A6211 Constant-Current 3-Ampere PWM Dimmable Buck Regulator LED Driver 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 Package 8-pin SOICN with exposed thermal pad (suffix LJ): 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. Applications: •General Illumination •Scanners and multi-function printers (light bars) •Architectural lighting •Industrial Lighting •Display case lighting / MR16 Not to scale Typical Application Circuit VIN (6 to 48 V) GND C1 Enable/PWM Dimming (100 Hz to 2 kHz) 1 8 SW A6211 R1 2 7 TON BOOT 3 6 EN PAD GND 4 VCC 5 CS VIN C4 LED+ D1 ... EN L1 C5 LED– RSENSE A6211-DS, Rev. 2 Constant-Current 3-Ampere PWM Dimmable Buck Regulator LED Driver A6211 Selection Guide Operating Ambient Temperature, TA Part Number A6211GLJTR-T –40ºC to 105ºC Package Packing 8-pin SOICN with exposed thermal pad 3000 pieces per 13-in reel Absolute Maximum Ratings Characteristic Symbol Rating Unit –0.3 to 50 V VBOOT –0.3 to VIN + 8 V VSW –1.5 to VIN + 0.3 V –0.3 to 14 V –0.3 to VIN + 0.3 V Supply Voltage Notes VIN Bootstrap Drive Voltage Switching Voltage Linear Regulator Terminal VCC Enable and TON Voltage VEN , VTON Current Sense Voltage VCC to GND VCS –0.3 to 7 V –40 to 105 ºC TJ(max) 150 ºC Tstg –55 to 150 ºC Operating Ambient Temperature TA Maximum Junction Temperature Storage Temperature G temperature range Thermal Characteristics*may require derating at maximum conditions, see application section for optimization Characteristic Symbol Package Thermal Resistance (Junction to Ambient) RθJA Package Thermal Resistance (Junction to Pad) RθJP Test Conditions* Value Unit On 4-layer PCB based on JEDEC standard 35 ºC/W On 2-layer generic test PCB with 0.8 in.2 of copper area each side 62 ºC/W 2 ºC/W *Additional thermal information available on the Allegro® website. Pin-out Diagram Terminal List Table 8 VIN 1 7 TON 2 SW BOOT PAD EN 3 CS 4 6 5 GND VCC Number Name 1 VIN Supply voltage input terminals Function 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 – PAD Exposed pad for enhanced thermal dissipation; connect to GND Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 2 Constant-Current 3-Ampere PWM Dimmable Buck Regulator LED Driver A6211 Functional Block Diagram CVCC CBOOT VCC VIN VIN BOOT L1 LED String D1 SW VREG 5.3 V VCC UVLO Average On-Time Current Generator TON On-Time Timer Off-Time Timer Gate Drive UVLO Shutdown RON Level Shift EN + IC and Driver Control Logic CCOMP 0.2 V Current Limit Off-Time Timer – – Buck Switch Current Sense + VIL = 0.4 V VIH = 1.8 V + + VCC UVLO – – Thermal Shutdown ILIM CS PAD GND RSENSE Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 3 Constant-Current 3-Ampere PWM Dimmable Buck Regulator LED Driver A6211 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 Input Supply Voltage VIN Undervoltage Lockout Threshold VIN Undervoltage Lockout Hysteresis VIN VUVLO IIN IINSD Buck Switch Current Limit Threshold ISWLIM Buck Switch On-Resistance R BOOT Undervoltage Lockout Threshold Test Conditions Typ. Max. Unit 6 – 48 V VIN increasing – 5.3 – V – 150 – mV VCS = 0.5 V, EN = high – 5 – mA EN shorted to GND – 1 10 µA 3.0 4.0 5.0 A VUVLO_HYS VIN decreasing VIN Pin Supply Current VIN Pin Shutdown Current Min. TA = 25°C DS(on) VBOOTUV VBOOT = VIN + 4.3 V, TA = 25°C, ISW = 1 A VBOOT to VSW increasing BOOT Undervoltage Lockout Hysteresis VBOTUVHYS VBOOT to VSW decreasing Switching Minimum Off-Time tOFFmin Switching Minimum On-TIme tONmin Selected On-Time tON VCS = 0 V VIN = 24 V, VOUT = 12 V, RON = 137 kΩ – 0.25 0.4 Ω 1.7 2.9 4.3 V – 370 – mV – 110 150 ns – 110 150 ns 800 1000 1200 ns 187.5 200 210 mV – 0.9 – µA 5.1 5.4 5.7 V 5 20 – mA – – V Regulation Comparator and Error Amplifier Load Current Sense Regulation Threshold VCSREG VCS decreasing, SW turns on Load Current Sense Bias Current ICSBIAS VCS = 0.2 V, EN = low Internal Linear Regulator VCC Regulated Output VCC Current Limit* VCC ICCLIM 0 mA < ICC < 5 mA, VIN > 6 V VIN = 24 V, VCC = 0 V Enable Input Logic High Voltage Logic Low Voltage VIH VEN increasing 1.8 VIL VEN decreasing – – 0.4 V EN Pin Pull-down Resistance RENPD VEN = 5 V – 100 – kΩ Maximum PWM Dimming Off-Time tPWML Measured while EN = low, during dimming control, and internal references are powered-on (exceeding tPWML results in shutdown) 10 17 – ms Thermal Shutdown Thermal Shutdown Threshold TSD – 165 – °C Thermal Shutdown Hysteresis TSDHYS – 25 – °C *The internal linear regulator is not designed to drive an external load Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 4 Constant-Current 3-Ampere PWM Dimmable Buck Regulator LED Driver A6211 Characteristic Performance VIN VIN VOUT VOUT C1,C2 C1,C2 iLED C3 iLED C3 VEN C4 C4 VEN t t Panel 1B. VIN = 24 V Panel 1A. VIN = 19 V VIN VOUT C1,C2 iLED C3 C4 VEN t Panel 1C. VIN = 30 V Figure 1: Startup waveforms from off-state at various input voltages; note that the rise time of the LED current depends on input/output voltages, inductor value, and switching frequency • 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 Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 5 Constant-Current 3-Ampere PWM Dimmable Buck Regulator LED Driver A6211 VIN VOUT C1,C2 iLED C3 C4 VEN t Panel 2A: Duty cycle = 50% and time scale = 1 ms/div VIN VOUT C1,C2 iLED C3 C4 VEN t Panel 2B: Duty cycle = 2% and time scale = 50 µs/div 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) Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 6 Constant-Current 3-Ampere PWM Dimmable Buck Regulator LED Driver A6211 95 95 VIN = 24 V, VOUT = 15 V VIN = 12 V, VOUT = 5.5 V 85 VIN = 12 V, VOUT = 3.5 V 80 fSW = 500 kHz 90 Efficiency, η (%) Efficiency, η (%) 90 75 fSW = 1 MHz 85 fSW = 2 MHz 80 75 70 70 0 0.5 1.0 1.5 2.0 2.5 3.0 0 0.5 LED Current, iLED (A) 1.0 1.5 2.0 2.5 3.0 LED Current, iLED (A) Figure 3: Efficiency versus LED Current at various LED voltages Operating conditions: fSW = 1 MHz Figure 4: Efficiency versus LED Current at various switching frequencies. Operating conditions: VIN = 12 V, VOUT = 5.5 V LED Current (A) 1 0.1 iLED = 3 A iLED = 2 A iLED = 1.4 A 0.01 0.001 0.1 1 10 100 Duty Cycle (%) Figure 5: Average LED Current versus PWM dimming percentage Operating conditions: VIN = 12 V, VOUT = 3.5 V, fSW = 1 MHz, fPWM = 200 Hz, L = 10 µH Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 7 Constant-Current 3-Ampere PWM Dimmable Buck Regulator LED Driver A6211 Functional Description 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: iLED = VCSREG / RSENSE where VCSREG = 0.2 V typical. 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 2.2 2.0 1.8 fsw (MHz) 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 0 20 40 60 80 100 120 140 160 180 200 220 240 260 RTON (kΩ) Figure 6: Switching Frequency versus RTON Resistance • During SW on-time: iRIPPLE = [(VIN – VOUT) / L] × tON = [(VIN – VOUT) / L] × T × D where D = tON / T. • During SW off-time: iRIPPLE = [(VOUT – VD) / L] × tOFF = [(VOUT – VD) / L] × T × (1 – D) Therefore (simplified equation for Output Voltage): VOUT = VIN × D – VD × (1 – D) If VD << VOUT , then VOUT ≈ VIN × D. More precisely: VOUT = (VIN – Iav × RDS(on) ) × D – VD × (1 – D) – RL × Iav where RL is the resistance of the inductor. CIN tON = k × (RON + RINT ) × ( VOUT / VIN ) fSW = 1 / [ k × (RON + RINT )] where k = 0.0139, with fSW in MHz, tON in µs, and RON and RINT (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 ( TON / Period ) of the PWM signal. A practical range for PWM 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 VIN voltage VSW VIN 0 –VD t iL A6211 VIN given by the following equation: i(max) MOS SW D L VOUT iL RSENSE iRIPPLE iav i(min) tON tOFF t T Figure 7: Simplified buck controller equations, and reference circuit and waveforms Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 8 Constant-Current 3-Ampere PWM Dimmable Buck Regulator LED Driver A6211 at any time, a series resistor (1 kΩ) is required to limit the