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MAX8505EEE Datasheet(PDF) 9 Page - Maxim Integrated Products |
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MAX8505EEE Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 15 page Detailed Description The MAX8505 is a high-efficiency synchronous buck regulator capable of delivering up to 3A of output current. It operates in PWM mode at a high fixed frequency of 500kHz or 1MHz, thereby reducing external component size. The MAX8505 operates from a 2.6V to 5.5V input voltage and can produce an output voltage from 0.8V to 0.85 ✕ VIN. Controller Block Function The MAX8505 step-down converter uses a PWM current-mode control scheme. An open-loop comparator compares the voltage-feedback error signal against the sum of the amplified current-sense signal and the slope compensation ramp. At each rising edge of the internal clock, the internal high-side MOSFET turns on until the PWM comparator trips. During this on-time, current ramps up through the inductor, sourcing current to the output and storing energy in the inductor. The current-mode feedback system regulates the peak inductor current as a function of the output-voltage error signal. Since the aver- age inductor current is nearly the same as the peak inductor current, the circuit acts as a switch-mode transconductance amplifier. To preserve inner-loop stability and eliminate inductor staircasing, a slope- compensation ramp is summed into the main PWM com- parator. During the second half of the cycle, the internal high-side N-channel MOSFET turns off, and the internal low-side N-channel MOSFET turns on. The inductor releases the stored energy as its current ramps down while still providing current to the output. The output capacitor stores charge when the inductor current exceeds the load current, and discharges when the inductor current is lower, smoothing the voltage across the load. Under overload conditions, when the inductor current exceeds the current limit (see the Current Limit section), the high-side MOSFET does not turn on at the rising edge of the clock and the low-side MOSFET remains on to let the inductor current ramp down. Current Sense An internal current-sense amplifier produces a current signal proportional to the voltage generated by the high- side MOSFET on-resistance and the inductor current (RDS(ON) ✕ ILX). The amplified current-sense signal and the internal slope-compensation signal are summed together into the comparator’s inverting input. The PWM comparator turns off the internal high-side MOSFET when this sum exceeds the feedback voltage from the voltage-error amplifier. 3A, 1MHz, 1% Accurate, Internal Switch Step-Down Regulator with Power-OK _______________________________________________________________________________________ 9 Pin Description PIN NAME FUNCTION 1, 3, 14, 16 LX Inductor Connection. Connect an inductor between these pins and the regulator output. All LX pins must be connected together externally. Connect a 3300pF ceramic capacitor from LX to PGND. 2, 4 IN Power-Supply Inputs. Ranges from 2.6V to 5.5V. Bypass with two ceramic 22µF capacitors to GND. All IN pins must be connected together externally. 5 BST Bootstrapped Voltage Input. High-side driver supply pin. Bypass to LX with a 0.1µF capacitor. Charged from IN with an external Schottky diode. 6VCC Supply Voltage and Gate-Drive Supply for Low-Side Driver. Decouple with a 10 Ω resistor and bypass to GND with 0.1µF. 7 POK Power-OK Output. Open-drain output of a window comparator that pulls POK low when the FB pin is outside the 0.8V ±12% range. 8 CTL Output Control. When at GND, the regulator is off. When at VCC, the regulator is operating at 1MHz. For a 500kHz application, raise the pin to 2/3 VCC. 9 COMP Regulator Loop Compensation. Connect a series RC network to GND. This pin is pulled to GND when the output is shut down, or in UVLO or thermal shutdown. 10 FB Feedback Input. This pin regulates to 0.8V. Use an external resistive-divider from the output to set the output voltage. 11 REF Place a capacitor at this pin to set the soft-start time. This pin goes to 0V when the part is shut down. 12 GND Ground 13, 15 PGND Power Ground. Connect this pin to GND at a single point. |
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