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Description

The A6211 is a single IC switching regulator that provides constant-current output to drive high-power LEDs. It integrates a high-side N-channel DMOS switch for DC-to-DC step- down (buck) conversion. A true average current is output using a cycle-by-cycle, controlled on-time method. Output current is user-selectable by an external current sense resistor. Output voltage is automatically adjusted to drive various numbers of LEDs in a single string. This ensures the optimal system efficiency. LED dimming is accomplished by a direct logic input pulse width modulation (PWM) signal at the enable pin. The device is provided in a compact 8-pin narrow SOIC package (suffix LJ) with exposed pad for enhanced thermal dissipation. It is lead (Pb) free, with 100% matte tin leadframe plating. A6211-DS Features and Benefits

  • Supply voltage 6 to 48 V
  • True average output current control
  • 3.0 A maximum output over operating temperature range
  • Cycle-by-cycle current limit
  • Integrated MOSFET switch
  • Dimming via direct logic input or power supply voltage
  • Internal control loop compensation
  • Undervoltage lockout (UVLO) and thermal shutdown protection
  • Low power shutdown (1 μA typical)
  • Robust protection against: ▫ Adjacent pin-to-pin short ▫ Pin-to-GND short ▫ Component open/short faults Constant-Current 3-Ampere PWM Dimmable Buck Regulator LED Driver Package 8-pin SOICN with exposed thermal pad (suffix LJ): Applications:
  • General Illumination
  • Scanners and multi-function printers (light bars)
  • Architectural lighting
  • Industrial Lighting
  • Display case lighting / MR16 Typical Application Circuit Not to scale A6211 GND VIN VIN (6 to 48 V) SW GND VCC A6211 BOOTTON EN CS C4 D1 L1 LED + LED– RSENSE EN Enable/PWM Dimming (100 Hz to 2 kHz) . . .

Constant-Current 3-Ampere PWM Dimmable Buck Regulator LED DriverA6211 2Allegro MicroSystems, Inc.

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A. Absolute Maximum Ratings Characteristic Symbol Notes Rating Unit Supply Voltage V IN –0.3 to 50 V Bootstrap Drive Voltage V BOOT –0.3 to VIN + 8 V Switching Voltage V SW –1.5 to VIN + 0.3 V Linear Regulator Terminal V CC VCC to GND –0.3 to 14 V Enable and TON Voltage V EN , VTON –0.3 to VIN + 0.3 V Current Sense Voltage V CS –0.3 to 7 V Operating Ambient Temperature T A G temperature range –40 to 105 ºC Maximum Junction Temperature T J(max) 150 ºC Storage Temperature T stg –65 to 125 ºC Selection Guide Part Number Operating Ambient Temperature, TA Package Packing A6211GLJTR-T –40ºC to 105ºC 8-pin SOICN with exposed thermal pad 3000 pieces per 13-in reel Thermal Characteristics may require derating at maximum conditions, see application information Characteristic Symbol Test Conditions* Value Unit Package Thermal Resistance, Junction to Ambient R θJA 4-layer PCB based on JEDEC standard 35 ºC/W Package Thermal Resistance, Junction to Pad RθJP 2 ºC/W *Additional thermal information available on the Allegro website. Pin-out Diagram Terminal List Table Number Name Function

1 VIN Supply voltage input terminals

2 TON Regulator on-time setting resistor terminal

3 EN Logic input for Enable and PWM dimming

4 CS Drive output current sense feedback

5 VCC Internal linear regulator output

6 GND Ground terminal

7 BOOT DMOS gate driver bootstrap terminal

8 SW Switched output terminals

–P A D Exposed pad for enhanced thermal dissipation; connect to GND SW BOOT GND VCC VIN TON EN CS PAD

Constant-Current 3-Ampere PWM Dimmable Buck Regulator LED DriverA6211 3Allegro MicroSystems, Inc. Worcester, Massachusetts 01615-0036 U.S.A. VIL = 0.4 V VIH = 1.8 V 0.2 V BOOT CVCC CBOOT L1 LED String VCC Shutdown VINVIN CCOMP EN TON RON CS GND RSENSE SW On-Time Current Generator On-Time Timer IC and Driver Control Logic Level Shift Gate Drive UVLO VCC UVLO Off-Time Timer VREG 5.3 V Buck Switch Current SenseCurrent Limit Off-Time Timer ILIM Thermal Shutdown VCC UVLO PAD Average Functional Block Diagram

