Constant-Current 3-Ampere PWM Dimmable

Buck Regulator LED Driver

A6211

9

Allegro MicroSystems, Inc.

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

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:

Dimming ratio = PWM on-time / PWM period

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-

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:

Schottky diode D1 (typically under 0.5 V).

Minimum and Maximum Output Voltages

For a given input voltage, the maximum output voltage depends

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

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

This is shown in figure 8.

Figure 8 shows how the minimum and maximum output volt-

If the required output voltage is lower than that permitted by the

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 8. Minimum and Maximum Output Voltage versus Switching

24

22

20

18

16

14

12

10

8

6

4

2

0

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

fsw (MHz)

9

8

7

6

5

4

3

2

1

0

0

0.5

1.0

1.5

2.0

2.5

3.0

iLED (A)