Electronic Components Datasheet Search |
|
MIC2571-1BMM Datasheet(PDF) 6 Page - Micrel Semiconductor |
|
MIC2571-1BMM Datasheet(HTML) 6 Page - Micrel Semiconductor |
6 / 12 page MIC2571 Micrel MIC2571 6 1997 Functional Description The MIC2571 switch-mode power supply (SMPS) is a gated oscillator architecture designed to operate from an input voltage as low as 0.9V and provide a high-efficiency fixed or adjustable regulated output voltage. One advantage of this architecture is that the output switch is disabled whenever the output voltage is above the feedback comparator threshold thereby greatly reducing quiescent current and improving efficiency, especially at low output currents. Refer to the Block Diagrams for the following discription of typical gated oscillator boost regulator function. The bandgap reference provides a constant 0.22V over a wide range of input voltage and junction temperature. The comparator senses the output voltage through an internal or external resistor divider and compares it to the bandgap reference voltage. When the voltage at the inverting input of the comparator is below 0.22V, the comparator output is high and the output of the oscillator is allowed to pass through the AND gate to the output driver and output switch. The output switch then turns on and off storing energy in the inductor. When the output switch is on (low) energy is stored in the inductor; when the switch is off (high) the stored energy is dumped into the output capacitor which causes the output voltage to rise. When the output voltage is high enough to cause the com- parator output to be low (inverting input voltage is above 0.22V) the AND gate is disabled and the output switch remains off (high). The output switch remains disabled until the output voltage falls low enough to cause the comparator output to go high. There is about 6mV of hysteresis built into the comparator to prevent jitter about the switch point. Due to the gain of the feedback resistor divider the voltage at V OUT experiences about 120mV of hysteresis for a 5V output. Appications Information Oscillator Duty Cycle and Frequency The oscillator duty cycle is set to 67% which is optimized to provide maximum load current for output voltages approxi- mately 3 × larger than the input voltage. Other output voltages are also easily generated but at a small cost in efficiency. The fixed oscillator frequency (options -1 and -2) is set to 20kHz. Output Waveforms The voltage waveform seen at the collector of the output switch (SW pin) is either a continuous value equal to V IN or a switching waveform running at a frequency and duty cycle set by the oscillator. The continuous voltage equal to V IN happens when the voltage at the output (V OUT) is high enough to cause the comparator to disable the AND gate. In this state the output switch is off and no switching of the inductor occurs. When V OUT drops low enough to cause the comparator output to change to the high state the output switch is driven by the oscillator. See Figure 1 for typical voltage waveforms in a boost application. 5V 0V 5V 0mA IPEAK VIN Time Figure 1. Typical Boost Regulator Waveforms Synchronization The SYNC pin is used to synchronize the MIC2571 to an external oscillator or clock signal. This can reduce system noise by correlating switching noise with a known system frequency. When not in use, the SYNC pin should be grounded to prevent spurious circuit operation. A falling edge at the SYNC input triggers a one-shot pulse which resets the oscillator. It is possible to use the SYNC pin to generate oscillator duty cycles from approximately 20% up to the nominal duty cycle. Current Limit Current limit for the MIC2571 is internally set with a resistor. It functions by modifying the oscillator duty cycle and fre- quency. When current exceeds 1.2A, the duty cycle is reduced (switch on-time is reduced, off-time is unaffected) and the corresponding frequency is increased. In this way less time is available for the inductor current to build up while maintaining the same discharge time. The onset of current limit is soft rather than abrupt but sufficient to protect the inductor and output switch from damage. Certain combina- tions of input voltage, output voltage and load current can cause the inductor to go into a continuous mode of operation. This is what happens when the inductor current can not fall to zero and occurs when: duty cycle V + V – V V + V – V OUT DIODE IN OUT DIODE SAT ≤ Time Current "ratchet" without current limit Current limit threshold Continuous current Discontinuous current Figure 2. Current Limit Behavior |
Similar Part No. - MIC2571-1BMM |
|
Similar Description - MIC2571-1BMM |
|
|
Link URL |
Privacy Policy |
ALLDATASHEET.COM |
Does ALLDATASHEET help your business so far? [ DONATE ] |
About Alldatasheet | Advertisement | Datasheet Upload | Contact us | Privacy Policy | Link Exchange | Manufacturer List All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |