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A6217KLJTR-1-T Datasheet(PDF) 8 Page - Allegro MicroSystems |
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A6217KLJTR-1-T Datasheet(HTML) 8 Page - Allegro MicroSystems |
8 / 19 page 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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