AME 1 AME5259A Rev.A.02 1.2A, 1.5MHz Synchronous Step-Down Converter The AME5259A is a high efficiency monolithic synchro- nous buck regulator using a constant frequency, current mode architecture. Capable of delivering 1.2A output cur- rent over a wide input voltage range from 2.5V to 5.5V. Supply current with no load is 400μA and drops to<1μA in shutdown. The 2.5V to 5.5V input Voltage range makes the AME5259A ideally suited for single Li-Ion battery- powered applications. 100% duty cycle provides low dropout operation, extending battery life in portable sys- tems. PWM pulse skipping mode operation provides very low output ripple voltage for noise sensitive applica- tions. At very light load, the AME5259A will automati- cally skip pulses in pulse skip mode operation to main- tain output regulation. The internal synchronous switch increases efficiency and eliminates the need for an external Schottky diode. Low output voltages are easily supported with the 0.6V feedback reference voltage. The AME5259A is available in small DFN-6D, QFN-16C and SOT-25 packages. Other features include soft start, lower internal refer- ence voltage with 2% accuracy, over temperature protec- tion, and over current protection. n General Description n Features n Applications n Typical Application l Cellular Telephones l Personal Information Applicances l Wireless and DSL Modems l MP3 Players l Portable Instruments l High Efficiency: Up to 95% l Shutdown Mode Draws < 1μA Supply Current l 2.5V to 5.5V Input Range l Adjustable Output From 0.6V to V IN l 1.0V, 1.2V. 1.5V, 1.6V, 1.8V, 2.5V and 3.3V Fixed/Adjustable Output Voltage l 1.2A Output Current l Low dropout Operation: 100% Duty Cycle l No Schottky Diode Required l 1.5MHz Constant Frequency PWM Operation l Green Product Meet RoHS Standard 2.2μH C OUT 10μF CER V IN VOUT Figure 1. Fixed Output Voltage High Efficiency Step -Down Conventer AME5259 A IN SW OUT EN GND 2.2μH CIN 4.7μF CER COUT 10μF CER VIN 1.8V V OUT =V FB (R1+R2)/R2 R1 150K V IN = 2.5V to 5.5V VOUT AME5259 A IN SW FB EN GND R2 75K Figure 2. Adjustable Output Voltage 1.8V at 1000mA Step-Down Requlator C FWD : 22pF~220pF CIN 4.7μF CER C FWD
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AME
1
AME5259A
Rev.A.02
1.2A, 1.5MHz SynchronousStep-Down Converter
The AME5259A is a high efficiency monolithic synchro-nous buck regulator using a constant frequency, currentmode architecture. Capable of delivering 1.2A output cur-rent over a wide input voltage range from 2.5V to 5.5V.
Supply current with no load is 400µA and drops to<1µAin shutdown. The 2.5V to 5.5V input Voltage range makesthe AME5259A ideally suited for single Li-Ion battery-powered applications. 100% duty cycle provides lowdropout operation, extending battery life in portable sys-tems. PWM pulse skipping mode operation providesvery low output ripple voltage for noise sensitive applica-tions. At very light load, the AME5259A will automati-cally skip pulses in pulse skip mode operation to main-tain output regulation.
The internal synchronous switch increases efficiencyand eliminates the need for an external Schottky diode.Low output voltages are easily supported with the 0.6Vfeedback reference voltage. The AME5259A is availablein small DFN-6D, QFN-16C and SOT-25 packages.
Other features include soft start, lower internal refer-ence voltage with 2% accuracy, over temperature protec-tion, and over current protection.
n General Description
n Features
n Applications
n Typical Application
l Cellular Telephonesl Personal Information Applicancesl Wireless and DSL Modemsl MP3 Playersl Portable Instruments
l High Efficiency: Up to 95%
l Shutdown Mode Draws < 1µA Supply Current
l 2.5V to 5.5V Input Rangel Adjustable Output From 0.6V to VIN
l 1.0V, 1.2V. 1.5V, 1.6V, 1.8V, 2.5V and 3.3V Fixed/Adjustable Output Voltage
l 1.2A Output Currentl Low dropout Operation: 100% Duty Cyclel No Schottky Diode Requiredl 1.5MHz Constant Frequency PWM Operationl Green Product Meet RoHS Standard
VIN=2.5 to 5.5V, in PWM modeFor Fixed Output Voltage
For Adjustable OutputVoltage
VFB=VIN
IOUT=0mA, VFB=1V
AME
9
AME5259A
Rev.A.02
1.2A, 1.5MHz SynchronousStep-Down Converter
n Detailed Description
Main Control Loop
AME5259A uses a constant frequency, current modestep-down architecture. Both the main (P-channelMOSFET) and synchronous (N-channel MOSFET)switches are intermal. During normal operation, the in-ternal top power MOSFET is turned on each cycle whenthe oscillator sets the RS latch, and turned off when thecurrent comparator resets the RS latch. While the topMOSFET is off, the bottom MOSFET is turned on untileither the inductor current starts to reverse as indicatedby the current reversal comparator IRCMP.