Constant-Current 3-Ampere PWM Dimmable Buck Regulator LED DriverA6211 4Allegro MicroSystems, Inc. Worcester, Massachusetts 01615-0036 U.S.A. ELECTRICAL CHARACTERISTICS Valid at VIN = 24 V, for TA = –40°C to 105°C, typical values at TA = 25°C; unless otherwise noted Characteristics Symbol Test Conditions Min. Typ. Max. Unit Input Supply Voltage V IN TA = 25°C 6 – 48 V VIN Undervoltage Lockout Threshold V UVLO VIN increasing – 5.3 – V VIN Undervoltage Lockout Hysteresis V UVLO_HYS VIN decreasing – 150 – mV VIN Pin Supply Current I IN VCS = 0.5 V, EN = high – 5 – mA VIN Pin Shutdown Current I INSD EN shorted to GND – 1 10 μA BOOT Switch Current Limit Threshold I SWLIM 3.0 4.0 5.0 A Buck Switch On-Resistance RDS(on) VBOOT = VIN + 4.3 V, TA = 25°C, ISW = 1 A – 0.25 0.4 Ω BOOT Undervoltage Lockout Threshold V BOOTUV VBOOT to VSW increasing 1.7 2.9 4.3 V BOOT Undervoltage Lockout Hysteresis VBOTUVHYS VBOOT to VSW decreasing – 370 – mV Switching Minimum Off-Time t OFFmin VCS = 0 V – 110 150 ns Switching Minimum On-TIme t ONmin – 110 150 ns Selected On-Time t ON VIN = 24 V, VOUT = 12 V, RON = 137 kΩ 800 1000 1200 ns Regulation Comparator and Error Amplifier Load Current Sense Regulation Threshold VCSREG VCS decreasing, SW turns on 187.5 200 210 mV Load Current Sense Bias Current I CSBIAS VCS = 0.2 V, EN = low – 0.9 – μA Internal Linear Regulator VCC Regulated Output V CC 0 mA < ICC < 5 mA, VIN > 6 V 5.0 5.3 5.6 V VCC Current Limit* I CCLIM VIN = 24 V, VCC = 0 V 5 20 – mA Enable Input Logic High Voltage V IH VEN increasing 1.8 – – V Logic Low Voltage V IL VEN decreasing – – 0.4 V EN Pin Pull-down Resistance R ENPD VEN = 5 V – 100 – k Ω Maximum PWM Dimming Off-Time t PWML Measured while EN = low, during dimming control, and internal references are powered-on (exceeding t PWML results in shutdown) 10 17 – ms Thermal Shutdown Thermal Shutdown Threshold T SD – 165 – °C Thermal Shutdown Hysteresis T SDHYS –2 5–° C *The internal linear regulator is not designed to drive an external load

Worcester, Massachusetts 01615-0036 U.S.A. Figure 1. Startup waveforms from off-state at various input voltages; note that the rise time of the LED current depends on

  • Operating conditions: LED voltage = 15 V, LED current = 1.3 A, R1 = 63.4 kΩ (frequency = 1 MHz in steady state), VIN = 19 V (panel 1A), 24 V (panel 1B) and 30 V (panel 1C)
  • Oscilloscope settings: CH1 (Red) = VIN (10 V/div), CH2 (Blue) = VOUT (10 V/div), CH3 (Green) = iLED (500 mA/div), CH4 (Yellow) = Enable (5 V/div), time scale = 50 μs/div t C1,C2 VIN VOUT iLED VEN t C1,C2 VIN VOUT iLED VEN

Worcester, Massachusetts 01615-0036 U.S.A. Figure 2. PWM operation at various duty cycles; note that there is no startup delay during PWM dimming operation

  • Operating conditions: at 200 Hz, VIN = 24 V, VOUT = 15 V, R1 = 63.4 kΩ, duty cycle = 50% (panel 2A) and 2% (panel 2B)
  • CH1 (Red) = VIN (10 V/div), CH2 (Blue) = VOUT (10 V/div), CH3 (Green) = iLED (500 mA/div), CH4 (Yellow) = Enable (5 V/div), time scale = 1 ms/div (panel 2A) and 50 μs/div (panel 2B) Panel 2A. Duty cycle = 50% and time scale = 1 ms/div Panel 2B. Duty cycle = 2% and time scale = 50 μs/div C1,C2 C1,C2 VIN VIN VOUT VOUT iLED iLED VEN VEN

