Constant-Current 3-Ampere PWM Dimmable

Buck Regulator LED Driver

A6211

11

Allegro MicroSystems, Inc.

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

SW

D1

LED+

LED–

L1

CS

SW

D1

LED+

LED–

L1

CS

C1

Figure 12. Ripple current and voltage, with and without shunt capacitor

Without output capacitor:

Ripple current through LED string is

proportional to ripple voltage at CS pin.

With a small capacitor across LED string:

Ripple current through LED string is reduced,

while ripple voltage at CS pin remains high.

Additional Notes on Ripple Current

• For stability, pick the inductor and switching frequency to

ensure the lowest inductor ripple current percentage is at least

• There is no hard limit on the highest ripple current percentage

allowed. A 60% ripple current is still acceptable, as long as both

the inductor and LEDs can handle the peak current (average cur-

rent × 1.3 in this case). However, care must be taken to ensure

the valley of the inductor ripple current never drops to zero at the

highest input voltage (which implies a 200% ripple current).

• In general, allowing a higher ripple current percentage enables

lower-inductance inductors to be used, which results in smaller

size and lower cost. The only down-side is the core loss of the

inductor increases with larger ripple currents. But this is typically

a small factor.

• If lower ripple current is required for the LED string, one solu-

tion is to add a small capacitor (such as 2.2 μF) across the LED

string from LED+ to LED– . In this case, the inductor ripple cur-

rent remains high while the LED ripple current is greatly reduced.

Output Filter Capacitor

The A6211 is designed to operate without an output filter capaci-

tor, in order to save cost. Adding a large output capacitor is not

recommended.

In some applications, it may be required to add a small filter

capacitor (up to several μF) across the LED string (between

LED+ and LED-) to reduce output ripple voltage and current. It is

important to note that:

• The effectiveness of this filter capacitor depends on many fac-

tors, such as: switching frequency, inductors used, PCB layout,

LED voltage and current, and so forth.

• The addition of this filter capacitor introduces a longer delay

in LED current during PWM dimming operation. Therefore the

maximum PWM dimming ratio is reduced.

• The filter capacitor should NOT be connected between LED+

and GND. Doing so may create instability because the control

loop must detect a certain amount of ripple current at the CS pin

for regulation.