A6211 ALLEGRO [Allegro MicroSystems], A6211 Datasheet - Page 8

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

A6211
The A6211 is a buck regulator designed for driving a high-current
LED string. It utilizes average current mode control to maintain
constant LED current and consistent brightness. The LED current
level is easily programmable by selection of an external sense
resistor, with a value determined as follows:
where V
Switching Frequency
The A6211 operates in fixed on-time mode during switching. The
on-time (and hence switching frequency) is programmed using
an external resistor connected between the VIN and TON pins, as
Figure 6. Switching Frequency versus R
i
LED
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
= V
Figure 7. Simplified buck controller equations, and reference circuit and waveforms
0
CSREG
CSREG
20
C
• During SW on-time:
• During SW off-time:
where D = t
Therefore (simplified equation for Output Voltage):
More precisely:
where R
If V
IN
= 0.2 V typical.
40
i
i
D
/ R
RIPPLE
RIPPLE
V
V
<< V
OUT
OUT
VIN
60
SENSE
L
is the resistance of the inductor.
OUT
= [(V
= [(V
= V
= (V
ON
MOS
80 100 120 140 160 180 200 220 240 260
IN
IN
, then V
/ T.
IN
OUT
× D – V
– I
– V
A6211
av
– V
OUT
× R
D
OUT
R
) / L] × t
D
) / L] × t
DS(on)
TON
× (1 – D)
SW
≈
D
TON
V
) × D – V
(kΩ)
IN
OFF
ON
× D.
Resistance
= [(V
= [(V
L
RSENSE
D
IN
OUT
× (1 – D) – R
– V
Constant-Current 3-Ampere PWM Dimmable
i
V
L
– V
OUT
OUT
D
) / L] × T × (1 – D)
) / L] × T × D
Functional Description
L
× I
av
i(max)
given by the following equation:
where k = 0.0139, with f
(internal resistance, 5 kΩ) in kΩ (see figure 6).
Enable and Dimming
The IC is activated when a logic high signal is applied to the EN
(enable) pin. The buck converter ramps up the LED current to a
target level set by RSENSE.
When the EN pin is forced from high to low, the buck converter
is turned off, but the IC remains in standby mode for up to 10 ms.
If EN goes high again within this period, the LED current is
turned on immediately. Active dimming of the LED is achieved
by sending a PWM (pulse-width modulation) signal to the EN
pin. The resulting LED brightness is proportional to the duty cycle
( T
dimming frequency is between 100 Hz ( Period = 10 ms) and
2 kHz. At a 200 Hz PWM frequency, the dimming duty cycle can
be varied from 100% down to 1% or lower.
If EN is low for more than 17 ms, the IC enters shutdown mode
to reduce power consumption. The next high signal on EN will
initialize a full startup sequence, which includes a startup delay
of approximately 130 μs. This startup delay is not present during
PWM operation.
The EN pin is high-voltage tolerant and can be directly connected
to a power supply. However, if EN is higher than the V
i(min)
V
–V
V
SW
IN
i
ON
i
0
D
av
L
t
f
ON
SW
/ Period ) of the PWM signal. A practical range for PWM
= k × (R
= 1 / [ k × (R
Buck Regulator LED Driver
t
ON
ON
+ R
ON
T
INT
+ R
SW
) × ( V
115 Northeast Cutoff
1.508.853.5000; www.allegromicro.com
INT
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
in MHz, t
t
)]
OFF
OUT
/ V
ON
IN
in μs, and R
)
t
t
i
RIPPLE
ON
IN
and R
voltage
INT
8