AME5258-AEV180

AME AME5258 n General Description The AME5258 is a high efficiency monolithic synchronous buck regulator using a constant frequency, current mode architecture. The device is available in an adjustable version and fixed output voltages of 1.2V, 1.8V, 2.5V and 3.3V. Supply current with no load is 300µA and drops to 7.8% by turning the main switch off and keeping it off until the fault is removed. Pulse Skipping Mode Operation At light loads, the inductor current may reach zero or reverse on each pulse. The bottom MOSFET is turned off by the current reversal comparator, IRCMP, and the switch voltage will ring. This is discontinuous mode operation, and is normal behavior for the switching regulator. Short-Circuit Protection When the output is shorted to ground, the frequency of the oscillator is reduced to about 210kHz, 1/7 the nominal frequency. This frequency foldback ensures that the inductor current has more time to decay, thereby preventing runaway. The oscillator's frequency will progressively increase to 1.5MHz when VFB or VOUT rises above 0V. 1.5MHz, 600mA Synchronous Buck Converter Dropout Operation As the input supply voltage decreases to a value approaching the output voltage, the duty cycle increases toward the maximum on-time. Further reduction of the supply voltage forces the main switch to remain on for more than one cycle until it reaches 100% duty cycle. The output voltage will then be determined by the input voltage minus the voltage drop across the P-channel MOSFET and the inductor. An important detail to remember is that at low input supply voltages, the RDS(ON) of the P-channel switch increases (see Typical Performance Characteristics). Therefore, the user should calculate the power dissipation when the AME5258 is used at 100% duty cycle with low input Voltage. n Application Information Inductor Selection For most applications, the value of the inductor will fall in the range of 1µH to 4.7µH. Its value is chosen based on the desired ripple current. Large value inductors lower ripple current and small value inductors result in higher ripple currents. Higher V IN or V OUT also increases the ripple current as shown in equation 1. A reasonable starting point for setting ripple current is IL = 240mA (40% of 600mA). ∆ IL= VOUT 1 ⋅ VOUT (1 − ) f ⋅L VIN The DC current rating of the inductor should be at least equal to the maximum load current plus half the ripple current to prevent core saturation. Thus, a 720mA rated inductor should be enough for most applications (600mA+ 120mA). For better efficiency, choose a low DC-resistance inductor. 8 Rev.A.05 AME AME5258 Inductor Core Selection Once the value for L is known, the type of inductor must be selected. High efficiency converters generally cannot afford the core loss found in low cost powdered iron cores, forcing the use of more expensive ferrite or mollypermalloy cores. Actual core loss is independent of core size for a fixed inductor value but it is very dependent on the inductance selected. As the inductance increases, core losses decrease. Unfortunately, increased inductance requires more turns of wire and therefore copper losses will increase. Ferrite designs have very low core losses and are preferred at high switching frequencies, so design goals can concentrate on copper loss and preventing saturation. Ferrite core material saturates "hard", which means that inductance collapses abruptly when the peak design current is exceeded. This result in an abrupt increase in inductor ripple current and consequent output voltage ripple. Do