A6211 ALLEGRO [Allegro MicroSystems], A6211 Datasheet - Page 9

• Manufacturer Part Number: A6211
• Description: Constant-Current 3-Ampere PWM Dimmable Buck Regulator LED Driver
• Manufacturer: ALLEGRO [Allegro MicroSystems]

A6211
at any time, a series resistor (1 kΩ) is required to limit the current
flowing into the EN pin. This series resistor is not necessary if
EN is driven from a logic input.
PWM Dimming Ratio
The brightness of the LED string can be reduced by adjusting the
PWM duty cycle at the EN pin as follows:
For example, by selecting a PWM period of 5 ms (200 Hz PWM
frequency) and a PWM on-time of 50 μs, a dimming ratio of 1%
can be achieved.
In an actual application, the minimum dimming ratio is deter-
mined by various system parameters, including: V
inductance, LED current, switching frequency, and PWM
frequency. As a general guideline, the minimum PWM on-time
should be kept at 50 μs or longer. A shorter PWM on-time is
acceptable under more favorable operating conditions.
Output Voltage and Duty Cycle
Figure 7 provides simplified equations for approximating output
voltage. Essentially, the output voltage of a buck converter is
approximately given as:
where D is the duty cycle, and V
Schottky diode D1 (typically under 0.5 V).
Figure 8. Minimum and Maximum Output Voltage versus Switching
Frequency (V
Dimming ratio = PWM on-time / PWM period
V
D = t
OUT
24
22
20
18
16
14
12
10
8
6
4
2
0
0
ON
= V
IN
/ (t
0.2
IN
= 24 V, i
ON
× D – V
0.4
+ t
OFF
LED
0.6
D1
= 2 A, minimum t
)
× (1 – D ) ≈ V
0.8
V
V
OUT
OUT
f sw (MHz)
D1
1.0
(max) (V)
(min) (V)
is the forward drop of the
1.2
ON
IN
× D, if V
and t
1.4
Constant-Current 3-Ampere PWM Dimmable
OFF
1.6
IN
= 150 ns)
D1
, V
<< V
1.8
OUT
IN
,
2.0
Minimum and Maximum Output Voltages
For a given input voltage, the maximum output voltage depends
on the switching frequency and minimum t
t
cycle is 85%. So for a 24 V input, the maximum output is 20.3 V.
This means up to 6 LEDs can be operated in series, assuming
V
The minimum output voltage depends on minimum t
switching frequency. For example, if the minimum t
and f
means with V
To a lesser degree, the output voltage is also affected by other
factors such as LED current, on-resistance of the high-side
switch, DCR of the inductor, and forward drop of the low-side
diode. The more precise equation is shown in figure 7.
As a general rule, switching at lower frequencies allows a wider
range of V
This is shown in figure 8.
Figure 8 shows how the minimum and maximum output volt-
ages vary with LED current (assuming R
DCR = 0.1 Ω, and diode V
If the required output voltage is lower than that permitted by the
minimum t
in order to maintain the correct duty cycle. This means that the
switching frequency will drop lower when necessary, while the
LED current is kept in regulation at all times.
Figure 9. Minimum and Maximum Output Voltage versus i
(V
OFF
f
IN
= 3.3 V or less for each LED.
(min) = 150 ns and f
= 9 V, f
SW
9
8
7
6
5
4
3
2
1
0
0
= 1 MHz, then the minimum duty cycle is 15%. That
SW
Buck Regulator LED Driver
OUT
ON
= 1 MHz, minimum t
IN
, the controller will automatically extend the t
, and hence more flexible LED configurations.
0.5
= 24 V, the minimum V
SW
1.0
f
115 Northeast Cutoff
1.508.853.5000; www.allegromicro.com
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
= 1 MHz, then the maximum duty
= 0.6 V).
V
V
ON
OUT
OUT
i LED (A)
1.5
and t
(max) (V)
(min) (V)
OFF
OUT
DS(on)
= 150 ns)
2.0
OFF
= 3.2 V (one LED).
. For example, if
= 0.4 Ω, inductor
LED
2.5
ON
ON
current
= 150 ns
and
OFF
3.0
,
9