AN2754 Application note Buck high-brightness LED driver based on the ST1S10 step-down DC-DC converter voltage regulator Introduction High-brightness LEDs are becoming a prominent source of light because of their long life, ruggedness, design flexibility, small size and energy efficiency. LEDs are now available in higher and higher wattages per package (1 W, 3 W and 5 W) with currents up to 1.5 A. At these current levels, the traditional means of limiting current with a resistor is not sufficiently accurate nor efficient. Today, single-dice, white HBLEDs capable of delivering up to 90 lm/W of light are available. A typical 1 W white LED delivers an optical efficiency of 30 lm/W, whereas a typical 60 W light bulb delivers 15 lm/W. It is known that the brightness of an LED is proportional to the forward current, so the best way to supply LEDs is to control the forward current to get good matching of the output light. LED manufacturers specify the characteristics (such as lumens, beam pattern) of their devices at a specified forward current (IF), not at a specific forward voltage (VF). This application note describes how to implement a constant current control to drive highbrightness LEDs by a step-down DC-DC converter voltage regulator. A switching regulator is the best choice for driving HBLEDs when high efficiency and low power dissipation are required. The circuit uses the ST1S10 high-efficiency buck converter configured to drive a single HBLED in constant current mode. The ST1S10 is a step-down monolithic power switching regulator which needs few external passive components and it is capable of delivering 3 A. An internal oscillator fixes the switching oscillation at 900 kHz, and it is possible to synchronize the switching frequency with an external clock from 400 kHz to 1.2 MHz. This application note includes a schematic diagram, bill of material (BOM), and test data. August 2008 Rev 1 1/20 www.st.com Contents AN2754 Contents 1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2 Buck topology switching power supply . . . . . . . . . . . . . . . . . . . . . . . . . 6 3 Design example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 4 3.1 Design parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.2 Power stage selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.3 Current sense . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.4 Inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.5 Output capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 3.6 Input capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Description of the board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 4.1 Input/output connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 5 Schematic and bill of material (1 A LED current) . . . . . . . . . . . . . . . . . 14 6 Board layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 7 Typical application waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 8 2/20 7.1 Startup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 7.2 Switching waveform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 7.3 PWM dimming using the enable function . . . . . . . . . . . . . . . . . . . . . . . . . 18 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 AN2754 List of figures List of figures Figure 1. Figure 2. Figure 3. Figure 4. Figure 5. Figure 6. Figure 7. Figure 8. Figure 9. Figure 10. Figure 11. Figure 12. Figure 13. Figure 14. Figure 15. Constant voltage control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Constant current control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Constant current control with VSENSE amplification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Buck topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Buck converter circuit while the switch is in position 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Inductor current waveform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Buck converter circuit while the switch is in position 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Output impedence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Schematic - LED current 1 A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Assembly layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Top layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Bottom layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Startup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Steady-state operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 PWM dimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 3/20 Background 1 AN2754 Background When designing a power supply for a white high-brightness LED, the main requirements are efficiency, size and cost of the complete solution. A standard buck converter is the best choice for providing a constant current because only the buck converter among the switching topologies has an average inductor current that is equal to the average load current. For this reason, the conversion of a constant voltage into constant current is much easier. LEDs are current-driven devices whose brightness is proportional to their forward current. Forward current can be controlled in two ways: voltage mode and current mode. The first method uses the LED V-I curve to determine what voltage has to be applied to the LED in order to generate the desired forward current. This is typically accomplished by applying a voltage source and using a ballast resistor as shown in Figure 1. This method has two serious drawbacks. The first is that every change in LED forward voltage creates a change in LED current. The second problem is the power lost across the ballast resistor which reduces the efficiency. Figure 1. Constant voltage control Equation 1 ⎛ R ⎞ VOUT = VFB ⎜⎜1 + H ⎟⎟ = n × VF _ MAX + IF × RFB ⎝ RL ⎠ LEDs are PN junction devices with a steep I - V curve. For this reason, driving an LED with a voltage source can lead to large swings of forward current in response to even a small change in forward voltage. In general, to meet the needs of a driver for an HBLED, the current output must be in the ±5% to ±20% range. The best way to drive the LEDs is to control the forward current so that it eliminates changes in current due to variations in forward voltage, which translates into a constant LED brightness. Figure 2 illustrates the configuration of a typical buck converter driver circuit. The value of current-sense resistor (RSENSE) depends on the desired LED current and the feedback voltage that the buck converter requires. Multiple LEDs should be connected in a series configuration to keep an identical current flowing in each LED. 4/20 AN2754 Background Figure 2. Constant current control VOUT Voltage Regulator FB RSENSE Equation 2 IF = VFB RFB Accuracy and efficiency are the two main goals of the current sensing even if they are in direct conflict. The higher the sense voltage is, the higher the signal-noise ratio, but the higher the power dissipated on RSENSE. To reduce the power dissipated in the series resistance, Figure 3 shows a simple method of amplifying the current sense signal by using a single supply op-amp. This method allows the user to select the current sense resistor RSENSE according to the desired power dissipation while setting the average value of IF with the gain of the op-amp. Figure 3. Constant current control with VSENSE amplification VOUT Voltage Regulator + FB OUT - RFB RIN RSENSE Equation 3 IF = VFB ⎛ R R SENSE ⋅ ⎜⎜1 + FB RIN ⎝ ⎞ ⎟⎟ ⎠ 5/20 Buck topology switching power supply 2 AN2754 Buck topology switching power supply The buck topology switching power supply is an efficient voltage regulator which produces an output voltage always less than or equal to the source voltage in the same polarity. The first step of conversion is to generate a chopped version of input source. A single-pole double-throw (SPDT) switch is connected as shown in Figure 4. Figure 4. Buck topology The switch output voltage is equal to the converter input voltage when the switch is in position 1 and equal to zero when the switch is position 2. The position is varied periodically at a frequency of 1/T, where T represents the switching cycle period. The ratio of the on-time to the period is referred to as the duty cycle D. So the switch output is a rectangular waveform having amplitude equal to the source voltage, frequency equal to 1/T and duty cycle equal to D. By inserting a low-pass filter between the (SPDT) switch and the load, a basic buck topology is formed. The DC value of switch output voltage is simply the source voltage multiplied by the duty cycle. The L-C filter cutoff is selected to pass the desired lowfrequency components of the switch output but also to attenuate the high-frequency switching harmonics. A power stage can operate in continuous or discontinuous inductor current mode. Continuous inductor current mode is characterized by current flowing