PAM2841 Datasheet, PDF (10/14 Pages) Power Analog Micoelectronics – 1.5ASW Current, 40V Precision WLED Driver

PAM2841
1.5A SW Current, 40V Precision WLED Driver
VIN
PWM
C1
10uF
0
1uF C2
L1
1
2
22uH
D1
12
U12
1
8
PGND SW
2 _VIN
OOVVP ⁷
3
ENA
6
FB
4 _{COMP GND} 5
PAM2841
C4
10n
C3
1uF
R1
910k
0
R2
27k
Figure. 1
2). Using an External PWM Signal to NMOS
Gate.
When PWM signal is at high level, N MOSFET
turned on, then pull comp pin down, then the
LED current should be zero. When PWM signal
is at low level, N MOSFET turned off. The circuit
uses resistor R1 to set the on state current. The
average LED current changes proportionally
with the duty cycle of the PWM signal. A 100%
duty cycle turns off the PAM2841 and leads to
zero LED current. A 0% duty cycle generates full
current.
I =I * _{AVE STATE} (T_ON –T_STA ) _RTUP /(T + _ON T ) _OFF
where T_ON: on time of a period
T : _STARTUP 0.08ms
T : _OFF off time of a period
I : _STATE on state current (full current)
Also the recommend frequency is between 100
and 500Hz. Frequency<100Hz can naturally
causes LEDs to blink visibly.
VIN
ENA
PWM
C1
10uF
0
1uF C2
Q1
L1
1
2
22uH
D1
12
U12
1
8
PGND SW
2 _VIN
3 _ENA
OOVVP ⁷
FB ⁶
4 _{COMP GND} 5
PAM2841
C4
10n
C3
1uF
R1
910k
0
R2
27k
R3
10
Figure. 2
3). Using a DC Voltage
For some applications, a simple and direct way
to control brightness is use an external variable
DC voltage to vary the voltage drop on feedback
resistor. This will make the PAM2841 adjust the
output current to follow the change of feedback
voltage. The circuit is shown in Figure 3. As the
DC voltage increases, the voltage drop on R4
increases and the voltage drop on R3
decreases. Thus, the LED current decreases.
The selection of R4 and R5 will make the current
from the variable DC source much smaller than
the LED current and much larger than the FB pin
current. For VCC range from 0V to 2V, the
selection of resistors in Figure 3 gives dimming
control of LED current from 0mA to 20mA.
VIN
ENA
L1
1
2
22uH
C1
10uF
0
1uF C2
U33
1 _{PGND SW} 8
2 _VIN
OOVVP ⁷
3 _ENA
FB ⁶
4 _{COMP GND} 5
PAM2841
D1
12
C4
10n
C3
1uF
R1
910k
0
R2
27k
R4
R5 ^5k
R3
90k
10
Figure. 3 ^DC
4). Using a Filtered PWM Signal
The filtered PWM signal can be considered as an
adjustable DC voltage. Such regulated signal is
often with some grade of ripple because of some
simple configuration of circuit. With appropriated
arrangement of PWM frequency and level, and
filter parameters, it can be used to replace the
variable DC voltage source in dimming control.
The circuit is shown in Figure 4.
VIN
ENA
L1
1
2
22uH
C1
10uF
0
1uF C2
U33
1
8
PGND SW
2
VIN
OOVVP ⁷
3 _ENA
FB ⁶
4
5
COMP GND
PAM2841
D1
12
C3
1uF
R1
910k
0
R2
27k
C4
10n
R4
R5 ^5k
R3
90k
10
5V
0V
R6
PWM
10k
Figure. 4
C5
100nF
0
Power Analog Microelectronics,Inc
www.poweranalog.com
10
08/2008 Rev 1.3