not allow the core to saturate! Different core materials and shapes will change the size/current and price/current relationship of an inductor. Toroid or shielded pot cores in ferrite or permalloy materials are small and don't radiate energy but generally cost more than powdered iron core inductors with similar characteristics. The choice of which style inductor to use mainly depends on the price vs. size requirements and any radiated field/EMI requirements. CIN and COUT Selection The input capacitance, CIN, is needed to filter the trapezoidal current at the source of the top MOSFET. To prevent large ripple voltage, a low ESR input capacitor sized for the maximum RMS current should be used.RMS current is given by : 1.5MHz, 600mA Synchronous Buck Converter Several capacitors may also be paralleled to meet size or height requirements in the design. The selection of COUT is determined by the effective series resistance (ESR) that is required to minimize voltage ripple and load step transients, as well as the amount of bulk capacitance that is necessary to ensure that the control loop is stable. Loop stability can be checked by viewing the load transient response as described in a later section. The output ripple, VOUT, is determined by :  1  ∆ VOUT ≤ ∆ IL ESR +  8 f ⋅ C OUT   The output ripple is highest at maximum input voltage since IL increases with input voltage. Multiple capacitors placed in parallel may be needed to meet the ESR and RMS current handling requirements. Dry tantalum, special polymer, aluminum electrolytic and ceramic capacitors are all available in surface mount packages. Special polymer capacitors offer very low ESR but have lower capacitance density than other types. Tantalum capacitors have the highest capacitance density but it is important to only use types that have been surge tested for use in switching power supplies. Aluminum electrolytic capacitors have significantly higher ESR but can be used in cost-sensitive applications provided that consideration is given to ripple current ratings and long term reliability. Ceramic capacitors have excellent low ESR characteristics but can have a high voltage coefficient and audible piezoelectric effects. The high Q of ceramic capacitors with trace inductance can also lead to significant ringing Using Ceramic Input and Output Capacitors Higher values, lower cost ceramic capacitors are now becoming available in smaller case sizes. Their high ripple current, high voltage rating and low ESR make them ideal for switching regulator applications. However, care must be taken when these capacitors are used at the input and output. When a ceramic capacitor is used at the input and the power is supplied by a wall adapter through long wires, a load step at the output can induce ringing at the input, VIN. At best, this ringing can couple to the output and be mistaken as loop instability. At worst, a sudden inrush of current through the long wires can potentially cause a voltage spike at VIN large enough to damage the part. 9 IRMS = I OUT ( max ) ⋅ VOUT ⋅ VIN VIN VOUT −1 This formula has a maximum at V IN = 2V OUT, where IRMS = IOUT/2. This simple worst-case condition is commonly used for design because even significant deviations do not offer much relief. Note that ripple current ratings from capacitor manufacturers are often based on only 2000 hours of life which makes it advisable to further derate the capacitor, or choose a capacitor rated at a higher temperature than required. Rev.A.05 AME AME5258 Output Voltage Programming The output voltage is set by an external resistive divider according to the following equation : 1.5MHz, 600mA Synchronous Buck Converter VIN 2.5V to 5.5V IN AME5258 EN COUT 4.7µF CER GND 604K FB 604K SW 22pF 2.2µH VOUT 1.2V C OUT 10µF CER VOUT = V REF ⋅ (1 + R2 ) R1 Where VREF equals to 0.6V typical. The resistive divider allows the FB pin to sense a fraction of the output voltage as shown in Figure 4. 