a8430 Allegro MicroSystems, Inc., a8430 Datasheet - Page 6

• Manufacturer Part Number: a8430
• Description: White Led Driver Constant Current Step-up Converter
• Manufacturer: Allegro MicroSystems, Inc.

Component Selection
The component values shown in schematic 1 are sufficient
for most applications. To reduce the output ripple the
inductor may be increased, but in most cases this results in
excessive board area and cost.
Inductor Selection
1.2 MHz, the optimal inductor value for most cases is 22 µH.
The inductor should have low winding resistance, typically
< 1 Ω, and the core should have low losses when operating
at 1.2 MHz. For worst case conditions, high output voltage
and current and low input voltage, the inductor should be
rated at the switch current limit, I
operation is required a derating factor will have to be
considered. In some cases, where lower inductor currents
are expected, the current rating can be decreased. Several
inductor manufacturers have and are developing suitable
small-size inductors, including: Murata, Panasonic, Sumida,
Taiyo Yuden, and TDK.
Diode Selection
voltage to reduce conduction losses. In addition, it should
have a low capacitance to reduce switching losses. Schottky
diodes can provide both these features, if carefully
selected. The forward voltage drop is a natural advantage
for Schottky diodes, and it reduces as the current rating
increases. However, as the current rating increases, the
diode capacitance also increases. As a result, the optimal
selection is usually the lowest current rating above the circuit
maximum. With the A8430, a current rating in the range from
100 mA to 200 mA is usually sufficient.
Capacitor Selection
low, ceramic capacitors are the best choice for use with the
A8430. To reduce performance variation as temperature
changes, low drift capacitor types, such as X7R and X5R,
should be used. Suitable capacitors are available from: Taiyo
Yuden, Murata, Kemet, and AVX.
Dimming Control
LED brightness can be controlled either by modifying the
voltage at the top of the sense resistor (R1) to control the
LED current, I
the EN pin to chop the output.
White LED Driver Constant Current Step-up Converter
LOAD
. The diode should have a low forward
. With an internal PWM frequency of
, directly, or by using a PWM signal on
. Because the capacitor values are
SWLIM
. If high temperature
Application Information
www.allegromicro.com
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 [phone]
Feedback modulation
between the FB pin and R1 (the sense resistor), as shown
in schematic 2, the LED current, I
decrease. As V
across R2 also increases. This causes the voltage at FB to
increase, and the A8430 reduces I
increases further, the current drops to zero, and R2 maintains
the full 95 mV on FB. Reducing V
across R2 until, at 95 mV on V
and the current level is defined by R1. Reducing V
95 mV causes I
drop across R2 in the reverse direction. This continues until,
at zero volts on V
At that point, I
where R1 is the resistance of the sense resister (Ω).
PWM Control
by a filtered PWM signal as shown in schematic 3. In this
case, a 0% duty cycle (PWM = 0 V) corresponds to full
brightness and a 100% duty cycle causes the LED current,
I
LOAD
, to go to zero.
Schematic 2. Dimming control with dc voltage
feedback modulation
Li-ion
2.5V to
4.2V
. LED dimming control can also be generated
LOAD
LOAD
C
(control voltage) increases, the voltage drop
C
C1
1µF
, there is approximately 5 mV across R2.
(mA), is defined as:
I
to increase further, due to the voltage
LOAD
VIN
. By adding a voltage drop
Enable
EN
= 100 mV / R1
A8430
V
GND
L1
22µH
C
C
, there is no drop across R2
LOAD
LOAD
C
FB
diminishes the voltage
SW
R3
90kΩ
R2
5kΩ
, can be made to
to compensate. As V
D1
R1
6.3Ω
A8430
C
C2
0.22µF
below
C
6