Technology ACE703 White LED PFM Step-Up DC-DC Converter Description The ACE703 is a PFM step-up DC-DC converter specifically designed to drive white LEDs with a constant current. It can deliver stable constant output current from 0mA to 500mA by adjusting the external resistor. The ACE703 can drive one or more LEDs in parallel connection, also it can drive two in series, several in parallel connection from one or two battery cells. With overvoltage limit protection circuit interiorly, the chip and the external circuits will be safe even if the load is not connected. The device also can deliver steady constant output voltage from 2.5V to 6.0V by adjusting the external resistor. The ACE703 integrates stable reference circuits and it uses trimming technology in the process, so it can afford high precision and low temperature-drift coefficient output current or output voltage. The ACE703 devices are available in SOT-89-5 package. Features • • • • • • Low start-up voltage (when the output current is 1mA)--------------------------0.8V The converter can output constant voltage from 2.5V to 6.0V or it can output constant current from 0 to 500mA by adjusting the external resistor. Output current accuracy -----------±10％ Low temperature-drift coefficient of the output current------------------±100ppm/℃ Only four external components are necessary: an inductor, a Schottky diode, an output filter capacitor and a resistor. High conversion efficiency (When Vin=2.5V, for 1W LED)----------80％ Application • • • Power source for white LED Supply constant current Power source for a single or dual-cell battery-powered equipments Absolute Maximum Ratings Parameter Symbol Max Unit Input supply voltage LX pin voltage -0.3~10 V -0.3 ~ Vout+0.3 V CE pin voltage IFB pin voltage Lx pin output current -0.3 ~ Vout+0.3 V -0.3 ~ Vout+0.3 V 1.5 A Power Dissipation PD@T=25 OC SOT-89-5 0.5 Junction temperature Storage temperature Operating free-air temperature 150 - 40 to 125 -20 ~ 85 W O C C O C O VER 1.2 1 Technology ACE703 White LED PFM Step-Up DC-DC Converter Packaging Type SOT-89-5 5 1 4 2 3 Pin SOT-89-5 Function IFB 1 Current feedback pin VOUT 2 Output pin, power supply for internal circuits CE 3 Chip enable pin (active high) LX 4 Switching pin (Nch open drain) GND 5 Ground pin Ordering information Selection Guide ACE703 XX XX + H Halogen - free Pb - free AN : SOT-89-5 Feedback Voltage 10…100mV / 20…200mV / 25…250 mV / 30…300 mV / 35…350 mV / 40…400 mV VER 1.2 2 Technology ACE703 White LED PFM Step-Up DC-DC Converter Block Diagram ACE703 Recommended Work Conditions Item Min Nom Max Unit Input Voltage Range 0.8 Vout V Inductor 10 22 100 μH Input capacitor 0 ≧22 μF Output capacitor 47 100 220 μF Compensation capacitor 10 22 47 μF Operating junction temperature -20 85 ℃ *Suggestion: Tantalum capacitor is recommended to reduce the ripple of the output voltage VER 1.2 3 Technology ACE703 White LED PFM Step-Up DC-DC Converter Electrical Characteristics (Vin＝2.5V， VCE=Vout＝3.3V，R＝33Ω，TA=25°C, unless otherwise noted.) Symbol Item Test conditions VIFB Feedback voltage Iout=100mA Vstart Start-up voltage Vhold Hold-on voltage IDD2*5 Quiescent current drawn from power source Inoload*6 Current with no load ILX ILXleak LX switching current LX leakage current Ileak Fosc Maxdty η VCEH VCEL IOM Stand-by current Oscillator frequency Oscillator duty cycle Efficiency CE “H” threshold voltage CE “L” threshold voltage Maximum output current Iout=1mA， Vin：0→2V Iout=1mA， Vin：2→0V LX hung in air, VCE=VIFB= Vout=3.3V, Vin=2.5V OUT pin and IFB pin without any load, VIFB=0 Vlx=0.4V，VIFB=0 Vout=Vlx=VIFB=6V, Reference data Min Typ. Max 80 100 120 180 200 220 230 250 270 280 300 320 330 350 370 380 400 420 0.8 0.6 0.7 0.6 V 20 30 uA 100 150 uA 1 mA uA 0.5 uA 400 80 kHZ ％ ％ V V mA < 0.1 300 70 mV V 700 Vout=3.3V, VCE=0, LX、IFB hung in air VIFB=0， On（Vlx “L”）side Iout=300mA VCE：0→2V VCE：2→0V VIFB=0，Vout≡3.3V 