0.6V ≤ VOUT ≤ 5.5V Figure 5: 1.2V Step-Down Regulator R2 FB VIN 3.3V to 5.5V 2.2µH IN AME5258 EN FB 475K GND 316K SW 22pF VOUT 1.5V C OUT 10µF CER AME5258 GND R1 Figure 4: Setting the AME5258 Output Voltage Thermal Considerations In most applications the AME5258 does not dissipate much heat due to its high efficiency. But, in applications where the AME5258 is running at high ambient temperature with low supply voltage and high duty cycles, such as in dropout, the heat dissipated may exceed the maximum junction temperature of the part. If the junction temperature reaches approximately 160O C, both power switches will be turned off and the SW node will become high impedance. To avoid the AME5258 from exceeding the maximum junction temperature, the user will need to do some thermal analysis. The goal of the thermal analysis is to determine whether the power dissipated exceeds the maximum junction temperature of the part. The temperature rise is given by: COUT 4.7µF CER Figure 6: 1.5V Step-Down Regulator VIN 2.7V to 5.5V 2.2µH IN AME5258 EN FB 1M GND 316K SW 22pF VOUT 2.5V C OUT 10µF CER C OUT 4.7µF CER TR = ( PD)( θJA ) Where PD is the power dissipated by the regulator and θJA is the thermal resistance from the junction of the die to the ambient temperature. Figure 7: 2.5V Step-Down Regulator 10 Rev.A.05 AME AME5258 VIN 3.3V to 5.5V 2.2µH IN AME5258 EN COUT 4.7µF CER GND 240K FB 960K SW 22pF VOUT 3V C OUT 10µF CER 1.5MHz, 600mA Synchronous Buck Converter VIN 3.6V to 5.5V 2.2µH IN AME5258 EN C OUT 4.7µF CER GND 196K FB 887K SW 22pF VOUT 3.3V C OUT 10µ F CER Figure 8: 3V Step-Down Regulator PC Board Layout Checklist Figure 9: 3.3V Step-Down Regulator When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the AME5258. These items are also illustrated graphically in Figures 10 and Figures 11 . Check the following in your layout: 1. The power traces, consisting of the GND trace, the SW trace and the V IN trace should be kept short, direct and wide. 2. Does the V FB pin connect directly to the feedback resistors? The resistive divider R1/R2 must be connected between the (+) plate of COUT and ground. 3. Does the (+) plate of CIN connect to V IN as closely as possible? This capacitor provides the AC current to the internal power MOSFETs. 4. Keep the switching node, SW, away from the sensitive VFB node. 5. Keep the (-) plates of CIN and COUT as close as possible. VIN L1 IN SW CFWD VOUT VIN L1 IN SW VOUT AME5258 EN FB + COUT AME5258 EN + COUT R2 V OUT GND - + CIN GND R1 + CIN - - Figure 10: AME5258 Adjustable Voltage Regulator Layout Diagram Rev.A.05 Figure 11: AME5258 Fixed Voltage Regulator Layout Diagram 11 AME AME5258 Start-UP form Shutdown 1.5MHz, 600mA Synchronous Buck Converter Pluse Skipping Mode RUN 2V /Div SW 5V /Div VOUT 1V/Div VOUT 10mV/Div IL 500mA/Div 200µ S/Div IL 20mA/Div VIN=3.6V VOUT=1.8V IOUT =50mA 1µS/Div VIN=3.6V VOUT=1.8V ILOAD=600mA Pluse Skipping Mode Pluse Skipping Mode SW 5V/Div SW 5V/Div VOUT 10mV/Div IL 20mA/Div VOUT 10mV/Div IL 20mA/Div VIN=3.6V VOUT=1.8V IOUT=10mA 1µ S/Div VIN=3.6V VOUT=1.8V IOUT=20mA 1µS/Div Load Step 100 95 Efficiency vs Input voltage IOUT=100mA IOUT=200mA Ef fi cienc y(%) VOUT 100mV/Div AC COUPLED 