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LT3476 Datasheet(PDF) 10 Page - Linear Technology |
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LT3476 Datasheet(HTML) 10 Page - Linear Technology |
10 / 12 page LT3476 10 3476fa APPLICATIO S I FOR ATIO Programming the LED Current The LED Current is programmed using an external sense resistor in series with the load. This method allows flex- ibility in driving the load (i.e., sensing one of several parallel strings) while maintaining good accuracy. The VADJ input sets the voltage regulation threshold across the external sense resistor between 10mV and 120mV. A 1.05V reference output (REF) is provided to drive the VADJ pins either 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 overtemperature 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 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. VREF 20k 25k 470 PTC VADJ1-4 1.05V 3476 F01 Figure 2. Overtemperature Protect Circuit |
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