LT3476 Просмотр технического описания (PDF) - Linear Technology

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LT3476

High Current Quad Output LED Driver

Linear Technology 

LT3476

Applications Information

through a resistor divider, or connected directly to REF to

give the full-scale threshold of 105mV. A DAC may also be

used to drive the VADJ pins. The VADJ pins should not be

left open. If the VADJ input is connected to a voltage higher

than 1.25V, the default regulation threshold across CAP

and LED is 125mV (typ). The VADJ pin can also be used

in conjunction with a PTC thermistor to provide overtem-

perature protection for the LED load as shown in Figure 2.

Dimming Control

There are two methods to control the current source for

dimming using the LT3476. The first method, popular

with LED applications, uses the PWM pin to modulate the

VREF

1.05V

20k

25k

VADJ1-4

470

PTC

3476 F01

Figure 2. Overtemperature Protect Circuit

current source between zero and full current to achieve

a precisely programmed average current. To make this

method of current control more accurate, during the qui-

escent phase the switch demand current is stored on the

VC node. This feature minimizes recovery time when the

PWM signal goes high. The minimum PWM on- or off-time

will depend on the choice of operating frequency through

the RT input pin. For best current accuracy, the minimum

PWM low or high time should be at least ten switching

cycles. This guideline has two reasons: first to allow the

output to reach steady state before shutting off, and second

because the oscillator is not synchronized to the PWM

signal and there may be as much as one switching cycle

delay from PWM going high to the start of switching. This

delay, however, does not apply to the negative transition

of the PWM signal. The minimum PWM low/high time can

be reduced to five switching cycles if a disconnect switch

is used in the LED current path.

The second method of dimming control uses the VADJ pin

to linearly adjust the current sense threshold during the

PWM high state. The LED current programming feature

augments the PWM dimming control, possibly increasing

total dimming range by a factor of ten.

3476fb

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