PAM2845 Datasheet, PDF (11/14 Pages) Power Analog Micoelectronics – 40V High Power Boost Convertor with 6 Channel Constant Current Source

PAM2845
40V High Power Boost Convertor
with 6 Channel Constant Current Source
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 Vout must be higher than total LED
voltage. For 10-LED application, R1=9.1kΩ,
R2=270kΩ, Vout=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_fN ) _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.
Power Analog Microelectronics,Inc
www.poweranalog.com
11
08/2008 Rev 1.0