Datasheet LT3476 (Analog Devices) - 10

LT3476
ApplicAtions inForMAtion
through a resistor divider, or connected directly to REF to current source between zero and full current to achieve give the full-scale threshold of 105mV. A DAC may also be a precisely programmed average current. To make this used to drive the VADJ pins. The VADJ pins should not be method of current control more accurate, during the qui- left open. If the VADJ input is connected to a voltage higher escent phase the switch demand current is stored on the than 1.25V, the default regulation threshold across CAP VC node. This feature minimizes recovery time when the and LED is 125mV (typ). The VADJ pin can also be used PWM signal goes high. The minimum PWM on- or off-time in conjunction with a PTC thermistor to provide overtem- will depend on the choice of operating frequency through perature protection for the LED load as shown in Figure 2. the RT input pin. For best current accuracy, the minimum PWM low or high time should be at least ten switching
Dimming Control
cycles. This guideline has two reasons: first to allow the There are two methods to control the current source for output to reach steady state before shutting off, and second dimming using the LT3476. The first method, popular because the oscillator is not synchronized to the PWM with LED applications, uses the PWM pin to modulate the 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 V 1.05V REF of the PWM signal. The minimum PWM low/high time can 20k 25k be reduced to five switching cycles if a disconnect switch is used in the LED current path. VADJ1-4 The second method of dimming control uses the VADJ pin to linearly adjust the current sense threshold during the 470 PTC PWM high state. The LED current programming feature 3476 F01 augments the PWM dimming control, possibly increasing total dimming range by a factor of ten.
Figure 2. Overtemperature Protect Circuit
3476fb 10 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts