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AME5140AEEVADJZ Datasheet(PDF) 10 Page - Analog Microelectronics |
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AME5140AEEVADJZ Datasheet(HTML) 10 Page - Analog Microelectronics |
10 / 22 page AME, Inc. 10 1.6 MHz Boost Converter with 30V Internal FET Switch AME5140 Rev.G.01 n Detailed Description The AME5140 is a switching converter IC that operates at a fixed frequency (1.6MHz) for fast transient response over a wide input voltage range and incorporates pulse-by- pulse current limiting protection. Operation can be best understood by referring to Figure 4. Because this is cur- rent mode control, a 33m Ω sense resistor in series with the switch FET is used to provide a voltage (which is pro- portional to the FET current) to both the input of the pulse width modulation (PWM) comparator and the current limit amplifier. At the beginning of each cycle, the S-R latch turns on the FET. As the current through the FET increases, a voltage (proportional to this current) is summed with the ramp com- ing from the ramp generator and then fed into the input of the PWM comparator. When this voltage exceeds the volt- age on the other input (coming from the Gm amplifier), the latch resets and turns the FET off. Since the signal coming from the Gm amplifier is derived from the feedback (which samples the voltage at the output), the action of the PWM comparator constantly sets the correct peak current through the FET to keep the output voltage in regulation. Q1 and Q2 align with R3 - R6 form a bandgap voltage reference used by the IC to hold the output in regulation. The currents flowing through Q1 and Q2 will be equal, and the feedback loop will adjust the regulated output to main- tain this. Because of this, the regulated output is always maintained at a voltage level equal to the voltage at the FB node "multiplied up" by the ratio of the output resistive di- vider. The current limit comparator feeds directly into the flip- flop that drives the switch FET. If the FET current reaches the limit threshold, the FET is turned off and the cycle ter- minated until the next clock pulse. The current limit input terminates the pulse regardless of the status of the output of the PWM comparator. n Application Hints Selecting The External Capacitors The best capacitors for use with the AME5140 are multilayer Ceramic capacitors. They have the lowest ESR (equivalent series resistance) and highest resonance frequency, which makes them optimum for use with high frequency switching Converters. When selecting a ce- ramic capacitor, only X5R and X7R dielectric types should be used. Other types such as Z5U and Y5F have such severe loss of capacitance due to effects of temperature variation and applied voltage, they may provide as little as 20% of rated capacitance in many typical applica- tions. Always consult capacitor manufacturer ’s data curves before selecting a capacitor. High-quality ceramic capacitors can be obtained from Taiyo-Yuden, AVX, and Murata. Selecting The Output Capacitor A single ceramic capacitor of value 4.7 µF to 10µF will provide sufficient output capacitance for most applica- tions. If larger amounts of capacitance are desired for improved line support and transient response, tantalum capacitors can be used. Aluminum electrolytic with ul- tra low ESR such as Sanyo Oscon can be used, but are usually prohibitively expensive. Typical AI electrolytic capacitors are not suitable for switching frequencies above 500kHz due to significant ringing and temperature rise due to self-heating from ripple current. An output ca- pacitor with excessive ESR can also reduce phase mar- gin and cause instability. In general, if electrolytic are used, it is recommended that. They be paralleled with ceramic capacitors to reduce ringing, switching losses, and output voltage ripple. Selecting The Input Capacitor An input capacitor is required to serve as an energy reservoir for the current which must flow into the coil each time the switch turns ON. This capacitor must have extremely low ESR, so ceramic is the best choice. We recommend a nominal value of 4.7 µF, but larger val- ues can be used. Since this capacitor reduces the amount of voltage ripple seen at the input pin, it also reduces the amount of EMI passed back along that line to other circuitry. |
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