0.9 Unit 350 75 80 0.9 0.3 500 0.6 Note: 1.Diode:schottky type, such as:1N5817,1N5819,1N5822 2.Inductor:22uH(R<0.5Ω) 3.Capacitor:100uF (Tantalum type) 4.Compensation capacitor:22uF (Tantalum type) 5.IDD2 is quiescent current drawn from power source, It is the minimum current of the chip without any dynamic current 6.Ino load is the current with no load in constant current application, it’s also the current consumed by the converter itself, including the chip and external circuits, when Vin=2.5V,Vout=VOM VER 1.2 4 Technology ACE703 White LED PFM Step-Up DC-DC Converter Typical Application (1) Constant output current application Vin D 1N5819 L White LED 27uH LX OUT ACE703 Power Iout Cin 47uF CE C1 22uF Tan GND IFB Cout 100uF Tan Rc (A) Drive only one white LED ACE703 (B) Drive two white-LED in series and several in parallel connection (2) Constant output voltage application ACE703 VER 1.2 5 Technology ACE703 White LED PFM Step-Up DC-DC Converter Detailed description The ACE703 series are boost structure, voltage-type pulse-frequency modulation (PFM) step-up DC-DC converter. It can deliver constant current or constant voltage. In constant current application, only five external components are necessary to supply output current from 0 to 500mA:an inductor, an output filter capacitor, a shottky diode, a compensation capacitor and a resistor which adjusts the output current. In constant voltage application, to deliver adjustable and low noise output voltage from 2.5V to 6.0V, only six external components are necessary as well: an inductor, a output filter capacitor, a shottky diode, a compensation capacitor and two resistors for output voltage detect. The ACE703 device consists of resistors for output voltage detection and trimming, a start-up voltage circuit, an oscillator, a reference circuit, a PFM control circuit, a switch protection circuit, a voltage limit protection circuit and a driver transistor. ● How to determine Rc in constant current application condition We define output current is IOUT ，then Rc can be determined by Rc=VIFB/IOUT For example, if we need Iout=100mA,and we choose the chip of VIFB=200mV,thus Rc=200mV/100mA=2Ω ● How to determine R1,R2 in constant voltage application We define output voltage is Vout, then R1/R2=VOUT/VIFB-1,R2 is in the range of 1KΩ~100KΩ.If we need Vout =3.3V,and we choose the ACE703 of VIFB=400mV and select R2=4KΩ,thus R1=29KΩ. In constant voltage application, we suggest using the high VIFB type ACE703,because there is voltage ripple in existence on feedback voltage. When VIFB is high, the infection of the ripple is less than that of low VIFB type, so the output is more stable. The PFM control circuit is the core of the ACE703 IC. This block controls power switch on/off duty cycle to stabilize output voltage by calculating results of other blocks which sense input voltage, output voltage, output current and load conditions. In PFM modulation system, the frequency and pulse width is fixed. The duty cycle is adjusted by skipping pulses, so that switch on-time is changed based on the conditions such as input voltage, output current and load. The oscillate block inside ACE703 provides fixed frequency and pulse width wave. High-gain differential error amplifier guarantees stable output voltage at difference input voltage and load. In order to reduce ripple and noise, the error amplifier is designed with high bandwidth. Though at very low load condition, the quiescent current of chip do affect efficiency certainly. The four main energy loss of Boost structure DC-DC converter in full load are the ESR of inductor, the voltage of Schottky diode, on resistor of internal N-channel MOSFET and its driver. In order to improve the efficiency, ACE703 integrates low on-resistor N-channel MOSFET and well designed driver circuits. The switch energy loss is limited at very low level. VER 1.2 6 Technology ACE703 White LED PFM