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A6217KLJTR-1-T Datasheet(PDF) 8 Page - Allegro MicroSystems

Part # A6217KLJTR-1-T
Description  Automotive-Grade, Constant-Current PWM Dimmable Buck Regulator LED Driver
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Manufacturer  ALLEGRO [Allegro MicroSystems]
Direct Link  http://www.allegromicro.com
Logo ALLEGRO - Allegro MicroSystems

A6217KLJTR-1-T Datasheet(HTML) 8 Page - Allegro MicroSystems

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Automotive-Grade, Constant-Current
PWM Dimmable Buck Regulator LED Driver
A6217 and
A6217-1
8
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
The A6217 is a buck regulator designed for driving a high-current
LED string. It uses 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:
iLED = VCSREG / RSENSE
where VCSREG = 0.2 V typical.
Switching Frequency
The A6217 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
given by the following equations:
tON = k × (RON + RINT) × ( VOUT / VIN )
fSW = 1 / [ k × (RON + RINT)] + c
where k = 0.014 and c = 0.09, with fSWinMHz,tON in µs, and
RON and RINT(internalresistance,6kΩ)inkΩ(seefigure6).
TominimizethepeaksofswitchingfrequencyharmonicsinEMC
measurement, a dithering feature is implemented. The dithering
range is internally set at ±5%. The actual switching frequency is
swept linearly between 0.95 × fSW and 1.05 × fSW, where fSW is
the programmed switching frequency. The rate of modulation for
fSWisfixedinternallyat~11kHz.
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 12 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
(tON/Period) of the PWM signal. A practical range for PWM dim-
FUNCTIONAL DESCRIPTION
Figure 6: Average Switching Frequency versus RON Resistance
(VIN = 12 V, VOUT = ~7 V, iLED = 1 A)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
0
20
40
60
80 100 120 140 160 180 200 220 240 260 280 300 320
RON (kΩ)
Figure 7: Simplified Buck Controller Equations
• During SW on-time:
iRIPPLE = [(VIN –VOUT) / L] × tON = [(VIN –VOUT) / L] × tSW × D
iRIPPLE = [(VOUT –VD) / L] × tOFF = [(VOUT –VD) / L] × tSW × (1 – D)
VOUT = VIN × D – VD × (1 – D)
VOUT = (VIN – iav × RDS(on)) × D – VD × (1 – D) – RL × iav
where D = tON / tSW.
• During SW off-time:
Therefore (simplified equation for Output Voltage):
More precisely:
Where RL is the resistance fo the inductor.
If VD << VOUT, then VOUT VIN × D.
VSW
iL
t
t
VIN
i(max)
iav
i(min)
0
iRIPPLE
tON
tOFF
tSW
–VD
CIN
VIN
A6217
SW
VOUT
RSENSE
L
iL
MOS
D


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