APW7071CI-TRL Ver la hoja de datos (PDF) - Anpec Electronics
Número de pieza
componentes Descripción
Lista de partido
APW7071CI-TRL Datasheet PDF : 15 Pages
| |||
APW7071
Application Information
Setting the LEDs Current
In figure 1, the converter regulates the voltage on FB pin,
connected with the cathod of the lowest LED and the
current-sense resistor R1, at 0.25V(typical). Therefore,
the current (I ), flowing via the LEDs and the R1, is cal-
LED
culated by the following equation:
ILED = 0.25V
R1
Brightness Control
The brightness of the LEDs is controlled by adjusting the
LED current. There are three following recommended
methods to adjust the brightness of the LEDs :
a. Using an adjustable DC voltage applied to the R3 is
shown in figure. 2.
In figure 2, an additional network (R2 and R3) is con-
nected between the FB, the junction of the LED cathode
and R1. An adjustable DC voltage (V ), connected with
ADJ
R3, injects a constant current (I1, I1=(V -0.25V)/R3) into
ADJ
the FB node when the FB voltage is regulated at 0.25V.
Therefore, the voltage across R1 is reduced by the offset
voltage (I1xR2), reducing the LED current and brightness.
The LED current is calculated by the following equation:
0.25V - R2 × (VADJ − 0.25)
ILED =
R3
R1
With the VADJ from 0V to 3.3V, the LED current can be
controlled from 0mA to 20mA.
VIN
2.7V~6V
C1
4.7µ F
D1
L1
MSCD052
4.7µ H
6
VIN
1
SW
C2
0.1µ F
VOUT
2
GND
5
OVP
4
3
R2
SHDN
FB
10K
R3
120K
R1
13
VADJ=3.3V, LED off
VADJ=0V, LED=20mA
Figure. 2 Brightness Control by an adjustable DC
voltage
b. Using a PWM signal to apply to SHDN
An external PWM signal applied to SHDN pin cyclically
turns on or off the converter. The average current through
the LEDs will increase proportionally to the duty cycle
of the PWM signal. Due to the soft-start duration, the
PWM signal with frequency from 100Hz to 300Hz is
recommended.
D1
L1
MSCD052
VIN
VOUT
2.7V~6V
4.7µH
C1
4.7µF
6
VIN
1
SW
C2
0.1µF
2
GND
5
OVP
PWM 100Hz~300Hz
brightness
control
4
SHDN
3
FB
Duty=100%, LED=20mA
R1
Duty=0%, LED off
13
Figure. 3 Brightness Control by applying a PWM signal
to SHDN
c. Using a filtered PWM signal
In figure. 4, the brightness control can be achieved by
applying a PWM signal to an RC filter (R4 and C3) to
generate a filtered PWM signal instead of the V . The
ADJ
PWM signal with frequency above 5kHz is recommended.
VIN
2.7V~6V
C1
4.7µF
L1
4.7µH
6
VIN
1
SW
D1
MSCD052
C2
0.1µF
VOUT
2
GND
5
OVP
4
3
R2
SHDN
FB
PWM
3.3V
R3
10K
120K
R1
13
brightness
0V
control
R4
Duty=100%, LED off
10K
Duty=0%, LED=20mA
C3
0.1uF
Figure. 4 Brightness Control by a filtered PWM signal
Inductor Selection
The inductor together with the load current (IOUT), internal
peak current (I ), input (V ) and output voltage (V ) of
PK
IN
OUT
the application determines the switching frequency of the
Copyright © ANPEC Electronics Corp.
9
Rev. A.3 - Jul., 2008
www.anpec.com.tw
Share Link: 