A6211 ALLEGRO [Allegro MicroSystems], A6211 Datasheet - Page 11

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

A6211
Additional Notes on Ripple Current
• For stability, pick the inductor and switching frequency to
ensure the lowest inductor ripple current percentage is at least
12.5% during worst case (at the lowest V
• 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.
V
SW
IN
Without output capacitor:
Ripple current through LED string is
proportional to ripple voltage at CS pin.
D1
V
ripple
L1
CS
Figure 12. Ripple current and voltage, with and without shunt capacitor
LED+
IN
LED–
R
).
SENSE
Constant-Current 3-Ampere PWM Dimmable
I
ripple
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.
V
SW
IN
Buck Regulator LED Driver
With a small capacitor across LED string:
Ripple current through LED string is reduced,
while ripple voltage at CS pin remains high.
D1
V
ripple
L1
CS
C1
115 Northeast Cutoff
1.508.853.5000; www.allegromicro.com
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
LED+
LED–
R
SENSE
I
ripple
11