A8483 Datasheet, PDF (4/9 Pages) Allegro MicroSystems – 1.2 MHz Step-up Converter for Display Bias Supply

A8483 ^{1.2 MHz Step-up Converter for Display Bias Supply}
Functional Description
Pin Functions
VIN. Supply to the control circuit. A bypass capacitor, C1, must
be connected from close to this pin to GND.
GND. Ground reference connected directly to the ground plane.
The feedback resistor should have a separate connection directly
to this point.
EN. Voltage lower than 0.4 V disables the A8483 and puts the
control circuit into the low-power sleep mode. Voltage greater
than 1.5 V fully enables the A8483.
SW. Low-side switch connection between the inductor, L1,
and ground. Because rapid changes of current occur at this
pin, the traces on the PCB that are connected to this pin should
be minimized. In addition, L1 and the diode, D1, should be
connected as close to this pin as possible.
FB. Feedback pin for OLED voltage control or WLED current
control. The reference voltage is 615 mV. Connect the feedback
resistor close to this pin to minimize noise.
Device Operation
The A8483 uses a 1.2 MHz constant switching frequency current-
mode control scheme to regulate the output voltage or current
through the load.
A typical OLED bias supply is shown in figure 7. For driving
OLEDs, output voltage is sensed by the FB pin through a voltage
divider network. Output voltage, V_OUT (V), is set according to the
following equation:
V_{OUT =} ^{R1 + R2} _{0.615 .}
(1)
R₂
A typical WLED backlight supply is shown in figure 8. The load
current, I_LOAD, is set by the selecting the external sense resistor,
R1, to produce 615 mV at the desired load, for example:
R₁ = 0.615 I_LOAD .
(2)
Voltage sensed across the FB pin is compared with the internal
615 mV reference to produce an error signal. The switch current
is sensed by the internal sense resistor and compared to the
error signal for current mode PWM control. As the error signal
increases, I_LOAD , increases to increase either output voltage,
V_OUT, or current, I_OUT , such that the FB pin voltage follows the
internal 615 mV reference voltage.
As I_LOAD is reduced, the energy required in the inductor, L1, also
reduces, resulting in the inductor current dropping to zero for
low load current levels. This is known as Discontinuous mode
operation and results in some low-frequency ripple. The average
load current will, however, remain regulated down to zero. In
Discontinuous mode, when I_LOAD drops to zero, the voltage at
the SW pin rings, due to the resonant LC circuit formed by L1
and the switch and diode D1 capacitance. This ringing is low-
frequency and is not harmful. It can be damped with a resistor
across the inductor but this will reduce efficiency and is not
recommended.
L1
5
10 µH ₁
VIN
SW
VSUPPLY C1
1 µF
Li-ion
2.5 V to
5.5 V
A8483
EN GND FB
423
Enable
VOUT
D1
R1
820 kΩ
R2
39 kΩ
C2
1 µF
Figure 7. OLED Bias Supply. Use for cell phones, MP3 players, DSCs,
and PDAs. (Circuit also shown in figure 1.)
L1
5
10 µH ₁
VIN
SW
VSUPPLY C1
1 µF
Li-ion
2.5 V to
5.5 V
A8483
EN GND FB
423
Enable
Cf
0.01 µF
VOUT
D1
R3
24 kΩ
R2
5 kΩ
R1
47 Ω
C2
0.22 µF
VC
Analog Voltage or
PWM
Figure 8. WLED Backlighting. (Circuit also shown in figure 5.)
A8483-DS, Rev. 1
Allegro MicroSystems, Inc.
4
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 [phone]
www.allegromicro.com