continuously in the inductor during the entire switching cycle in steady-state operation. In discontinuous mode the inductor current drops to zero for a portion of the switching cycle. In this section we will derive the voltage conversion relationship for the continuous conduction mode buck power stage. In continuous conduction mode, the power stage assumes two states per switching cycle. The ON state is when the high-side switch is ON and the low-side switch is OFF. Figure 5. Buck converter circuit while the switch is in position 1 During the ON state the voltage applied on the inductor is given by: Equation 4 v L = Vs − Vout 6/20 AN2754 Buck topology switching power supply We can find the inductor current by integrating the inductor voltage waveform. Equation 5 iL (nT + t ) = i(nT ) + Figure 6. VS − Vout ⋅t L Inductor current waveform iL iL (nT + t ') = i(nT ) + VS − VOUT ⋅ t' L iL (t ' ') = i(nT + DT ) − VOUT ⋅ t' ' L IOUT nT t’ nT+DT t’’ (n+1)T t Equation 6 IL max = i(nT ) + VS − Vout ⋅ DT L The inductor current increase during the ON state is given by: Equation 7 ∆iL = VS − Vout ⋅ DT L The OFF state is when the high side is OFF and the low side ON. Figure 7. Buck converter circuit while the switch is in position 2 7/20 Buck topology switching power supply AN2754 The inductor voltage during the OFF state is given by: Equation 8 VL = −Vout The inductor current during the OFF state is given by: Equation 9 iL (t ) = i(nT + DT ) − Vout ⋅t L The inductor current decrease during the OFF state is given by: Equation 10 ∆iL = Vout ⋅ (1 − D)T L The volt-time product of each switch state must be equal in steady-state operation, so the current increase during the ON state and the current decrease during the OFF state must be equal. Equation 11 VS − Vout V ⋅ DT = out ⋅ (1 − D)T L L From the above equation we obtain the continuous conduction mode buck voltage conversion relationship. Equation 12 Vout = D × Vs To guarantee continuous mode, the following equation must be satisfied: Equation 13 ILavg = ILOAD ≥ ∆iL 2 The following relationship provides the minimum value of the inductance which is necessary to guarantee the fixed ripple current in continuous mode: Equation 14 L MIN = [VinMAX − (RDSON + ESRL ) ⋅ ILavg − Vout] ⋅ ∆IL ⋅ FSW Vout VinMAX The function of the output capacitor is both filtering the AC current and providing the charge that is necessary to supply the load during the transients. Constant current drivers are free of load transient by design. For this reason, the capacitor is only needed to obtain a lower current ripple amplitude across the LEDs. The value of the output capacitor is chosen to 8/20 AN2754 Buck topology switching power supply reduce the ripple current on the LEDs branch. To calculate the ripple current that flows through the LEDs it is necessary to estimate the impedances of the branches of both the LED and output capacitor (Zo, Zc). In this procedure let us suppose that the triangular shape of the ripple current on the inductor is approximately sinusoidal. Figure 8. Output impedence Equation 15 Z O = rD + RFB Equation 16 Z C = ESR + 1 2 ⋅ π ⋅ Fsw ⋅ Cout The following equation can be used to estimate the impedance of the output capacitor which guarantees the desired ripple current on the LEDs for a given inductor ripple current: Equation 17 ZC = ∆iLED × ZO ∆iL − ∆iLED 9/20 Design example 3 AN2754 Design example This section outlines a step-by-step procedure for the design of a constant current control by means of a switching step-down voltage regulator. The aim is to maintain both high efficiency and good accuracy. The following design procedure is helpful in selecting the component values of the application. 3.1 Design parameters LED manufacturers generally recommend values for ∆IF ranging from ±5% to ±20% of IF . The higher LED ripple allows the use of smaller inductors and smaller output capacitors. The advantages of higher ripple current are reductions in the solution size and cost. Lower ripple current requires more inductor output and more capacitor output. The advantages of a lower ripple current are reductions in heating of the LED itself and a greater range of the average LED current before the current limit is reached. The application is designed to supply up to four HBLEDs. The LED used in the application is a Lumides LUXEON III Emitter LXHL-PW09 with a typical forward voltage of 3.7 V at 700 mA. Table 1 provides a summary of the specifications of a particular