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MAX8887EZK29T Datasheet(PDF) 6 Page - Maxim Integrated Products |
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MAX8887EZK29T Datasheet(HTML) 6 Page - Maxim Integrated Products |
6 / 9 page Detailed Description The MAX8887/MAX8888 are low-dropout, low-quies- cent-current linear regulators designed primarily for battery-powered applications. The devices supply loads up to 300mA and are available in several fixed output voltages in the 1.5V to 3.3V range. The MAX8887 is optimized for low-noise operation, while the MAX8888 includes an open-drain POK output flag. As illustrated in Figure 1, the MAX8888 consists of a 1.25V reference, error amplifier, P-channel pass tran- sistor, and internal feedback voltage divider. Internal P-Channel Pass Transistor The MAX8887/MAX8888 feature a 0.5 Ω P-channel MOSFET pass transistor. Unlike similar designs using PNP pass transistors, P-channel MOSFETs require no base drive, which reduces quiescent current. PNP- based regulators also waste considerable current in dropout when the pass transistor saturates and use high base drive currents under large loads. The MAX8887/MAX8888 do not suffer from these problems and consume only 55µA of quiescent current under heavy loads as well as in dropout. Output Voltage Selection The MAX8887/MAX8888 are supplied with various fac- tory-set output voltages ranging from 1.5V to 3.3V. The part number’s two-digit suffix identifies the nominal out- put voltage. For example, the MAX8887EZK33 has a preset output voltage of 3.3V (see the Ordering Infor- mation). Shutdown Drive SHDN low to enter shutdown. During shutdown, the output is disconnected from the input and supply current drops to 0.1µA. When in shutdown, POK and OUT are driven low. SHDN can be pulled as high as 6V, regardless of the input and output voltages. Power-OK Output The power-OK output (POK) pulls low when OUT is less than 93% of the nominal regulation voltage. Once OUT exceeds 93% of the nominal voltage, POK goes high impedance. POK is an open-drain N-channel output. To obtain a logic level output, connect a pullup resistor from POK to OUT. A 100k Ω resistor works well for most applications. POK can be used as a power-on-reset (POR) signal to a microcontroller (µC) or to drive other logic. Adding a capacitor from POK to ground creates POK delay. When the MAX8887 is shut down, POK is held low independent of the output voltage. If unused, leave POK grounded or unconnected. Current Limit The MAX8887/MAX8888 monitor and control the pass transistor’s gate voltage, limiting the output current to 0.8A (typ). This current limit is reduced to 500mA (typ) when the output voltage is below 93% of the nominal value to provide foldback current limiting. Thermal Overload Protection Thermal overload protection limits total power dissipa- tion in the MAX8887/MAX8888. When the junction tem- perature exceeds TJ =+170°C, a thermal sensor turns off the pass transistor, allowing the device to cool. The thermal sensor turns the pass transistor on again after the junction temperature cools by 20°C, resulting in a pulsed output during continuous thermal overload con- ditions. Thermal overload protection protects the MAX8887/MAX8888 in the event of fault conditions. For continuous operation, do not exceed the absolute maxi- mum junction-temperature rating of TJ=+150°C. Operating Region and Power Dissipation The MAX8887/MAX8888’s maximum power dissipation depends on the thermal resistance of the IC package and circuit board. The temperature difference between the die junction and ambient air, and the rate of air flow. The power dissipated in the device is P = IOUT ✕ (VIN- VOUT). The maximum allowed power dissipation is 727mW or: PMAX = (TJ(MAX) - TA) / ( θJC + θCA) where TJ(MAX)-TA is the temperature difference between the MAX8887/MAX8888 die junction and the surrounding air; θJC is the thermal resistance from the junction to the case; and θCA is the thermal resistance from the case through PC board, copper traces, and other materials to the surrounding air. Refer to Figure 2 for the MAX8887/MAX888 valid oper- ating region. Noise Reduction For the MAX8887 only, an external 0.01µF bypass capacitor at BP creates a lowpass filter for noise reduc- tion. The MAX8887 exhibits 42µVRMS of output voltage noise with CBP = 0.01µF and COUT = 2.2µF (see the Typical Operating Characteristics). Applications Information Capacitor Selection and Regulator Stability Connect a 2.2µF ceramic capacitor between IN and ground and a 2.2µF ceramic capacitor between OUT and ground. The input capacitor (CIN) lowers the source impedance of the input supply. Reduce noise and improve load-transient response, stability, and power-supply rejection by using a larger ceramic out- put capacitor such as 4.7µF. The output capacitor’s (COUT) equivalent series resis- tance (ESR) affects stability and output noise. Use out- Low-Dropout, 300mA Linear Regulators in SOT23 6 _______________________________________________________________________________________ |
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