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MAX8727ETB Datasheet(PDF) 7 Page - Maxim Integrated Products |
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MAX8727ETB Datasheet(HTML) 7 Page - Maxim Integrated Products |
7 / 13 page Detailed Description The MAX8727 is a highly efficient power supply that employs a current-mode, fixed-frequency, pulse-width modulation (PWM) architecture for fast transient response and low-noise operation. The device regu- lates the output voltage through a combination of an error amplifier, two comparators, and several signal generators (Figure 2). The error amplifier compares the signal at FB to 1.24V and varies the COMP output. The voltage at COMP determines the current trip point each time the internal MOSFET turns on. As the load changes, the error amplifier sources or sinks current to the COMP output to command the inductor peak cur- rent necessary to service the load. To maintain stability at high duty cycles, a slope-compensation signal is summed with the current-sense signal. At light loads, this architecture allows the MAX8727 to “skip” cycles to prevent overcharging the output voltage. In this region of operation, the inductor ramps up to a peak value of approximately 50mA, discharges to the output, and waits until another pulse is needed again. Output Current Capability The output current capability of the MAX8727 is a func- tion of current limit, input voltage, operating frequency, and inductor value. Because of the slope compensa- tion used to stabilize the feedback loop, the inductor current limit depends on the duty cycle. The current limit is determined by the following equation: ILIM = (1.26 - 0.35 x D) x ILIM_EC where ILIM_EC is the current limit specified at 75% duty cycle (see the Electrical Characteristics) and D is the duty cycle. The output current capability depends on the current- limit value and is governed by the following equation: where ILIM is the current limit calculated above, η is the regulator efficiency (85% nominal), and D is the duty cycle. The duty cycle when operating at the current limit is: where VDIODE is the rectifier diode forward voltage and RON is the on-resistance of the internal MOSFET. D VV V VI R V OUT IN DIODE OUT LIM ON DIODE = −+ −× + II DV fL V V OUT MAX LIM IN OSC IN OUT () . =− ×× × ⎡ ⎣ ⎢ ⎢ ⎤ ⎦ ⎥ ⎥ ×× 05 η TFT-LCD Step-Up DC-DC Converter _______________________________________________________________________________________ 7 GND LX IN FREQ FB COMP 4 µA 5 µA N ERROR COMPARATOR ERROR AMPLIFIER SKIP COMPARATOR SS CLOCK SKIP BIAS SHDN MAX8727 Σ CURRENT SENSE CONTROL AND DRIVER LOGIC SOFT- START SLOPE COMPEN- SATION OSCILLATOR ∞ 1.24V Figure 2. MAX8727 Functional Diagram |
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