application. Table 1. Performance specification summary Input source 4 AA batteries Symbol Value Unit Vin 6 V White LED LUXEON III Emitter LXHL-PW09 LED forward voltage VF 3.7 at 700 mA V ILEDavg = Average inductor current ILED 1 A Ripple current on the inductor (%ILavg) ∆IL = % ILED 60 % 10 % Ripple current on the LED branch (%∆ILED) ∆ILED = % ILED 3.2 Power stage selection For the power stage we use the ST1S10 which is a general-purpose voltage regulator stepdown DC-DC converter which has been optimized for high-efficiency small-sized equipment. A high switching frequency (900 kHz) allows the use of tiny surface-mount components. The synchronous rectification is implemented in order to obtain efficiency higher than 90%. The ST1S10 provides up to 3 A over an input voltage range from 2.5 V to 16 V. The minimum input voltage to maintain regulation, depending on the load current and output voltage, can be calculated as: Equation 18 Vin min = 10/20 VOUT + ILED ⋅ (RL + RDS _ ON max ) DMAX AN2754 Design example where RDS (on) is the maximum PMOS switch-on resistance, RL is the DC resistance of the inductor and VOUT is the nominal output voltage. 3.3 Current sense Amplifying the sensed voltage is a way to reduce the power loss in the current sense resistor. The operational amplifier selected for this application must be able to work with a common mode input voltage close to zero. The selected device is the TS951, a rail-to-rail BiCMOS operational amplifier. The value of the current sense resistor is determined by two factors: power dissipation on RSENSE and the threshold level for amplifier input. Smaller RSENSE reduces power dissipation but the detection of the feedback signal is more difficult. In order to keep the power dissipation for the current sensing at a minimum value, a good choice for the sense voltage with a forward current of 1 A is 100 mV. The ratio between feedback voltage and sense voltage gives the value of the gain of the amplification stage: Equation 19 Gain = VFB ILED × R SENSE Gain = 3.4 0. 8 =8 1× 0.1 Inductor selection The buck power stage is designed to operate in continuous mode for load current greater than 30% of full load. We choose an inductor value producing a maximum peak-peak ripple current equal to sixty percent of the maximum load current. This limits the RMS current in the output filter capacitor and, as a second order effect, keeps the core losses in the inductor reasonable. By using a single HBLED in conjunction with the chosen current sense resistor the output voltage is given by the following equation: Equation 20 VOUT = VF + VSENSE = 3.8 V + 0.1 V = 3.9 V Let us set the value of the ripple current equal to 60% of the average current: Equation 21 ∆IL = 0.6 × ILEDAVG = 0.6 × 1 = 0.6 A The minimum value of the output current to guarantee continuous mode is given by: Equation 22 ILED min CCM = ∆IL / 2 = 0.6 / 2 = 300 mA 11/20 Design example AN2754 The minimum value of inductance which guarantees a ripple current of 300 mA can be calculated using Equation 14: Equation 23 L MIN = 3.5 [VinMAX − (RDSON + ESRL ) ⋅ ILavg − Vout ] ⋅ ∆IL ⋅ FSW Vout VinMAX Output capacitor selection The target tolerance for the LED ripple current is 10% of the forward current. In this particular example the forward current is 1 A and the ripple current on LEDs branch is 100 mA. In current-mode converters, the load consists of the dynamic resistance of the diodes, rD and the operating point resistance VO/ILED. Typical values for rD are provided by LED manufacturers and for those do not, it must be determined by examining the slope of the I-V curve that is provided in all LED datasheets. The LUXEON III Emitter LXHL-PW09 datasheet gives a typical value for the dynamic resistance rD of 0.8 Ω at 700 mA. Given the ripple current on the inductor with the equation below, it is possible to calculate the ZC impedance to guarantee a ripple current on LEDs branch equal to 10% of IF : Equation 24 ZC = ZC = Z O ⋅ ∆iLED ∆iL − ∆iLED (0.8 + 0.1) × 0.05 = 0.18 Ω (0.3 − 0.05) A ceramic capacitor is used and the required capacitance is selected based on the impedance at 900 kHz. Equation 25 C = 1/ [2 × π × (Z C − ESR ) × fSW ] [ ] C = 1 / 2 × π × (0.18 − 0.01) × 900 ⋅ 10 3 = 1 3.6 Input capacitor selection Because of the pulsating input current nature of the buck converter, a low ESR input capacitor is required. A good input voltage filtering is important for minimizing the interference with other circuits caused by high input voltage spikes. The following equation is used to calculate the input ripple voltage due to capacitance and ESR: Equation 26 ∆Vinpp = Iin ⋅ RESR + Iin ⋅ D Fsw ⋅ C A 4.7 µF input capacitor is sufficient for effective input voltage filtering. 