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MAX8868EUKXY-T Datasheet(PDF) 7 Page - Maxim Integrated Products |
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MAX8868EUKXY-T Datasheet(HTML) 7 Page - Maxim Integrated Products |
7 / 8 page Low-Noise, Low-Dropout, 150mA Linear Regulators in SOT23 _______________________________________________________________________________________ 7 An external bypass capacitor connected to the BP pin reduces noise at the output. Additional blocks include a current limiter, reverse battery protection, thermal sen- sor, and shutdown logic. The MAX8868 also includes an auto-discharge function, which actively discharges the output voltage to ground when the device is placed in shutdown mode. Output Voltage The MAX8867/MAX8868 are supplied with factory-set output voltages from 2.5V to 5V, in 100mV increments. Except for the MAX886_EUK29 and the MAX886_EUK32 (which have an output voltage preset at 2.84V and 3.15V, respectively), the two-digit suffix allows the cus- tomer to choose the output voltage in 100mV increments. For example, the MAX8867EUK33 has a preset output voltage of 3.3V. (see Expanded Ordering Information). Internal P-Channel Pass Transistor The MAX8867/MAX8868 feature a 1.1 Ω typical P-chan- nel MOSFET pass transistor. This provides several advantages over similar designs using PNP pass tran- sistors, including longer battery life. The P-channel MOSFET requires no base drive, which reduces quies- cent current considerably. PNP-based regulators waste considerable current in dropout when the pass transis- tor saturates. They also use high base-drive currents under large loads. The MAX8867/MAX8868 do not suf- fer from these problems and consume only 100µA of quiescent current whether in dropout, light-load, or heavy-load applications (see the Typical Operating Characteristics). Current Limit The MAX8867/MAX8868 include a current limiter, which monitors and controls the pass transistor’s gate voltage, limiting the output current to 390mA. For design purposes, consider the current limit to be 160mA minimum to 500mA maximum. The output can be shorted to ground for an indefinite amount of time without damaging the part. Thermal-Overload Protection Thermal-overload protection limits total power dissipa- tion in the MAX8867/MAX8868. When the junction tem- perature exceeds TJ = +170°C, the thermal sensor signals the shutdown logic, turning off the pass transis- tor and allowing the IC to cool. The thermal sensor will turn the pass transistor on again after the IC’s junction temperature cools by 20°C, resulting in a pulsed output during continuous thermal-overload conditions. Thermal-overload protection is designed to protect the MAX8867/MAX8868 in the event of fault conditions. For continual operation, do not exceed the absolute maxi- mum junction-temperature rating of TJ = +150°C. Operating Region and Power Dissipation The MAX8867/MAX8868’s maximum power dissipation depends on the thermal resistance of the case and cir- cuit board, the temperature difference between the die junction and ambient air, and the rate of air flow. The power dissipation across the device is P = IOUT (VIN - VOUT). The maximum power dissipation is: PMAX = (TJ - TA) / ( θJB + θBA) where TJ - TA is the temperature difference between the MAX8867/MAX8868 die junction and the surrounding air, θJB (or θJC) is the thermal resistance of the pack- age, and θBA is the thermal resistance through the printed circuit board, copper traces, and other materi- als to the surrounding air. The GND pin of the MAX8867/MAX8868 performs the dual function of providing an electrical connection to ground and channeling heat away. Connect the GND pin to ground using a large pad or ground plane. Reverse Battery Protection The MAX8867/MAX8868 have a unique protection scheme that limits the reverse supply current to 1mA when either VIN or VSHDN falls below ground. Their cir- cuitry monitors the polarity of these two pins and dis- connects the internal circuitry and parasitic diodes when the battery is reversed. This feature prevents device damage. Noise Reduction An external 0.01µF bypass capacitor at BP, in conjunction with an internal 200k Ω resistor, creates a 80Hz lowpass filter for noise reduction. The MAX8867/MAX8868 exhibit 30µVRMS of output voltage noise with CBP = 0.01µF and COUT = 10µF. Start-up time is minimized by a power-on circuit that pre-charges the bypass capacitor. The Typical Operating Characteristics show graphs of Noise vs. BP Capacitance, Noise vs. Load Current, and Output Noise Spectral Density. __________Applications Information Capacitor Selection and Regulator Stability Normally, use a 1µF capacitor on the MAX8867/ MAX8868’s input and a 1µF to 10µF capacitor on the output. Larger input capacitor values and lower ESRs provide better supply-noise rejection and line-transient response. Reduce noise and improve load-transient response, stability, and power-supply rejection by using large output capacitors. For stable operation over the full temperature range and with load currents up to 150mA, a minimum of 1µF is recommended. Note that some ceramic dielectrics exhibit large capacitance and |
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