90 85 80 75 70 65 IL 500mA/Div IOUT =600mA IOUT =10mA IOUT 500mA/Div 20µS/Div VIN=3.6V VOUT=1.8V ILOAD=0mA to 600mA 60 55 50 2.5 3 3.5 4 4.5 5 5.5 Input Voltage(V) 12 Rev.A.05 AME AME5258 Oscillator Frequency VS Temperature 2.0 1.9 1.8 1.7 1.6 1.5 1.4 1.3 1.2 -50 1.5MHz, 600mA Synchronous Buck Converter Oscillator Frequency VS Supply Voltage 2.0 1.9 1.8 Fr equency(MHz ) -25 0 +25 +50 o Frequency( MHz) 1.7 1.6 1.5 1.4 1.3 1.2 +75 +100 +125 1.1 2.5 3.5 4.5 5.5 Temperature( C) Supply Voltage (V) VFB vs Temperature 0.615 0.612 RDS(ON) vs Input voltage 0.7 0.6 0.609 Main Switch VF B(V) 0.603 0.600 0.597 0.594 0.591 0.588 0.585 -50 -25 0 +25 +50 o RDS(O N) (mΩ) 0.606 0.5 0.4 0.3 0.2 Synchronous Switch 0.1 0 2.5 +75 +100 +125 3.5 4.5 5.5 6.5 Temperature( C) Input Voltage(V) RDS(ON) vs Temperature 0.80 0.75 0.70 0.65 VIN=3.6V VIN =4.2V VIN =2.7V Efficiency vs Load Current 100 90 80 V IN =2.7V RDS(ON) (mΩ) Ef ficie ncy( %) 0.60 0.55 0.50 0.45 0.40 0.35 0.30 0.25 0.20 -50 -25 0 +25 +50 +75 +100 +125 70 60 50 40 V IN =3.3V V IN =4.2V Main Switch Synchronous Switch 30 20 1 V OUT=1.2V 10 100 1000 Temperature( oC) I OUT (mA) 13 Rev.A.05 AME AME5258 Efficiency vs Load Current 100 90 V IN =2.7V 80 1.5MHz, 600mA Synchronous Buck Converter Efficiency vs Load Current 95 90 85 VIN =2.7V Eff iciency( %) Ef f iciency(%) 70 60 50 40 VIN=3.3V 80 75 70 V IN =4.2V 65 60 V IN =3.6V V IN =4.2V 30 20 1 VOUT =1.5V 10 100 1000 55 50 1 10 100 VOUT =1.8V 1000 IOUT (mA) IOUT ( mA) Efficiency vs Load Current 100 95 90 VIN=2. 7V Output Voltage vs Load Current 1.844 1.834 1.824 Eff iciency(% ) 85 VO UT (V) VOUT =2.5V 1000 80 75 70 65 60 55 50 1 10 100 V IN =3.6V V IN =4.2V 1.814 1.804 1.794 1.784 1.774 0 100 200 300 400 500 600 700 800 900 IOUT (mA) I OUT (mA) Current Limit vs Input Voltage 1800 1700 Cur re nt L imit (A ) 1600 1500 1400 1300 1200 1100 1000 2.5 3.0 3. 5 4.0 Temperature ( C) 4.5 o 5. 0 5.5 14 Rev.A.05 AME AME5258 n Date Code Rule Month Code 1: January 7: July 2: February 8: August 3: March 9: September 4: April A: October 5: May B: November 6: June C: December 1.5MHz, 600mA Synchronous Buck Converter A A A A A A A A A A A A A A A A A A A A Marking A M A M A M A M A M A M A M A M A M A M X X X X X X X X X X X X X X X X X X X X Year xxx0 xxx1 xxx2 xxx3 xxx4 xxx5 xxx6 xxx7 xxx8 xxx9 n Tape and Reel Dimension SOT-25 P W AME PIN 1 AME Carrier Tape, Number of Components Per Reel and Reel Size Package SOT-25 Carrier Width (W) 8.0±0.1 mm Pitch (P) 4.0±0.1 mm Part Per Full Reel 3000pcs Reel Size 180±1 mm Rev.A.05 15 AME AME5258 n Package Dimension SOT-25 Top View D θ1 Side View 1.5MHz, 600mA Synchronous Buck Converter SYMBOLS A A1 b D E e H L θ1 S1 A L MILLIMETERS MIN 0.90 0.00 0.30 2.70 1.40 INCHES MIN 0.0354 0.0000 0.0118 0.1063 0.0551 MAX 1.30 0.15 0.55 3.10 1.80 MAX 0.0512 0.0059 0.0217 0.1220 0.0709 H E 1.90 BSC 2.60 3.00 0.07480 BSC 0.10236 0.11811 0.0146BSC o PIN1 S1 e Front View 0.37BSC 0 o 10 0 o 10 o 0.95BSC 0.0374BSC b 16 A1 Rev.A.05 www.ame.com.tw E-Mail: sales@ame.com.tw Life Support Policy: These products of AME, Inc. are not authorized for use as critical components in life-support devices or systems, without the express written approval of the president of AME, Inc. AME, Inc. reserves the right to make changes in the circuitry and specifications of its devices and advises its customers to obtain the latest version of relevant information. © AME, Inc. , February 2009 Document: 1265-DS5258-A.05 Corporate Headquarter AME, Inc. 2F, 302 Rui-Guang Road, Nei-Hu District Taipei 114, Taiwan. Tel: 886 2 2627-8687 Fax: 886 2 2659-2989