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MAX877 Arkusz danych(PDF) 6 Page - Maxim Integrated Products |
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MAX877 Arkusz danych(HTML) 6 Page - Maxim Integrated Products |
6 / 12 page _______________Detailed Description Operating Principle The MAX877/MAX878/MAX879 combine a switch-mode regulator with an NPN bipolar power switch and current limit, a precision voltage reference, and a synchronous rectifier—all in a single monolithic device. In shutdown mode, the internal rectifier is completely turned off and disconnects the load from the source. Only two external components are required in addition to the input bypass capacitor—a 22µH inductor, and a 100µF filter capacitor. A minimum-off-time, current-limited, pulse-frequency- modulation (PFM) control scheme combines the high output power and efficiency of pulse-width modulation (PWM) with the low quiescent currents of traditional PFM pulse skippers. External conditions (inductor value, load, and input volt- age) determine the way the converter operates, as fol- lows: At light loads, the current through the inductor starts at zero, rises to a peak value, and drops down to zero in each cycle (discontinuous-conduction mode). In this case, the switching frequency is governed by a pair of one-shots, which set a maximum on-time inversely pro- portional to VIN [tON = 8.8/(VIN - 0.25)] and a minimum off-time (1.3µs for MAX877/MAX879, or 2.3µs for MAX878). With a 22µH inductor, LX’s peak current is about 400mA and is independent of input voltage. Efficiency at light loads is improved because of lower peak currents. At very light loads , more energy is stored in the coil than is required by the load in each cycle. The converter regulates by skipping entire cycles. Efficiency is typically 65% to 75% in the pulse-skipping mode. Pulse-skipping waveforms can be irregular, and the output waveform contains a low-frequency component. Larger, low equiv- alent-series-resistance (ESR) filter capacitors can help reduce the ripple voltage if needed. At heavy loads above approximately 100mA, the con- verter enters continuous-conduction mode, where cur- rent always flows in the inductor. The switch ON state is controlled on a cycle-by-cycle basis, either by the tON(max) time or the preset current limit in the switch. This prevents exceeding the switch current rating or sat- urating the inductor. At very heavy loads, the inductor current self-oscillates between this peak current limit and some lower value governed by the minimum off-time, the inductance value, and the input/output differential. 5V/3.3V/3V/Adjustable-Output, Step-Up/Step-Down DC-DC Converters 6 _______________________________________________________________________________________ FUNCTION 1 Sets switch current-limit input. Connect to IN for 1A current limit. A resistor from ILIM to IN sets lower peak inductor currents. 2 Input supply. 3 Analog ground. Not internally connected to PGND. PIN ILIM IN 4 Power ground must be low impedance; solder directly to ground plane or star ground. Connect to AGND, close to the device. 5 1A NPN power switch collector and active-rectifier PNP emitter. 6 Voltage output. Connect filter capacitor close to pin. 7 Shutdown input disables power supply when low. Also disconnects load from input. Threshold is set at VIN/2. Connect to IN for normal operation. AGND PGND LX 8 No connect, not internally connected. OUT – S — H — D — N – N.C. (MAX877) ______________________________________________________________Pin Description NAME FB (MAX879) Feedback input for adjustable-output operation. Connect to an external voltage divider between VOUT and AGND. SEL (MAX878) Selects the main output voltage: 3.3V when connected to AGND, 3.0V when left open. |
Podobny numer części - MAX877 |
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Podobny opis - MAX877 |
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