a8430 Allegro MicroSystems, Inc., a8430 Datasheet - Page 5

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

Device Operation
The A8430 uses a constant-frequency, current-mode control
scheme to regulate the current through the load. The load
current produces a voltage across the external sense resistor
(R1) and the input at the FB pin. This voltage is then
compared to the internal 95 mV reference to produce an error
signal. The switch current is sensed by the internal sense
resistor and compared to the load current error signal. As the
load current increases, the error signal diminishes, reducing
the maximum switch current and thus the current delivered
to the load. As
rises, increasing the maximum switch current and thus
increasing the current delivered to the load.
To set
reference value of 95 mV is produced at the desired load. To
do so, select a resistance value for the sense resistor, R1 (Ω),
such that:
where
120
110
100
90
80
70
60
50
40
30
20
10
0
the load current,
I
5
LOAD
White LED Driver Constant Current Step-up Converter
Vin = 3V, 3 LED
is the target load current (mA).
the load current
Power Dissipation versus I
10
R1 = 95 mV /
Vin = 5V, 3 LED
ensure that the required internal
I
OUT
15
decreases, the error signal
(mA)
I
LOAD
Vin = 3V, 4 LED
OUT
20
Vin = 5V, 4 LED
www.allegromicro.com
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 [phone]
25
The table below shows typical values for R1. Note that the
resistance value is from the standard E96 series.
As load current is reduced, the energy required in the
inductor diminishes, 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, however, remains
regulated down to zero.
In Discontinuous mode, when the inductor current drops to
zero, the voltage at the SW pin rings, due to the capacitance
in the resonant LC circuit formed by the inductor and the
capacitance of the switch and the diode. This ringing is
low-frequency and is not harmful. It can be damped with a
resistor across the inductor, but this reduces efficiency and is
not recommended.
Target Load Current
(
I
(mA)
LOAD
10
12
15
20
5
)
Sense Resistor (R1)
19.1
9.53
7.87
6.34
4.75
(Ω)
A8430
5