current flowing into the EN pin. This series resistor is not necessary if EN is driven from a logic input. PWM Dimming Ratio The brightness of the LED string can be reduced by adjusting the PWM duty cycle at the EN pin as follows: Dimming ratio = PWM on-time / PWM period For example, by selecting a PWM period of 5 ms (200 Hz PWM frequency) and a PWM on-time of 50 µs, a dimming ratio of 1% can be achieved. In an actual application, the minimum dimming ratio is determined by various system parameters, including: VIN , VOUT , inductance, LED current, switching frequency, and PWM frequency. As a general guideline, the minimum PWM on-time should be kept at 50 µs or longer. A shorter PWM on-time is acceptable under more favorable operating conditions. Output Voltage and Duty Cycle Figure 7 provides simplified equations for approximating output voltage. Essentially, the output voltage of a buck converter is approximately given as: VOUT = VIN × D – VD1 × (1 – D ) ≈ VIN × D, if VD1<< VIN D = tON / (tON + tOFF ) where D is the duty cycle, and VD1 is the forward drop of the Schottky diode D1 (typically under 0.5 V). Minimum and Maximum Output Voltages For a given input voltage, the maximum output voltage depends on the switching frequency and minimum tOFF . For example, if tOFF(min) = 150 ns and fSW = 1 MHz, then the maximum duty cycle is 85%. So for a 24 V input, the maximum output is 20.3 V. This means up to 6 LEDs can be operated in series, assuming Vf = 3.3 V or less for each LED. The minimum output voltage depends on minimum tON and switching frequency. For example, if the minimum tON = 150 ns and fSW = 1 MHz, then the minimum duty cycle is 15%. That means with VIN = 24 V, the minimum VOUT = 3.2 V (one LED). To a lesser degree, the output voltage is also affected by other factors such as LED current, on-resistance of the high-side switch, DCR of the inductor, and forward drop of the low-side diode. The more precise equation is shown in figure 7. As a general rule, switching at lower frequencies allows a wider range of VOUT , and hence more flexible LED configurations. This is shown in figure 8. Figure 8 shows how the minimum and maximum output voltages vary with LED current (assuming RDS(on) = 0.4 Ω, inductor DCR = 0.1 Ω, and diode Vf = 0.6 V). If the required output voltage is lower than that permitted by the minimum tON , the controller will automatically extend the tOFF , in order to maintain the correct duty cycle. This means that the switching frequency will drop lower when necessary, while the LED current is kept in regulation at all times. 24 9 22 8 20 7 VOUT(max) (V) 16 VOUT ( V ) VOUT ( V ) 18 14 12 10 VOUT(max) (V) 6 5 4 3 8 6 2 VOUT(min) (V) 4 VOUT(min) (V) 1 2 0 0 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 fsw (MHz) Figure 8: Minimum and Maximum Output Voltage versus Switching Frequency (VIN = 24 V, iLED = 2 A, minimum tON and tOFF = 150 ns) 0 0.5 1.0 1.5 2.0 2.5 3.0 iLED (A) Figure 9: Minimum and Maximum Output Voltage versus iLED current (VIN = 9 V, fSW = 1 MHz, minimum tON and tOFF = 150 ns) Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 9 Constant-Current 3-Ampere PWM Dimmable Buck Regulator LED Driver A6211 Thermal Budgeting The A6211 is capable of supplying a 3 A current through its high-side switch. However, depending on the duty cycle, the conduction loss in the high-side switch may cause the package to overheat. Therefore care must be taken to ensure the total power loss of package is within budget. For example, if the maximum temperature rise allowed is ∆T = 50 K at the device case surface, then the maximum power dissipation of the IC is 1.4 W. Assuming the maximum RDS(on) = 0.4 Ω and a duty cycle of 85%, then the maximum LED current is limited to 2 A approximately. At a lower duty cycle, the LED current can be higher. Fault Handling The A6211 is designed to handle the following faults: •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. As