Pulse Skipping Mode Operation
At light loads, the inductor current may reach zero orreverse on each pulse. The bottom MOSFET is turnedoff by the current reversal comparator, IRCMP, and theswitch voltage will ring. This is discontinuous mode op-eration, and is normal behavior for the switching regula-tor.
Short-Circuit Protection
When the output is shorted to ground, the frequency ofthe oscillator is reduced to about 180KHz. This frequencyfoldback ensures that the inductor current hsa more timedo decay, thereby preventing runaway. The oscillator sfrequency will progressively increase to 1.5MHz when VFB
or VOUT rises above 0V.
Dropout Operation
As the input supply voltage decreases to a value ap-proaching the output voltage, the duty cycle increasestoward the maximum on-time. Further reduction of thesupply voltage forces the main switch to remain on formore than one cycle until it reaches 100% duty cycle.The output voltage will then be determined by the inputvoltage minus the voltage drop across the P-channelMOSFET and the inductor.
The basic AME5259A application circuit is shown inTypical Application Circuit. External component selec-tion is determined by the maximum load current and be-gins with the selection of the inductor value and followedby CIN and COUT.
Inductor Selection
For a given input and output voltage, the inductor valueand operating frequency determine the ripple current. Theripple current DIL increases with higher VIN and decreaseswith higher inductance.
A reasonable starting point for setting ripple current is∆IL=0.4(lmax). The DC current rating of the inductorshould be at least equal to the maximum load currentplus half the ripple current to prevent core saturation. Forbetter efficiency, choose a low DC-resistance inductor.
CIN and COUT Selection
The input capacitance, CIN is needed to filter the trap-ezoidal current at the source of the top MOSFET. Toprevent large voltage transients, a low ESR inputcapacitorsized for the maximum RMS current must beused. The maximum RMS capacitor current is given by:
This formula has a maximum at VIN=2VOUT, whereIRMS=IOUT/2. This simple worst-case condition is com-monly used for design because even significant devia-tions do not offer much relief. Note that the capacitormanufacturer ripple current ratings are often based on 2000hours of life. This makes it advisable to further derate thecapacitor, or choose a capacitor rated at a higher tem-perature than required.
The selection of COUT is determined by the effective seriesresistance(ESR) that is required to minimize voltage rippleand load step transients. The output ripple, VOUT, is de-termined by:
Higher values, lower cost ceramic capacitors are nowbecoming available in smaller case sizes. Their high ripplecurrent, high voltage rating and low ESR make them idealfor switching regulator applications. However, care mustbe taken when these capacitors are used at the input andoutput. When a ceramic capacitor is used at the inputand the power is supplied by a wall adapter through longwires, a load step at the output can induce ringing at theinput, VIN. At best, this ringing can couple to the outputand be mistaken as loop instability. At worst, a suddeninrush of current through the long wires can potentiallycause a voltage spike at VIN large enough to damage thepart.
Output Voltage Programming
The output voltage is set by an external resistive divideraccording to the following equation :
Where VREF equals to 0.6V typical. The resistive di-vider allows the FB pin to sense a fraction of the outputvoltage as shown in Figure 4.
)1(R2
R1VREFVOUT ++++==== ⋅⋅⋅⋅
Thermal Considerations
In most applications the AME5259A does not dissipatemuch heat due to its high efficiency. But, in applicationswhere the AME5259A is running at high ambient tem-perature with low supply voltage and high duty cycles,such as in dropout, the heat dissipated may exceed themaximum junction temperature of the part. If the junc-tion temperature reaches approximately 160OC, bothpower switches will be turned off and the SW node willbecome high impedance. To avoid the AME5259A fromexceeding the maximum junction temperature, the userwill need to do some thermal analysis. The goal of thethermal analysis is to determine whether the power dissi-pated exceeds the maximum junction temperature of thepart. The temperature rise is given by:
Where PD is the power dissipated by the regulator andθJA is the thermal resistance from the junction of the dieto the ambient temperature.
When laying out the printed circuit board, the following checklist should be used to ensure proper operation of theAME5259A. These items are also illustrated graphically in Figures 10 and Figures 11 . Check the following in yourlayout:
1. The power traces, consisting of the GND trace, the SW trace and the VIN trace should be kept short, direct and wide.
2. Does the VFB pin connect directly to the feedback resistors? The resistive divider R2/R1 must be connected between the (+) plate of COUT and ground.
3. Does the (+) plate of CIN connect to VIN 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.
BSC: Basic. Represents theoretical exact dimension or
dimension target.
2. Dimensions in Millimeters.
3. General tolerance +0.05mm unless otherwise specified.
n Lead Pattern
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 presidentof AME, Inc.
AME, Inc. reserves the right to make changes in the circuitry and specifications of its devices andadvises its customers to obtain the latest version of relevant information.
AME, Inc. , January 2014Document: 1283-DS5259A-A.02