Design Article
Combined voltage, current control loops simplify LED, solar apps
Xin Qi, Linear Technology
11/19/2012 3:00 PM EST
LED driver with PWM dimming ratio
Using an input referred LED string allows the LT3796 to act as a buck mode controller as shown in Figure 4. The 1MHz operating frequency enables a high PWM dimming ratio. The OPENLED regulation voltage is set to
through the independent current sense amplifier at CSP, CSN and CSOUT pins. During the PWM off phase, the LT3796 disables all internal loads to the VC pin and preserves the charge state. It also turns off the PMOS switch M2 to disconnect the LED string from the power path and prevent the output capacitor from discharging. These features combine to greatly improve the LED current recovery time when PWM signal goes high. Even with a 100Hz PWM input signal, this buck mode LED driver can achieve a 3000:1 dimming ratio as illustrated in Figure 5.
Using an input referred LED string allows the LT3796 to act as a buck mode controller as shown in Figure 4. The 1MHz operating frequency enables a high PWM dimming ratio. The OPENLED regulation voltage is set to
through the independent current sense amplifier at CSP, CSN and CSOUT pins. During the PWM off phase, the LT3796 disables all internal loads to the VC pin and preserves the charge state. It also turns off the PMOS switch M2 to disconnect the LED string from the power path and prevent the output capacitor from discharging. These features combine to greatly improve the LED current recovery time when PWM signal goes high. Even with a 100Hz PWM input signal, this buck mode LED driver can achieve a 3000:1 dimming ratio as illustrated in Figure 5.
Figure 4: A buck mode LED driver with 3000:1 PWM dimming ratio
Click on image to enlarge
Figure 5: 3000:1 PWM dimming ratio of the circuit in Figure 4 at VIN = 24V and PWM = 100Hz
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agk
11/20/2012 7:05 AM EST
This chip has multiple features tied together and designers of UPS,inverters also can benefit. If this chip could be combined with a digital technology with a simple user interface to program various parameters of currents and voltages then it will be more easy to incorporate into many systems.
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Jack.L
11/24/2012 10:16 PM EST
It's a somewhat simplistic view of maximum power point tracking and would apply to one panel, at one temperature. It would not achieve anywhere near 100% utilization in the real world as the maximum power point voltage will vary considerably with temperature ... which is impacted by solar irradiance as well.
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ZekeR
11/27/2012 6:24 PM EST
@Semiman_#1: True. If you wanted to achieve temperature-invariant MPPT with this chip, it looks like you'd have to detach Vs and CSN from INTVCC, and re-attach the pins to an external temperature-sensing diode. You'd need to tweak the resistor values accordingly. I haven't shopped around, but it would seem reasonable for solar panel makers to include a temperature-sensing diode in the center of the panel; the panel's MPPT should track the diode's open-circuit voltage fairly well. If that's not the case, then you can always glue a diode to the panel and drive it with a current source (resistor to INTVCC).
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