another example, the waveform in figure 11 shows the fault case where external Schottky diode D1 is missing or open. As LED current builds up, a larger-than-normal negative voltage is developed at the SW node during off-time. This voltage trips the missing Schottky detection function of the IC. The IC then shuts down immediately, and waits for a cool-down period before retry. Component Selections The inductor is often the most critical component in a buck converter. Follow the procedure below to derive the correct parameters for the inductor: 1. Determine the saturation current of the inductor. This can be done by simply adding 20% to the average LED current: iSAT ≥ 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 < iRIPPLE(pk-pk) / iLED < 0.3. 3. Calculate the inductance based on the following equations: L = (VIN – 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). C1 C1 VEN Negative voltage developed at SW pin during off-time VSW VOUT C2 VSW C2 C3 C3 iLED VOUT iLED C4 t Figure 10: A6211 overcurrent protection tripped in the case of a fault caused by the sense resistor pin shorted to ground; shows switch node, VSW (ch1, 10 V/div.), output voltage, VOUT (ch2, 10 V/div.), LED current, iLED (ch3, 1 A/div.), t = 100 µs/div. t Figure 11: Startup waveform with a missing Schottky diode; shows Enable, VEN (ch1, 5 V/div.), swtich node, VSW (ch2, 5 V/div.), output voltage, VOUT (ch3, 5 V/div.), LED current, iLED (ch4, 500 mA/div.), t = 100 µs/div. Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 10 Constant-Current 3-Ampere PWM Dimmable Buck Regulator LED Driver Inductor Selection Chart The chart in figure 12 summarizes the relationship between LED current, switching frequency, and inductor value. Based on this chart: Assuming LED current = 2 A and fSW =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. Additional Notes on Ripple Current • For stability, pick the inductor and switching frequency to ensure the lowest inductor ripple current percentage is at least 10% during worst case (at the lowest VIN). • 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 current × 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. 2.0 1.8 Switching Frequency, f SW (MHz) A6211 1.6 1.4 1.2 L=10 µH 1.0 L=15 µH 0.8 L=22 µH 0.6 L=33 µH 0.4 L=47 µH 0.2 0 0.0 0.5 1.0 1.5 2.0 LED Current, ILED 2.5 3.0 (A) Figure 12: Inductance selection based on ILED and fSW ; VIN = 24 V, VOUT = 12 V, ripple current = 30% • If lower ripple current is required for the LED string, one solution is to add a small capacitor (such as 2.2 µF) across the LED VIN VIN L1 LED+ Iripple SW D1 L1 LED+ Iripple SW D1 CS Vripple ... ... C1 CS LED– RSENSE Without output capacitor: Ripple current through LED string is proportional to ripple voltage at CS pin. Vripple LED– RSENSE With a small capacitor across LED string: Ripple current through LED string is reduced, while ripple voltage at CS pin remains high. Figure 13: Ripple current and voltage, with and without shunt capacitor Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 11 Constant-Current 3-Ampere PWM Dimmable Buck Regulator LED Driver A6211 string from LED+ to LED– . In this case, the inductor ripple current remains high while the LED ripple current is greatly reduced. Output Filter Capacitor The A6211 is designed to operate without an output filter capacitor, 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 factors, 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. VIN VIN VOUT VOUT iLED C1,C2 C3 VEN C4 C1,C2 iLED C3 VEN C4 t Panel 14A: Operation without using any output capacitor across the LED string t Panel 14B: Operation with a 0.68 µF ceramic capacitor connected across the LED string Figure 14. 