3 A, 2 MHz Buck-Regulating LED Driver

A6210

6

Allegro MicroSystems, Inc.

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

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:

=,

2

(4)

substituting values:

=

,

R2

183 mV













L

×

×

1

2

+

(5)

where:

.

V

V

×

1

f

SW

(6)

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:

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

0.39

=

.

==

12 + 0.58

24 + 0.58

(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

Average

LED Current

RIPPLE

Time

Valley Current

0

Figure 2. Current control

Figure 1. Switching frequency versus value of external resistor R1 on the

TON pin.

400

600

800

1000

1200

1400

1600

1800

2000

Resistor, R1 (kΩ)

1 LED

2 LEDs

3 LEDs

4 LEDs

5 LEDs