a8431 Allegro MicroSystems, Inc., a8431 Datasheet - Page 6

• Manufacturer Part Number: a8431
• Description: White Led Driver Constant Current Step-up Converter
• Manufacturer: Allegro MicroSystems, Inc.

A8431
Device Operation
The A8431 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 in schematic 1) 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
error signal 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
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, L1, diminishes, resulting in the inductor current
dropping to 0 A for low load-current levels. This is known
120
110
100
90
80
70
60
50
40
30
20
10
0
the load current,
I
5
LOAD
Vin = 3V, 3 LED
is the target load current (mA).
Power Dissipation versus I
10
R1 = 95 mV ⁄
Vin = 5V, 3 LED
the load current
ensure that the required internal
I
OUT
15
(mA)
I
LOAD
Vin = 3V, 4 LED
decreases, the
OUT
20
Vin = 5V, 4 LED
White LED Driver Constant Current
25
as Discontinuous mode operation, and results in some low-
frequency ripple. The average load current, however, remains
regulated down to 0 A.
In Discontinuous mode, when the inductor current drops to
0 A, 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.
Overvoltage Protection
An overvoltage event can occur when the LEDs become
disconnected or fail in an open state. In these cases, the
current flow through the sense resistor, R1, becomes 0 A and
thus the feedback voltage, V
compensates by increasing the on time of the switch, which
increases the output voltage.
The A8431 has built-in protection to guard against excessive
voltage on the SW pin. If the output voltage exceeds the
typical level of the Output Overvoltage Rising Limit, V
then the overvoltage protection circuitry shuts off the internal
switch until the output voltage falls below the Output
Overvoltage Falling Limit, V
operates normally. There is no need for an external Zener
diode for the A8431.
Target Load Current
(
I
(mA)
LOAD
10
12
15
20
5
)
115 Northeast Cutoff, Box 15036
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 [phone]
www.allegromicro.com
Step-up Converter
FB
OVPF
becomes 0 V. The A8431
. At this point, the A8431
Sense Resistor (R1)
19.1
9.53
7.87
6.34
4.75
(Ω)
OVPR
,
6