описания (PDF) - Linear Technology

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LT3486

Dual 1.3A White LED Step-Up Converters with Wide Dimming

Linear Technology 

LT3486

APPLICATIONS INFORMATION

Duty Cycle

The duty cycle for a step-up converter is given by:

D = VOUT + VD – VIN

VOUT + VD – VCESAT

where:

VOUT = Output voltage

VD = Schottky forward voltage drop

VCESAT = Saturation voltage of the switch

VIN = Input battery voltage

The maximum duty cycle achievable for LT3486 is 96%

(typ) when running at 1MHz switching frequency. It in-

creases to 98% (typ) when run at 200kHz and drops to

90% (typ) at 2MHz. Always ensure that the converter is

not duty-cycle limited when powering the LEDs at a given

switching frequency.

Setting the Switching Frequency

The LT3486 uses a constant frequency architecture that

can be programmed over a 200kHz to 2.5MHz range

with a single external timing resistor from the RT pin to

ground. The nominal voltage on the RT pin is 0.54V, and the

current that flows into the timing resistor is used to charge

and discharge an internal oscillator capacitor. A graph for

selecting the value of RT for a given operating frequency

is shown in the Figure 5.

1000

100

Operating Frequency Selection

The choice of operating frequency is determined by sev-

eral factors. There is a trade-off between efficiency and

component size. Higher switching frequency allows the

use of smaller inductors albeit at the cost of increased

switching losses and decreased efficiency.

Another consideration is the maximum duty cycle achiev-

able. In certain applications the converter needs to operate

at the maximum duty cycle in order to light up the maximum

number of LEDs. The LT3486 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.

The circuit of Figure 6a is operated with different values

of timing resistor (RT). RT is chosen so as to run the con-

verters at 800kHz (RT = 63.4k), 1.25MHz (RT = 39.1k) and

2MHz (RT = 21.5k). The CTRL pins are used to provide

dimming for the respective LED strings. The efficiency

comparison for different RT values is shown in Figure 6b.

5V CIN

10µF

8.06Ω

D1

D2

COUT1

2.2µF

25mA

OFF ON

REF

L1

10µH

L2

10µH

SW1

OVP1

VIN

SW2

OVP2

CTRL1

CTRL2

SHDN

PWM1

LT3486

REF

PWM2

FB1

VC1

FB2

GND RT VC2

COUT2

2.2µF

25mA

1.25V

CREF

0.1µF

2.8k

4.7nF

CIN: 10V, X7R

COUT1, COUT2: 35V, X5R

D1, D2: ZETEX ZHCS400

L1, L2: TOKO D53LC TYPE A

RT 2.8k

4.7nF

8.06Ω

3486 F06a

Figure 6a. 5V to 8/8 White LEDs

10

0

500 1000 1500 2000 2500

OSCILLATOR FREQUENCY (kHz)

3486 G09

Figure 5. Timing Resistor (RT) Value

3486fe

10

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