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AL9910SP-13 Datasheet(PDF) 7 Page - Diodes Incorporated |
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AL9910SP-13 Datasheet(HTML) 7 Page - Diodes Incorporated |
7 / 14 page AL9910/AL9910A UNIVERSAL HIGH VOLTAGE HIGH BRIGHTNESS LED DRIVER AL9910/AL9910A Document number: DS 35103 Rev. 2 - 2 7 of 14 www.diodes.com March 2011 © Diodes Incorporated Applications Information (Continued) Setting Operating Frequency The AL9910 is capable of operating over a 25 and 300 kHz switching frequency range. The switching frequency is programmed by connecting an external resistor between ROSC pin and ground. The corresponding oscillator period is: tOSC = 25 22 + osc R µs with ROSC in kΩ The switching frequency is the reciprocal of the oscillator period. Typical values for ROSC vary from 75kΩ to 1MΩ When driving smaller numbers of LEDs, care should be taken to ensure that tON > tBLANK. The simplest way to do this is to reduce/limit the switching frequency by increasing the ROSC value. Reducing the switching frequency will also improve the efficiency. When operating in buck mode the designer must keep in mind that the input voltage must be maintained higher than 2 times the forward voltage drop across the LEDs. This limitation is related to the output current instability that may develop when the AL9910 operates at a duty cycle greater than 0.5. This instability reveals itself as an oscillation of the output current at a sub- harmonic (SBO) of the switching frequency. The best solution is to adopt the so-called constant off-time operation as shown in Figure 2. The resistor (ROSC) is, connected to ground by default, to set operating frequency. To force the AL9910 to enter constant OFF time mode ROSC is connected to the gate of the external MOSFET. This will decrease the duty cycle from 50% by increasing the total period, tOFF + tON. AL9910/A GATE CS LD PWM_D GND R OSC V DD V IN Q1 R OSC V IN AL9910/A GATE CS LD PWM_D GND R OSC V DD V IN Q1 R OSC V IN Figure 2. Constant off-time configuration The oscillator period equation above now defines the AL9910 off time, tOFF. When using this mode the nominal switching frequency is chosen and from the nominal input and output voltages the off-time can be calculated: OSC nom IN nom OUT OFF f V V t 1 1 ) ( ) ( ∗ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ − = From this the timing resistor, ROSC, can be calculated: ( ) ) k ( 22 25 ) µs ( t R OFF OSC Ω − ∗ = |
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