Diodes PAM2846 - Manual (Page 10)

PAM2846
Document number: DSxxxxx Rev. 1 - 2
10 of 14
www.diodes.com
November 2012
© Diodes Incorporated
PAM2846
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 with PWM Signal to 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 10 s 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 Over Voltage Protection
The OVP pin is connected to the center tap of a resistive voltage divider (R
1
and R
2
in Typical Application diagram) from the high-voltage output.
V
OUT_LIMIT
= V
OV
(1 + R
2
/R
1
)
The recommend procedure is to choose R
2
= 300kΩ and R
1
= 9.2kΩ to set V
OUT_LIMIT
= 40V. This OVP pin is used for over voltage protect, not
for output voltage regulate.
LED Short Protection
The PAM2846 uses LED_OVP 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 PAM2846 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
Vf: LED forward voltage
N
MIN
: The minimum LED numbers among all strings.
LED Open Protection
The control loop is related to all six LED sinks. When one or more LED(s) is/are opened, the sink will have no current and the device will work in
unstable open loop state. The V
OUT
voltage will be limited by external resistor divider or 5V + V
f
*N
MIN
, whichever is lower.