Worcester, Massachusetts 01615-0036 U.S.A. Figure 3. Efficiency versus LED Current at various LED voltages Figure 4. Efficiency versus LED Current at various switching frequencies Figure 5. Average LED Current versus PWM dimming percentage

Worcester, Massachusetts 01615-0036 U.S.A. where VCSREG = 0.2 V typical. (internal resistance, 5 kΩ) in kΩ (see figure 6). is turned off, but the IC remains in standby mode for up to 10 ms. be varied from 100% down to 1% or lower. Figure 6. Switching Frequency versus RTON Resistance Figure 7. Simplified buck controller equations, and reference circuit and waveforms

  • During SW on-time: iRIPPLE = [(VIN – VOUT) / L] × tON = [(VIN – VOUT) / L] × T × D iRIPPLE = [(VOUT – VD) / L] × tOFF = [(VOUT – VD) / L] × T × (1 – D) VOUT = VIN × D – VD × (1 – D) VOUT = (VIN – Iav × RDS(on) ) × D – VD × (1 – D) – RL × Iav where D = tON / T. where RL is the resistance of the inductor.
  • During SW off-time: Therefore (simplified equation for Output Voltage): More precisely: If VD << VOUT , then VOUT ≈ VIN × D. VSW iL t t VIN i(max) iav i(min) iRIPPLE tON tOFF T –VD CIN VIN A6211 SW VOUT RSENSE L iL MOS D

Worcester, Massachusetts 01615-0036 U.S.A. EN is driven from a logic input. acceptable under more favorable operating conditions. Schottky diode D1 (typically under 0.5 V). cycle is 85%. So for a 24 V input, the maximum output is 20.3 V . f = 3.3 V or less for each LED. means with VIN = 24 V , the minimum VOUT = 3.2 V (one LED). diode. The more precise equation is shown in figure 7. OUT , and hence more flexible LED configurations. LED current is kept in regulation at all times. Figure 8. Minimum and Maximum Output Voltage versus Switching Figure 9. Minimum and Maximum Output Voltage versus iLED current

10Allegro MicroSystems, Inc. Worcester, Massachusetts 01615-0036 U.S.A. lower duty cycle, the LED current can be higher.

  • Pin-to-ground short
  • Pin-to-neighboring pin short
  • Pin open
  • External component open or short
  • Output short to GND The waveform in figure 10 illustrates how the A6211 responds in the case in which the current sense resistor or the CS pin is shorted to GND. Note that the SW pin overcurrent protection is tripped at around 3.75 A, and the part shuts down immediately. The part then goes through startup retry after approximately 380 μs of cool-down period. Component Selections The inductor is often the most critical component in a buck con- verter. Follow the procedure below to derive the correct param- eters for the inductor: 1. Determine the saturation current of the inductor. This can be done by simply adding 20% to the average LED current: i SAT ≥ iLED × 1.2 . 2. Determine the ripple current amplitude (peak-to-peak value). As a general rule, ripple current should be kept between 10% and 30% of the average LED current: 0.1 < i RIPPLE(pk-pk) / iLED < 0.3 . 3. Calculate the inductance based on the following equations: L = (V IN – VOUT ) × D × T / iRIPPLE , and D = (VOUT + VD1 ) / ( VIN + VD1 ) , where D is the duty cycle, T is the period 1/ fSW , and VD1 is the forward voltage drop of the Schottky diode D1 (see figure 7). Inductor Selection Chart The chart in figure 11 summarizes the relationship between LED current, switching frequency, and inductor value. Based on this chart: Assuming LED current = 2 A and f SW =1 MHz, then the minimum inductance required is L = 10 μH in order to keep the ripple current at 30% or lower. (Note: VOUT = VIN / 2 is the worst case for ripple current). If the switching frequency is lower, then either a larger inductance must be used, or the ripple current requirement has to be relaxed.

Figure 11. Inductance selection based on ILED and fSW ; VIN = 24 V, Figure 10. A6211 overcurrent protection tripped in the case of a fault

11Allegro MicroSystems, Inc. Worcester, Massachusetts 01615-0036 U.S.A. Figure 12. Ripple current and voltage, with and without shunt capacitor proportional to ripple voltage at CS pin. while ripple voltage at CS pin remains high.

  • 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 IN).
  • 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.