Step-Up DC-DC Converter Selection of the external components Thus it can be seen, the inductor and shottky diode affect the conversion efficiency greatly. The inductor and the capacitor also have great influence on the output voltage ripple of the converter. So it is necessary to choose a suitable inductor, a capacitor and a right shottky diode, to obtain high efficiency, low ripple and low noise. Before discussion, we define: D≣VOUT-VIN/VOUT (1) Inductor Selection Above all, we should define the minimum value of the inductor that can ensure the boost DC-DC to operate in the continuous current-mode condition. L min ≥ D (1 − D ) 2 RL 2f The above expression is got under conditions of continuous current mode, neglect Schottky diode’s voltage, ESR of both inductor and capacitor. The actual value is greater that it. If inductor’s value is less than Lmin，the efficiency of DC-DC converter will drop greatly, and the DC-DC circuit will not be stable. Secondly, consider the ripple of the output voltage, ΔI = D • Vin Lf Im ax = Vin DVin + 2 (1 − D ) RL 2 Lf If inductor value is too small, the current ripple through it will be great. Then the current through diode and power switch will be great. Because the power switch on chip is not ideal switch, the energy of switch will improve. he efficiency will fall. Thirdly, in general, smaller inductor values supply more output current while larger values start up with lower input voltage and acquire high efficiency. An inductor value of 3uH to 1mH works well in most applications. If DC-DC converter delivers large output current (for example: output current is great than 50mA),large inductor value is recommended in order to improve efficiency. If DC-DC must output very large current at low input supply voltage, small inductor value is recommended. The ESR of inductor will affect efficiency greatly. Suppose ESR value of inductor is Rl, Rload is load resistor, then the energy can be calculated by following expression: Δη ≈ rL Rload (1 − D) 2 For example: input 1.5V,output is 3.0V,Rload=20Ω,rL=0.5Ω,The energy loss is 10%. Consider all above, inductor value of 47uH.ESR<0.5Ω is recommended in most applications. Large value is recommended in high efficiency applications and smaller value is recommended. VER 1.2 7 Technology ACE703 White LED PFM Step-Up DC-DC Converter (2) Capacitor Selection Ignore ESR of capacitor, the ripple of output voltage is: r= ΔVout D = Vout Rload Cf So large value capacitor is needed to reduce ripple. But too large capacitor value will slow down system reaction and cost will improve. So 100uF capacitor is recommended. Larger capacitor value will be used in large output current system. If output current is small (<10mA), small value is needed. Consider ESR of capacitor, ripple will increase: r' = r + Im ax • RESR Vout When current is large, ripple caused by ESR will be main factor. It may be greater than 100mV.The ESR will affects efficiency and increase energy loss. So low-ESR capacitor (for example: tantalum capacitor) is recommend or connect two or more filter capacitors in parallel. (3) Diode Selection Rectifier diode will affects efficiency greatly，Though a common diode (such as 1N4148) will work well for light load, it will reduce about 5%~10% efficiency for heavy load, For optimum performance, a Schottky diode (such as 1N5817.1N5819.1N5822) is recommended. (4) Input Capacitor If supply voltage is stable, the DC-DC circuit can output low ripple, low noise and stable voltage without input capacitor. If voltage source is far away from DC-DC circuit, input capacitor value greater than 10uF is recommended. (5) Resistor R1.R2 for output voltage detect In constant current application,R1 and R2 can be selected by following expression: Vout R1 = +1 VIFB R2 For example, when VIFB=400mV,Vout=3.3V. If R2=4KΩ,then R1= Vout =29KΩ;if R2=40KΩ,then R1=290 KΩ. ( VIFB − 1) × R 2 (6) Selection of L.C and