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 Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 12 Constant-Current 3-Ampere PWM Dimmable Buck Regulator LED Driver A6211 Application Circuit The application circuit in figure 15 shows a design for driving a 15 V LED string at 1.3 A (set by RSENSE ). The switching frequency is 500 kHz, as set by R1. A 0.68 µF ceramic capacitor is added across the LED string to reduce the ripple current through the LEDs (as shown in figure 14B). VIN = 24 to 48 V GND C1 47 µF 50 V L1 10 µH / 2 A C2 4.7uF 50V 1 R1 3 4 TON EN CS A6211 SW BOOT PAD 8 7 C4 0.1 µF GND 6 VCC D1 60 V / 2 A C3 0.68 µF 50 V 5 C5 0.1 µF ... EN 140 kΩ 2 VIN LED+ LED string (≈15 V) LED– RSENSE 0.15 Ω Figure 15: Application circuit diagram Suggested Components Symbol Part Number Manufacturer C1 EMZA500ADA470MF80G United Chemi-Con C2 UMK316BJ475KL-T Taiyo Yuden C3 CGA5L2X5R1H684K160AA TDK L1 NR8040T100M Taiyo Yuden D1 B250A-13-F Diodes, Inc. RSENSE RL1632R-R150-F Susumu Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 13 A6211 Constant-Current 3-Ampere PWM Dimmable Buck Regulator LED Driver Component Placement and PCB Layout Guidelines PCB layout is critical in designing any switching regulator. A good layout reduces emitted noise from the switching device, and ensures better thermal performance and higher efficiency. The following guidelines help to obtain a high quality PCB layout. Figure 16 shows an example for components placement. Figure 17 shows the three critical current loops that should be minimized and connected by relatively wide traces. 1) When the upper FET (integrated inside the A6211) is on, current flows from the input supply/capacitors, through the upper FET, into the load via the output inductor, and back to ground as shown in loop 1. This loop should have relatively wide traces. Ideally this connection is made on both the top (component) layer and via the ground plane. 2) When the upper FET is off, free-wheeling current flows from ground through the asynchronous diode D1, into the load via the output inductor, and back to ground as shown in loop 2. This loop should also be minimized and have relatively wide traces. Ideally this connection is made on both the top (component) layer and via the ground plane. 3) The highest di/dt occurs at the instant the upper FET turns on and the asynchronous diode D1 undergoes reverse recovery as shown in loop 3. The ceramic input capacitors C2 must deliver this high instantaneous current. C1 (electrolytic capacitor) should not be too far off C2. Therefore, the loop from the ceramic input capacitor through the upper FET and asynchronous diode to ground should be minimized. Ideally this connection is made on both the top (component) layer and via the ground plane. 4) The voltage on the SW node (pin 8) transitions from 0 V to VIN very quickly and may cause noise issues. It is best to place the asynchronous diode and output inductor close to the A6211 to minimize the size of the SW polygon. Keep sensitive analog signals (CS, and R1 of switching frequency setting) away from the SW polygon. 6) For accurate current sensing, the LED current sense resistor RSENSE should be placed close to the IC. 7) Place the boot strap capacitor C4 near the BOOT node (pin 7) and keep the routing to this capacitor short. 8) When routing the input and output capacitors (C1, C2, and C3 if used), use multiple vias to the ground plane and place the vias as close as possible to the A6211 pads. 9) To minimize PCB losses and improve system efficiency, the input (VIN) and output (VOUT) traces should be wide and duplicated on multiple layers, if possible. Loop 1 Loop 2 Loop 3 L1 SW VIN Figure 16: Example layout for the A6211 evaluation board CIN D1 COUT LED Figure 17: Three different current loops in a buck converter Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 14 Constant-Current 3-Ampere PWM Dimmable Buck Regulator LED Driver A6211 10) Connection to the LED array should be kept short. Excessively long wires can cause ringing or oscillation. When the LED array is separated from the converter board and an output capacitor is used, the capacitor should be placed