AL9910S-13 Diodes Zetex, AL9910S-13 Datasheet - Page 8

• Manufacturer Part Number: AL9910S-13
• Manufacturer: Diodes Zetex

Specifications of AL9910S-13

• Lead Free Status / Rohs Status: Compliant

Applications Information
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 R
The switching frequency is the reciprocal of the oscillator period. Typical values for R
When driving smaller numbers of LEDs, care should be taken to ensure that t
reduce/limit the switching frequency by increasing the R
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
The best solution is to adopt the so-called constant off-time operation as shown in Figure 2. The resistor (R
to ground by default, to set operating frequency. To force the AL9910 to enter constant OFF time mode R
the gate of the external MOSFET. This will decrease the duty cycle from 50% by increasing the total period, t
The oscillator period equation above now defines the AL9910 off time, t
When using this mode the nominal switching frequency is chosen and from the nominal input and output voltages the off-time
can be calculated:
From this the timing resistor, R
AL9910/AL9910A/AL9910-5
Document number: DS 35103 Rev. 3 - 2
t
t
OSC
(SBO)
OFF
=
=
R
of the switching frequency.
⎛
⎜
⎜
⎝
osc
1
−
25
V
+
OUT
V
22
IN
(
nom
(
µs
nom
OSC
)
)
⎞
⎟
⎟
⎠
, can be calculated:
∗
(Continued)
with R
V
V
f
OSC
IN
IN
1
Fig. 2 Constant off-time configuration
OSC
in kΩ
UNIVERSAL HIGH VOLTAGE HIGH BRIGHTNESS LED
V
V
LD
LD
PWM_D
PWM_D
DD
DD
R
R
R
www.diodes.com
OSC
OSC
OSC
OSC
OSC
AL9910/A
AL9910/A
8 of 15
=
AL9910/AL9910A/AL9910-5
value. Reducing the switching frequency will also improve the
pin and ground. The corresponding oscillator period is:
GND
GND
(
t
V
V
OFF
IN
IN
(
µs
OFF
)
GATE
GATE
R
R
∗
25
OSC
OSC
.
CS
CS
)
−
ON
22
Q1
Q1
k (
> t
Ω
OSC
BLANK
)
vary from 75kΩ to 1MΩ
. The simplest way to do this is to
OSC
OSC
© Diodes Incorporated
OFF
is connected to
) is, connected
+ t
June 2011
DRIVER
ON
.