12/20 AN2754 4 Description of the board Description of the board The evaluation board is configured as constant current supply. Current regulation is accomplished by regulating the voltage across a current sense resistor 4.1 Input/output connection The following table describes the input/output connections. Table 2. Input/output connections Reference designator Name Description J1 LED cathode Output to cathode of LED J2 LED anode Output to anode of LED Supply/sync VIN_SW: Power input supply voltage to be tied to VIN_A. (VIN_SW max=18 V) VIN_A: Analog input supply voltage to be tied to VIN_SW. (VIN_ A max=18 V) SYNC: Synchronization and frequency select. Connect SYNC to GND for 900 kHz switching frequency or connect to an external clock from 400 kHz to 1.2 MHz. Enable Use this connector to enable and disable DC-DC converter. Connect a jumper between the ON pin and the center pin to enable the supply. Connect a jumper between the OFF pin and the center pin to disable the supply. If this pin is left open, the EVM does not operate correctly. This pin is also used for PWM dimming control of the LED current. J3 J4 13/20 Schematic and bill of material (1 A LED current) Schematic and bill of material (1 A LED current) Figure 9. Schematic - LED current 1 A CIN SW 4.7uF CIN A 100nF U1 Vin Vin 1 2 3 4 5 1 6 8 4 SW Vin_sw ST1S10 PGND AGND Vf b 5 Supply J2 L1 4.7uH Vin_a 1 2 7 VIN Rz1 330 Vin J1 Dz Cs 12V 100nF 1 2 J4 VIN EN GND LED ANODE COUT 2.2uF 3 Vin Inh Vin a Vin sw Pgnd Agnd SY N Sync J3 2 5 AN2754 LED CATHODE U3 TS951ILT 1 2 3 + R2 5.6k DZ2 C3 22nF 2.4V Rsense 0.1 RFB 6.8k Table 3. 14/20 RIN 1k Bill of material Quantity Reference Part/value PCB footprint 2 Cs,CIN A 100 nF SM/C_0805 1 CIN SW 4.7 µF SM/C_1210 1 COUT 2.2 µF SM/C_1210 1 C3 22 nF SM/C_0805 1 D3 12 V SM/D_1406 1 J1 LED cathode SIP/TM/L.200/2 1 J2 LED anode SIP/TM/L.200/2 1 J3 Supply SIP/TM/L.500/5 1 J4 Inh SIP/TM/L.300/3 1 L1 4.7 µF SM/L_2220 1 RFB 6.8 kΩ SM/R_0805 1 RIN 1 kΩ SM/R_0805 1 RSENSE 0.1 SM/R_0805 1 Rz 330 SM/R_0805 1 R2 5.6 kΩ SM/R_0805 1 U1 ST1S10 VFDFPN 8L 4X4 1 U2 TS951ILT SOT23-5 AN2754 6 Board layout Board layout Board layout is critical for all switched-mode power supplies. Figure 5, 6 and 7 show the board layout for the ST1S10 constant current control evaluation board. It is essential to keep the high switching current circulating paths as small as possible to reduce radiation and resonance problems. In general the following rules should be applied: ● Traces should be as short as possible ● L1, CIN, and COUT should be placed as close together as possible ● Especially the connection from the IC pin SW and the inductor must be kept short ● Ground areas should be as large as possible. If a 2-layer PCB is used, one layer should be assigned as the ground layer and good connectivity between both layers should be observed ● CIN should be placed close to pin (VS) of the chip; CIN directly to pin 4 Figure 10. Assembly layer 15/20 Board layout Figure 11. Top layer Figure 12. Bottom layer 16/20 AN2754 AN2754 Typical application waveforms 7 Typical application waveforms 7.1 Startup Figure 13. Startup Conditions: VIN: 6V; ILED: 1A; L=4.7uH; COUT=2.2uF. Ch3: Inductor current Ch4: LED current 7.2 Switching waveform Figure 14. Steady-state operation Conditions: VIN: 6V; ILED: 1A; L=4.7uH; COUT=2.2uF. CH1:SW Ch3: Inductor current Ch4: LED current 17/20 Typical application waveforms 7.3 AN2754 PWM dimming using the enable function Figure 15. PWM dimming Conditions: VIN: 7 V; IOUT: 1 A; L=4.7 µH; COUT=2.2 µF, Freq=100 Hz; duty cycle 50%. CH1: SW; CH2: Enable Ch3: inductor current Ch4:LED current; 18/20 AN2754 8 Revision history Revision history Table 4. Document revision history Date Revision 28-Aug-2008 1 Changes Initial release 19/20 AN2754 Please Read Carefully: Information in this document is provided solely in connection with ST products. STMicroelectronics NV and its subsidiaries (“ST”) reserve the right to make changes, corrections, modifications or improvements, to this document, and the products and services described herein at any time, without notice. All ST products are sold pursuant to ST’s terms and conditions of sale. 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