Diodes PAM2842 - Manual (Page 11)

PAM2842
Document number: DSxxxxx Rev. 1 - 2
11 of 17
www.diodes.com
October 2012
© Diodes Incorporated
PAM2842
A Product Line of
Diodes Incorporated
Also, the recommended PWM frequency is between 100Hz and 200Hz.
Application Information
(cont.)
● Method 2: LED Current Setting with PWM Signal Using COMP Pin (cont.)
Frequency <100Hz can cause the LEDs to blink visibly. As the COMP pin connects to a capacitor, it needs rise time. If frequency >200Hz, the
average LED current will have a large error when duty cycle is small (<50%).
It maybe generate the audible noise in this dimming condition.
● Method 3: LED Current Setting with PWM Signal using Sense Pin
This method is turn PWM signal to DC voltage, the output current can be adjusted. Because the LED current is a adjustable DC value, it will
cause LED color drift.
Low side current sense and high side current sense circuit is different. Please see Figure 2 and 3. It use the internal reference voltage, so PWM
dimming signal voltage is not considered, just meet the request of the MOSFET driving voltage.
Figure 2. PWM Dimming Use Sense Pin in Low Side Current Sense Figure 3. PWM Dimming Use Sense Pin in High Side Current
Sense
The RC filter (R1,R2,C1,C2) value is decided by dimming frequency, the divider resistor (R3,R4) is decided by dimming range.
Because final adjusted is a DC value, this method can avoid audible noise effectively and achieve better EMI performance than the second
method.
Setting the Output Limit Voltage
The OV pin is connected to the center tap of a resistive voltage divider from the high-voltage output to ground (see application schematic).
)
R
R
1 ( V V
DOWN
UP
OV LIMIT OUT
+ =
−
The recommend procedure is to choose R3 = 360K and R4 = 12K to set V
OUT_LIMIT
= 37.2V.
In boost and sepic circuit, when LED open or no load, the circuit will have no feedback, if no other measure be taken the switch voltage will be
very high and damage the switch, so this OV pin must be set carefully.
In buck circuit, the switch voltage is always small than input voltage, so the OV pin setting is not important in this condition.
This OV pin is used to limit output voltage to avoid breakdown of the switch other than to regulate output voltage. The setting value must keep
the switch voltage below 40V.
In sepic circuit, one must notice that the switch voltage equals V
IN
+V
O
.
This OV pin has a hysteresis voltage detect function, not latch-up function, so output voltage will have a overshoot when no load or load working
voltage is high than setting limit voltage. If the component parameter not match appropriately, the overshoot voltage will be too high and can
demage the switch.