Diodes PAM2845 - Manual (Page 10)

PAM2845
Document number: DSxxxxx Rev. 1 - 2
10 of 14
www.diodes.com
October 2012
© Diodes Incorporated
PAM2845
A Product Line of
Diodes Incorporated
Application Information
(cont.)
• Method 1: LED Current Setting with External Resistor R
ISET
The most basic means of setting the LED current is connecting a resistor between R
ISET
and GND. The LED current is decided by I
SET
Resistor.
I
LED
= 228/ R
ISET
•
Method 2: LED Current Setting Using PWM Signal to PWMD Pin
This circuit uses resistor R
ISET
to set the on state current and the average LED current, then proportional to the percentage of on-time when the
PWMD pin is logic low. Average LED current is approximately equal to:
I = (t
ON
*I)/ (t
ON
+ t
OFF
)
Also, the recommended PWM frequency is between 100Hz and 1kHz. Frequency <100Hz can cause the LEDs to blink visibly.
• Method 3: LED Current Setting with Single Wire Logic to ENA Pin
When the LEDs are enabled by high level, the LED current initially goes to I
LED
. Dimming is done by pulsing ENA low (500ns to 10µs pulse
width). Each pulse reduces the LED current by 1/32, so after one pulse the LED current is 31/32*I
LED
. The 32th pulse sets the LED current back
to I
LED
.
Figure 1 shows a timing diagram for EN.
Setting the Output Voltage
The FB pin is connected to the center tap of a resistive voltage divider (R1 and R2 in Figure 1) from the high-voltage output.
V
OUT
= V
FB
(1 + R2/R1)
The recommend procedure is to choose R2 = 300kΩ and R1 = 9.2kΩ to set V
OUT(MAX)
= 40V.
Generally the V
OUT
must be higher than total LED voltage. For 10-LED application, R1 = 9.1kΩ, R2 = 270kΩ, V
OUT
= 36.8V, higher than
V
LED
= 33V.
One or more of the LED1 - 6 pins could be floating if not used because the PAM2845 uses external resistor to set the output voltage. One or
more LEDx pins floating is just like change of output loadings.
LED Short Protection
The PAM2845 uses LED FB function to protect devices when one or more LED(s) is/are shorted.
V
LED
= V
OUT
– V
f
N
Normally V
LED
is around 0.4V and V
OUT
is decided by LED numbers. When one or more LED(s) is/are shorted, the PAM2845 will clamp V
OUT
to
make sure all LED pins’ voltage is less then 5V. With this function V
OUT
will be clamped at (5V+ V
f
N
MIN
).
Note:
V
LED
: LED pin voltage
V
OUT
: Output voltage
V
f
: LED forward voltage
N
MIN
: The minimum LED numbers among all strings.