on the converter board to reduce the effect of stray inductance from long wires. Optimizing Thermal Layout The features of the printed circuit board, including heat conduction and adjacent thermal sources such as other components, have a very significant effect on the thermal performance of the device. To optimize thermal performance, the following should be taken into account: Thermal Dissipation The amount of heat that can pass from the silicon of the A6211 to the surrounding ambient environment depends on the thermal resistance of the structures connected to the A6211. The thermal resistance, RθJA , is a measure of the temperature rise created by power dissipation and is usually measured in degrees Celsius per watt (°C/W). •The device exposed thermal pad should be connected to as much copper area as is available. The temperature rise, ΔT, is calculated from the power dissipated, PD , and the thermal resistance, RθJA , as: ΔT = PD × RθJA A thermal resistance from silicon to ambient, RθJA , of approximately 35°C/W can be achieved by mounting the A6211 on a standard FR4 double-sided printed circuit board (PCB) with a copper area of a few square inches on each side of the board under the A6211. Additional improvements in the range of 20% may be achieved by optimizing the PCB design. •Copper thickness should be as high as possible (for example, 2 oz. or greater for higher power applications). •The greater the quantity of thermal vias, the better the dissipation. If the expense of vias is a concern, studies have shown that concentrating the vias directly under the device in a tight pattern, as shown in figure 18, has the greatest effect. •Additional exposed copper area on the opposite side of the board should be connected by means of the thermal vias. The copper should cover as much area as possible. •Other thermal sources should be placed as remote from the device as possible •Place as many vias as possible to the ground plane around the anode of the asynchronous diode. Signal traces LJ package footprint 0.7 mm 0.7 mm LJ package exposed thermal pad Top-layer exposed copper Ø0.3 mm via Figure 18: Suggested PCB layout for thermal optimization (maximum available bottom-layer copper recommended) Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 15 Constant-Current 3-Ampere PWM Dimmable Buck Regulator LED Driver A6211 Package LJ, 8-Pin Narrow SOIC with Exposed Thermal Pad 4.90 ±0.10 0.65 8° 0° 8 B A 1 3.90 ±0.10 6.00 ±0.20 0.25 BSC SEATING PLANE GAUGE PLANE Branded Face SEATING PLANE 0.10 C 1.27 BSC 1 1.27 0.40 3.30 NOM 0.51 0.31 2.41 1.04 REF 2 8X C 5.60 2 3.30 C PCB Layout Reference View 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 1.70 MAX 0.15 0.00 1.27 1.75 0.25 0.17 2.41 NOM 8 A Terminal #1 mark area B Exposed thermal pad (bottom surface); dimensions may vary with device C 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) Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 16 Constant-Current 3-Ampere PWM Dimmable Buck Regulator LED Driver A6211 Revision History Revision Revision Date 1 October 21, 2013 2 March 27, 2014 Description of Revision Update applications information Revised VCC Spec and Suggested Components Table Copyright ©2010-2014, Allegro MicroSystems, Inc. Allegro MicroSystems, Inc. reserves the right to make, from time to time, such departures from the detail specifications as may be required to permit improvements in the performance, 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 any devices or systems, including but not limited to life support devices or systems, in which a failure of Allegro’s product can reasonably be expected to cause bodily harm. The information included herein is believed to be accurate and reliable. However, Allegro MicroSystems, Inc. assumes no responsibility for its use; nor for any infringement 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 Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 17