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MIC2179-3.3YSM Datasheet(PDF) 7 Page - Micrel Semiconductor |
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MIC2179-3.3YSM Datasheet(HTML) 7 Page - Micrel Semiconductor |
7 / 13 page June 2009 7 M9999-063009 Micrel, Inc. MIC2179 Functional Description Micrel’s MIC2179 is a synchronous buck regulator that oper- ates from an input voltage of 4.5V to 16.5V and provides a regulated output voltage of 1.25V to 16.5V. Its has internal power MOSFETs that supply up to 1.5A load current and operates with up to 100% duty cycle to allow low-dropout operation. To optimize efficiency, the MIC2179 operates in PWM and skip mode. Skip mode provides the best efficiency when load current is less than 150mA, while PWM mode is more efficient at higher current. PWM or skip-mode opera- tion is selected externally, allowing an intelligent system (i.e. microprocessor controlled) to select the correct operating mode for efficiency and noise requirements. During PWM operation, the MIC2179 uses current-mode control which provides superior line regulation and makes the control loop easier to compensate. The PWM switching frequency is set internally to 200kHz and can be synchronized to an external clock frequency up to 300kHz. Other features include a low-current shutdown mode, current limit, under- voltage lockout, and thermal shutdown. See the following sections for more detail. Switch Output The switch output (SW) is a half H-bridge consisting of a high-sideP-channelandlow-sideN-channelpowerMOSFET. TheseMOSFETshaveatypicalon-resistanceof150mΩwhen the MIC2179 operates from a 12V supply. Antishoot-through circuitry prevents the P-channel and N-channel from turning on at the same time. Current Limit The MIC2179 uses pulse-by-pulse current limiting to protect the output. During each switching period, a current limit com- parator detects if the P-Channel current exceeds 4.3A. When it does, the P-channel is turned off until the next switching period begins. Undervoltage Lockout Undervoltage lockout (UVLO) turns off the output when the input voltage (VIN) is to low to provide sufficient gate drive for the output MOSFETs. It prevents the output from turning on until VIN exceeds 4.3V. Once operating, the output will not shut off until VIN drops below 4.2V. Thermal Shutdown Thermal shutdown turns off the output when the MIC2179 junction temperature exceeds the maximum value for safe operation. After thermal shutdown occurs, the output will not turn on until the junction temperature drops approximately 10°C. Shutdown Mode The MIC2179 has a low-current shutdown mode that is con- trolled by the enable input (EN). When a logic 0 is applied to EN, the MIC2179 is in shutdown mode, and its quiescent current drops to less than 5µA. Internal Bias Regulator An internal 3.3V regulator provides power to the MIC2179 control circuits. This internal supply is brought out to the BIAS pin for bypassing by an external 0.01µF capacitor. Do not connect an external load to the BIAS pin. It is not designed to provide an external supply voltage. Frequency Synchronization The MIC2179 operates at a preset switching frequency of 200kHz. It can be synchronized to a higher frequency by con- necting an external clock to the SYNC pin. The SYNC pin is a logic level input that synchronizes the oscillator to the rising edge of an external clock signal. It has a frequency range of 220kHz to 300kHz, and can operate with a minimum pulse width of 500ns. If synchronization is not required, connect SYNC to ground. Power Good Flag The power good flag (PWRGD) is an error flag that alerts a system when the output is not in regulation. When the output voltage is 10% below its nominal value, PWRGD is logic low, signaling that VOUTis to low. PWRGD is an open-drain output that can sink 1mA from a pull-up resistor connected to VIN. Low-Dropout Operation Output regulation is maintained in PWM or skip mode even when the difference between VIN and VOUT decreases below 1V. As VIN – VOUT decreases, the duty cycle increases until it reaches 100%. At this point, the P-channel is kept on for several cycles at a time, and the output stays in regulation until VIN – VOUT falls below the dropout voltage (dropout voltage = P-channel on-resistance × load current). PWM-Mode Operation Refer to “PWM Mode Functional Diagram” which is a simpli- fied block diagram of the MIC2179 operating in PWM mode and its associated waveforms. When operating in PWM mode, the output P-channel and N- channel MOSFETs are alternately switched on at a constant frequency and variable duty cycle. A switching period begins when the oscillator generates a reset pulse. This pulse resets the RS latch which turns on the P-channel and turns off the N-channel. During this time, inductor current (IL1) increases and energy is stored in the inductor. The current sense ampli- fier (ISENSE Amp) measures the P-channel drain-to-source voltage and outputs a voltage proportional to IL1. The output of ISENSE Amp is added to a sawtooth waveform (corrective ramp) generated by the oscillator, creating a composite wave- form labeled ISENSE on the timing diagram. When ISENSE is greater than the error amplifier output, the PWM comparator will set the RS latch which turns off the P-channel and turns on the N-channel. Energy is then discharged from the induc- tor and IL1 decreases until the next switching cycle begins. By varying the P-channel on-time (duty cycle), the average inductor current is adjusted to whatever value is required to regulate the output voltage. The MIC2179 uses current-mode control to adjust the duty cycle and regulate the output voltage. Current-mode control has two signal loops that determine the duty cycle. One is an outer loop that senses the output voltage, and the other is a faster inner loop that senses the inductor current. Signals from these two loops control the duty cycle in the following way: VOUT is fed back to the error amplifier which compares the feedback voltage (VFB) to an internal reference voltage |
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