Datasheet LT3518 (Analog Devices) - 9
| Manufacturer | Analog Devices |
| Description | Full-Featured LED Driver with 2.3A Switch Current |
| Pages / Page | 20 / 9 — applications inForMation Dimming Control. Figure 2. Setting Output … |
| File Format / Size | PDF / 465 Kb |
| Document Language | English |
applications inForMation Dimming Control. Figure 2. Setting Output Voltage. Figure 3
Model Line for this Datasheet
- LT3518EFE#PBF LT3518EFE#PBF LT3518EFE#TRPBF LT3518EFE#TRPBF LT3518EUF#PBF LT3518EUF#PBF LT3518EUF#TRPBF LT3518EUF#TRPBF LT3518HFE#PBF LT3518HFE#PBF LT3518HFE#TRPBF LT3518HFE#TRPBF LT3518HUF#PBF LT3518HUF#PBF LT3518HUF#TRPBF LT3518HUF#TRPBF LT3518IFE#PBF LT3518IFE#PBF LT3518IFE#TRPBF LT3518IFE#TRPBF LT3518IUF#PBF LT3518IUF#PBF LT3518IUF#TRPBF LT3518IUF#TRPBF
Text Version of Document
LT3518
applications inForMation Dimming Control
When VCTRL is higher than 1V, the LED current is clamped There are two methods to control the current source for to be: dimming using the LT3518. The first method uses the 100mV PWM pin to modulate the current source between zero ILED = (3) RSENSE and full current to achieve a precisely programmed aver- age current. To make this method of current control more The LED current programming feature possibly increases accurate, the switch demand current is stored on the VC total dimming range by a factor of ten. node during the quiescent phase. This feature minimizes recovery time when the PWM signal goes high. To further improve the recovery time, a disconnect switch is used in 2V VREF the LED current path to prevent the output capacitor from 45.3k 49.9k discharging in the PWM signal low phase. The minimum CTRL PWM on or off time will depend on the choice of operating frequency through RT input pin or SYNC pin. When us- 5k PTC ing the SYNC function, the SYNC and PWM signals must 3518 F02 have the aligned rising edges to achieve the optimized high PWM dimming ratio. For best current accuracy, the
Figure 2
minimum PWM low or high time should be at least six switching cycles (3µs for fSW = 2MHz). Maximum PWM period is determined by the system and is unlikely to be The CTRL pin should not be left open (tie to VREF if not longer than 12ms. The maximum PWM dimming ratio used). The CTRL pin can also be used in conjunction with (PWM a PTC thermistor to provide overtemperature protection RATIO) can be calculated from the maximum PWM period (t for the LED load. MAX) and the minimum PWM pulse width (tMIN) as follows:
Setting Output Voltage
PWM tMAX (1) For a boost application, the output voltage can be set by RATIO = tMIN selecting the values of R1 and R2 (see Figure 3) according Example: to the following equation: t ⎛ R1 MAX = 9ms, tMIN = 3µs (fSW = 2MHz) V ⎞ OUT = ⎜ +1⎟ • 1.01V (4) PWM ⎝R2 ⎠ RATIO = 9ms/3µs = 3000:1 The second method of dimming control uses the CTRL pin to linearly adjust the current sense threshold during VOUT the PWM high state. When the CTRL pin voltage is less R1 than 1V, the LED current is: LT3518 FB R2 I VCTRL LED = (2) 10 •R 3518 F03 SENSE
Figure 3
3518ff For more information www.linear.com/LT3518 9 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 Typical Application Related Parts
applications inForMation Dimming Control
When VCTRL is higher than 1V, the LED current is clamped There are two methods to control the current source for to be: dimming using the LT3518. The first method uses the 100mV PWM pin to modulate the current source between zero ILED = (3) RSENSE and full current to achieve a precisely programmed aver- age current. To make this method of current control more The LED current programming feature possibly increases accurate, the switch demand current is stored on the VC total dimming range by a factor of ten. node during the quiescent phase. This feature minimizes recovery time when the PWM signal goes high. To further improve the recovery time, a disconnect switch is used in 2V VREF the LED current path to prevent the output capacitor from 45.3k 49.9k discharging in the PWM signal low phase. The minimum CTRL PWM on or off time will depend on the choice of operating frequency through RT input pin or SYNC pin. When us- 5k PTC ing the SYNC function, the SYNC and PWM signals must 3518 F02 have the aligned rising edges to achieve the optimized high PWM dimming ratio. For best current accuracy, the
Figure 2
minimum PWM low or high time should be at least six switching cycles (3µs for fSW = 2MHz). Maximum PWM period is determined by the system and is unlikely to be The CTRL pin should not be left open (tie to VREF if not longer than 12ms. The maximum PWM dimming ratio used). The CTRL pin can also be used in conjunction with (PWM a PTC thermistor to provide overtemperature protection RATIO) can be calculated from the maximum PWM period (t for the LED load. MAX) and the minimum PWM pulse width (tMIN) as follows:
Setting Output Voltage
PWM tMAX (1) For a boost application, the output voltage can be set by RATIO = tMIN selecting the values of R1 and R2 (see Figure 3) according Example: to the following equation: t ⎛ R1 MAX = 9ms, tMIN = 3µs (fSW = 2MHz) V ⎞ OUT = ⎜ +1⎟ • 1.01V (4) PWM ⎝R2 ⎠ RATIO = 9ms/3µs = 3000:1 The second method of dimming control uses the CTRL pin to linearly adjust the current sense threshold during VOUT the PWM high state. When the CTRL pin voltage is less R1 than 1V, the LED current is: LT3518 FB R2 I VCTRL LED = (2) 10 •R 3518 F03 SENSE
Figure 3
3518ff For more information www.linear.com/LT3518 9 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 Typical Application Related Parts