compensation capacitor C1 When the load current is large（eg: Iout=100mA）,the output voltage ripple will increase: ΔVout ≈ L 2 I L + Vout 2 − Vout C As we can see, When output current is large, the maximum current of the inductor IL will increase, thus the ripple of the output voltage will increase. To decrease the ripple, there are two ways: VER 1.2 8 Technology ACE703 White LED PFM Step-Up DC-DC Converter Firstly, select the appropriate value of L.C. It require the value of L*C be not too large, and the value of L/C be small. For example, select L=22uH,C=100uF;or L=10uH,C=47uF;etc. If select L/C>１（such as L=100uH,C=47uF）,it is improper in large current（larger than 100mA） application. Secondly, add a compensation capacitor C1 to reduce the ripple. However, the value of the compensation capacitor is different between constant current application and constant voltage mode.C1 and the resistor parallel connected with it will generate a pole, thus, it can compensate the syntonic frequency made by L.C. In constant voltage mode, if the resistor parallel connected with C1 is large,(eg: R1=290KΩ) then τ=R1‧ C1 is large, therefore, when C1≥0.1uF,the ripple can be decreased greatly. But in constant current mode, the resistor parallel connected with C1 is just the equivalent resistor of the white-LED, about 1Ω.Then,to maintain larger τ require larger compensation capacitor, normally between 10uF to 47uF.Many experiments have proved, when L=22uH,Cout=100uF,C1=22uF,the ripple of the output voltage, the output current, and the feedback voltage is smallest, and the characteristic of the constant current is excellent, even the efficiency is highest. Test circuits (1) Feedback voltage test circuit ACE703 (2) Start-up voltage test circuit（Iload=1mA） ACE703 VER 1.2 9 Technology ACE703 White LED PFM Step-Up DC-DC Converter (3) Hold-on voltage test circuit（Iload=1mA） ACE703 (4) Quiescent current test circuit ACE703 (5) Oscillator frequency and duty cycle test circuit ACE703 VER 1.2 10 Technology ACE703 White LED PFM Step-Up DC-DC Converter (6) LX switching current test circuit ACE703 (7) LX leakage current test circuit ACE703 (8) CE“H＂voltage test circuit ACE703 VER 1.2 11 Technology ACE703 White LED PFM Step-Up DC-DC Converter (9) CE“L＂voltage test circuit ACE703 (10) Maximum output voltage (with no load) test circuit ACE703 VER 1.2 12 Technology ACE703 White LED PFM Step-Up DC-DC Converter Typical Characteristic (Recommended operating conditions: Cin=47uF, Cout=22uF, Cout=100uF,L=22uH Topt=25℃,unless otherwise noted) 1. Output Voltage VS. Input Voltage ACE70320AN+ Output Current VS. Input Voltage Input Voltage (V) 3. Efficiency VS. Input Voltage ACE70320AN+ Efficiency VS. Input Voltage (Iout=200mA) Input Voltage (V) 5. Ripple Voltage VS. Input Voltage ACE70320AN+ Ripple Voltage VS. input Voltage(Iout=100mA) Input Voltage (V) 2. Efficiency VS. Input Voltage ACE70320AN+ Efficiency VS. Input Voltage (Iout=100mA) Input Voltage (V) 4. Efficiency VS. Input Voltage ACE70320AN+ Efficiency VS. Input Voltage (Iout=300mA) Input Voltage (V) 6. Ripple Voltage VS. Input Voltage ACE70320AN+ Ripple Voltage VS. input Voltage(Iout=200mA) Input Voltage (V) VER 1.2 13 Technology ACE703 White LED PFM Step-Up DC-DC Converter 7. Ripple Voltage VS. Input Voltage ACE70320AN+ Ripple Voltage VS. input Voltage(Iout=300mA) Input Voltage (V) VER 1.2 14 Technology ACE703 White LED PFM Step-Up DC-DC Converter Packing Information SOT-89-5 VER 1.2 15 Technology ACE703 White LED PFM Step-Up DC-DC Converter Notes ACE does not assume any responsibility for use as critical components in life support devices or systems without the express written approval of the president and general counsel of ACE Electronics Co., LTD. As sued herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and shoes failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. ACE Technology Co., LTD. http://www.ace-ele.com/ VER 1.2 16