4
Worcester, Massachusetts 01615-0036 (508) 853-5000
115 Northeast Cutoff, Box 15036
www.allegromicro.com
Data Sheet
Advance Information
26185.300b
A8430
Constant Current LED Driver Boost Converter
Component Selection
The component values shown in schematic 1 are suffi cient 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. With an internal PWM frequency of
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, ISWLIM. If high temperature 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. The diode should have a low forward 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 suffi cient.
Capacitor Selection. Because the capacitor values are 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
Application Information
Schematic 2. Dimming control with dc voltage
feedback modulation
A8430
VIN SW
EN GND FB
Li-ion
2.5V to
4.2V
C1
1µF
L1
22µH D1
C2
0.22µF
R1
6R3
VC
R3
90k
R2
5k
Enable Ω
Ω
Ω
LED current, ILOAD , directly, or by using a PWM signal on
the EN pin to chop the output.
Feedback modulation. By adding a voltage drop between
the FB pin and R1 (the sense resistor), as shown in sche-
matic 2, the LED current, ILOAD , can be made to decrease.
As VC (control voltage) increases, the voltage drop across
R2 also increases. This causes the voltage at FB to increase,
and the A8430 reduces ILOAD to compensate. As VC increases
further, the current drops to zero, and R2 maintains the full
95 mV on FB. Reducing VC diminishes the voltage across R2
until, at 95 mV on VC, there is no drop across R2 and the cur-
rent level is defi ned by R1. Reducing VC below 95 mV causes
ILOAD to increase further, due to the voltage drop across R2
in the reverse direction. This continues until, at zero volts
on VC, there is approximately 5 mV across R2. At that point,
ILOAD (mA), is defi ned as:
ILOAD = 100 mV / R1
where R1 is the resistance of the sense resister (Ω).
PWM Control. LED dimming control can also be gener-
ated by a fi ltered 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,
ILOAD , to go to zero.