12Allegro MicroSystems, Inc. Worcester, Massachusetts 01615-0036 U.S.A. Figure 13. Waveforms showing the effects of adding a small filter capacitor across the LED string

  • Operating conditions: at 200 Hz, VIN = 24 V, VOUT = 15 V, fSW = 500 kHz, L = 10 μH, duty cycle = 50%
  • CH1 (Red) = VIN (10 V/div), CH2 (Blue) = VOUT (10 V/div), CH3 (Green) = iLED (500 mA/div), CH4 (Yellow) = Enable (5 V/div), time scale = 1 ms/div Panel 13A. Operation without using any output capacitor across the LED string Panel 13B. Operation with a 0.68 μF ceramic capacitor connected across the LED string t C1,C2 VIN VOUT iLED VEN t C1,C2 VIN VOUT iLED VEN

13Allegro MicroSystems, Inc. Worcester, Massachusetts 01615-0036 U.S.A. the LEDs (as shown in figure 13B).

60 V / 2 A

Figure 14. Application circuit diagram

14Allegro MicroSystems, Inc. Worcester, Massachusetts 01615-0036 U.S.A. and ensures better thermal performance and higher efficiency. layout. Figure 15 shows an example for components placement. minimized and connected by relatively wide traces. shown in loop 1. This loop should have relatively wide traces. both the top (component) layer and via the ground plane. minimize the size of the SW polygon. quency setting) away from the SW polygon. SENSE should be placed close to the IC. and keep the routing to this capacitor short. as close as possible to the A6211 pads. cated on multiple layers, if possible. Figure 16. Three different current loops in a buck converterFigure 15. Example layout for the A6211 evaluation board

Constant-Current 3-Ampere PWM Dimmable Buck Regulator LED DriverA6211 15Allegro MicroSystems, Inc. Worcester, Massachusetts 01615-0036 U.S.A. 10) To improve thermal performance, use multiple layers for GND. Place as many vias as possible to the ground plane around the anode of the asynchronous diode. 11) The thermal pad under the A6211 must connect to the ground plane using multiple vias. More vias will insure lower operating temperature and higher efficiency. 12) Connection to the LED array should be kept short. Exces- sively long wires can cause ringing or oscillation. When the LED array is separated from the converter board and an output capaci- tor is used, the capacitor should be placed on the converter board to reduce the effect of stray inductance from long wires.

Constant-Current 3-Ampere PWM Dimmable Buck Regulator LED DriverA6211 16Allegro MicroSystems, Inc. Worcester, Massachusetts 01615-0036 U.S.A. 3.30 Reference land pattern layout (reference IPC7351 SOIC127P600X175-9AM); all pads a minimum of 0.20 mm from all adjacent pads; adjust as necessary to meet application process requirements and PCB layout tolerances; when mounting on a multilayer PCB, thermal vias at the exposed thermal pad land can improve thermal dissipation (reference EIA/JEDEC Standard JESD51-5) PCB Layout Reference View C 1.27 5.602.41 1.75 0.65

2.41 NOM

3.30 NOM

C SEATING PLANE

1.27 BSC

A Terminal #1 mark area B C B C SEATING PLANEC0.10

0.25 BSC

1.04 REF

1.70 MAX

For Reference Only; not for tooling use (reference MS-012BA) Dimensions in millimeters Dimensions exclusive of mold flash, gate burrs, and dambar protrusions Exact case and lead configuration at supplier discretion within limits shown 4.90 ±0.10 0.51 0.31 0.15 0.00 0.25 0.17 1.27 0.40 Exposed thermal pad (bottom surface); dimensions may vary with device A Branded Face Package LJ, 8-Pin Narrow SOIC with Exposed Thermal Pad

Constant-Current 3-Ampere PWM Dimmable Buck Regulator LED DriverA6211 17Allegro MicroSystems, Inc. Worcester, Massachusetts 01615-0036 U.S.A. Copyright ©2010-2012, Allegro MicroSystems, Inc. Allegro MicroSystems, Inc. reserves the right to make, from time to time, such de par tures from the detail spec i fi ca tions as may be required to per- mit improvements in the per for mance, reliability, or manufacturability of its products. Before placing an order, the user is cautioned to verify that the information being relied upon is current. Allegro’s products are not to be used in life support devices or systems, if a failure of an Allegro product can reasonably be expected to cause the failure of that life support device or system, or to affect the safety or effectiveness of that device or system. The in for ma tion in clud ed herein is believed to be ac cu rate and reliable. How ev er, Allegro MicroSystems, Inc. assumes no re spon si bil i ty for its use; nor for any in fringe ment of patents or other rights of third parties which may result from its use. For the latest version of this document, visit our website: www.allegromicro.com