Datasheet LT3517 (Analog Devices) - 8

LT3517
operation
The LT3517 is a constant frequency, current mode regula- through RSENSE. If the error amplifier’s output increases, tor with an internal power switch. Operation can be best more current is delivered to the output; if it decreases, understood by referring to the Block Diagram in Figure 1. At less current is delivered. The current regulated in RSENSE the start of each oscillator cycle, the SR latch is set, which can be adjusted by changing the input voltage VCTRL. turns on the Q1 power switch. A voltage proportional to The current sense amplifier provides rail-to-rail current the switch current is added to a stabilizing ramp and the sense operation. The FB voltage loop is implemented by resulting sum is fed into the positive terminal of the PWM the amplifier A2. When the voltage loop dominates, the comparator, A4. When this voltage exceeds the level at the error amplifier and the amplifier A2 regulate the FB pin to negative input of A4, the SR latch is reset, turning off the 1.01V (constant-voltage mode). power switch. The level at the negative input of A4 is set Dimming of the LED array is accomplished by pulsing the by the error amplifier A3. A3 has two inputs, one from the LED current using the PWM pin. When the PWM pin is voltage feedback loop and the other one from the current low, switching is disabled and the error amplifier is turned loop. Whichever feedback input is lower takes precedence, off so that it does not drive the VC pin. Also, all internal and forces the converter into either constant-current or loads on the VC pin are disabled so that the charge state constant-voltage mode. The LT3517 is designed to transi- of the VC pin will be saved on the external compensation tion cleanly between these two modes of operation. The capacitor. This feature reduces transient recovery time. current sense amplifier senses the voltage across RSENSE When the PWM input again transitions high, the demand and provides a pre-gain to amplifier A1. The output of A1 current for the switch returns to the value just before is simply an amplified version of the difference between PWM last transitioned low. To further reduce transient the voltage across RSENSE and the lower of VCTRL/10 recovery time, an external PMOS is used to disconnect or 100mV. In this manner, the error amplifier sets the the LED array current loop when PWM is low, stopping correct peak switch current level to regulate the current CFILT from discharging.
applications inForMation Dimming Control
minimum PWM low or high time should be at least four There are two methods to control the current source for switching cycles (2µs for fSW = 2MHz). Maximum PWM dimming using the LT3517. The first method uses the period is determined by the system and is unlikely to be PWM pin to modulate the current source between zero longer than 12ms. The maximum PWM dimming ratio and full current to achieve a precisely programmed aver- (PWMRATIO) can be calculated from the maximum PWM age current. To make this method of current control more period (tMAX) and the minimum PWM pulse width (tMIN) accurate, the switch demand current is stored on the VC as follows: node during the quiescent phase. This feature minimizes PWM tMAX (1) recovery time when the PWM signal goes high. To further RATIO = tMIN improve the recovery time, a disconnect switch is used in the LED current path to prevent the output capacitor from Example: discharging in the PWM signal low phase. The minimum tMAX = 10ms, tMIN = 2µs (fSW = 2MHz) PWM on or off time will depend on the choice of operating PWM frequency through RT input pin or SYNC pin. When us- RATIO = 10ms/2µs = 5000:1 ing the SYNC function, the SYNC and PWM signals must The second method of dimming control uses the CTRL have the aligned rising edges to achieve the optimized pin to linearly adjust the current sense threshold during high PWM dimming ratio. For best current accuracy, the 3517fh 8 For more information www.linear.com/LT3517 Document Outline Features Applications Typical Application Description 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 Related Parts