A6210 Datasheet, PDF (6/11 Pages) Allegro MicroSystems – 3 A, 2 MHz Buck-Regulating LED Driver

A6210
3 A, 2 MHz Buck-Regulating LED Driver
of physically small, low value inductors. An output capacitor is
not necessary either to reduce the ripple current or to close the
control loop.
High efficiencies are achieved via drive circuits optimized to
minimize switching losses and the current sense voltage has a
typical voltage drop of only 183 mV. The current in the LED
string can be pulse width modulated (PWM) via the DIS (Dis-
able/Enable) pin. See figure 4.
The actual current control is maintained on the valley of the cur-
rent ripple. The average LED current is the valley level plus half
the inductor ripple current, as shown in figure 2.
To avoid potential mistriggering issues, it is recommended that
the ripple current that flows through the sense resistor (R2) does
not develop a ripple voltage of less than 20 mV.
The average LED current can be found from:
I_av
=
I_VALLEY +
I_RIPPLE
2
,
(4)
substituting values:
I_{av =}
183 mV
R2

+ _

1
2
×
V_IN
–V_LED
L
×
t_on



,
(5)
where:
t_{on =}
V_LED +V_f
V_IN +V_f
×
1
f_SW
.
(6)
Note: V_f is the forward voltage drop of the recirculation diode
and sense resistor (R2). The valley current is determined by the
sense voltage (183 mV) divided by the sense resistor.
Worked example
This example uses the brief specification outlined in the typical
application circuit on page 1. The following information is used
as a starting point:
V_IN = 24 V ,
3 LEDs producing V_LED = 12 V ,
I_LED = 500 mA, and
LED ripple current, I_RIPPLE = 60 mA .
The duty cycle can be found initially. Assume the forward voltage
drop of the re-circulation diode is 400 mV, and that the sense
resistor is 183 mV. Then:
D
=
V_LED +V_f
V_IN +V_f
=
12 + 0.58
24 + 0.58
=
0.39
.
(7)
One of the objectives is to maximize the switching frequency to
minimize the inductor value. When driving at very high switching
frequencies, the duty cycle may be limited due to the minimum
2000
1800
1600
1400
1200
1000
800
600
400
10⁴
1 LED
2 LEDs
3 LEDs
4 LEDs
5 LEDs
10⁵
10⁶
Resistor, R1 (kΩ)
Figure 1. Switching frequency versus value of external resistor R1 on the
TON pin.
t_on + t_off = 1/ f_SW
t_on
t_off
Valley Current
0
Time
Figure 2. Current control
1_{/2 IRIPPLE}
Average
LED Current
Allegro MicroSystems, Inc.
6
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com