AL9910 datasheet(8/16 Pages) DIODES

8 / 16 page

AL9910/AL9910A/AL9910-5

UNIVERSAL HIGH VOLTAGE HIGH BRIGHTNESS LED DRIVER

AL9910/AL9910A/AL9910-5

Document number: DS35103 Rev. 7 - 2

8 of 16

www.diodes.com

February 2012

Applications Information (cont.)

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 =

Rosc +

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.

Fig. 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

(

)

nom

(

OUT

OFF

∗

⎟

⎠

⎞

⎜

⎝

⎛

−

From this the timing resistor, ROSC, can be calculated:

(

)

(

)

µs

(

OFF

OSC

−

∗

AL9910/A GATE

PWM_D

GND

OSC

AL9910/A GATE

PWM_D

GND

OSC

AL9910 Arkusz danych(PDF) 8 Page - Diodes Incorporated

AL9910 Arkusz danych(HTML) 8 Page - Diodes Incorporated

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Arkusz danych Pobierz

Link URL