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MAX1793EUE25 Datasheet(PDF) 9 Page - Maxim Integrated Products |
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MAX1793EUE25 Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 11 page Thermal-overload protection is designed to safeguard the MAX1793 in the event of fault conditions. For con- tinuous operation, do not exceed the absolute maxi- mum junction-temperature rating of TJ = +150°C. Operating Region and Power Dissipation Maximum power dissipation of the MAX1793 depends on the thermal resistance of the case and circuit board, the temperature difference between the die junction and ambient air, and the rate of air flow. The power dis- sipated by the device is: P = IOUT (VIN – VOUT). The resulting maximum power dissipation is: PMAX = [(TJ(MAX) – TA)/( θJC + θCA)] where (TJ(MAX) – TA) is the temperature difference between the maximum allowed die junction (+150°C) and the surrounding air, θJC (junction-to-case) is the thermal resistance of the package chosen, and θCA is the thermal resistance from the case through the PCB, copper traces, and other materials to the surrounding air. Figure 4 shows allowable power dissipation for typical PCBs at +25°C, +50°C, and +70°C ambient temperatures. The MAX1793 TSSOP-EP package features an exposed thermal pad on its underside. This pad lowers the ther- mal resistance of the package by providing a direct heat conduction path from the die to the PCB. Additionally, the ground pin (GND) also channels heat. Connect the exposed thermal pad and GND to circuit ground by using a large pad (minimum 1in2 recommended), or mul- tiple vias to the ground plane. Applications Information Capacitor Selection and Regulator Stability Capacitors are required at the input and output of the MAX1793. Connect a 4.7µF capacitor between IN and ground (CIN) and a 6.8µF capacitor between OUT and ground (COUT). CIN serves only to lower the source impedance of the input supply and may be smaller than 4.7µF when the MAX1793 is powered from regulated power supplies or low-impedance batteries. The output capacitor’s equivalent series resistance (ESR) affects stability and output noise. COUT ESR should be 0.5 Ω or less to ensure stability and optimum transient response. Surface-mount ceramic capacitors have very low ESR and are commonly available in values up to 10µF. Other low-ESR (< 0.5 Ω) capacitors, such as sur- face-mount tantalum or OS-CON, may also be used. Connect CIN and COUT as close to the IC as possible to minimize the impact of PCB trace inductance. Noise, PSRR, and Transient Response The MAX1793 output noise is typically 115µVRMS during normal operation. This is suitably low for most applica- tions. See the output noise plot in the Typical Operating Characteristics. The MAX1793 is designed to achieve low dropout volt- age and low quiescent current in battery-powered sys- tems while still maintaining good noise, transient response, and AC rejection. See the Typical Operating Characteristics for a plot of Power-Supply Rejection Ratio (PSRR) vs. Frequency. When operating from very noisy sources, supply noise rejection and transient response can be improved by increasing the values of the input and output capacitors and employing passive postfiltering. Input-Output (Dropout) Voltage A regulator’s minimum input-to-output voltage differential (dropout voltage) determines the lowest usable supply voltage. In battery-powered systems, this determines the useful end-of-life battery voltage. Since a p-channel MOSFET is used as the pass device, dropout voltage is the product of RDS(ON) and load current (see the Electrical Characteristics and Dropout Voltage vs. Load Current in the Typical Operating Characteristics). The MAX1793 operating current typically remains below 225µA in dropout. Low-Dropout, Low IQ, 1A Linear Regulator _______________________________________________________________________________________ 9 0 0.6 0.8 1.6 0.4 0.2 1.0 1.2 1.4 1.8 2.0 1 03 24 5 6 MAXIMUM OUTPUT CURRENT vs. INPUT-OUTPUT DIFFERENTIAL VOLTAGE INPUT-OUTPUT DIFFERENTIAL VOLTAGE (V) MAXIMUM CONTINUOUS CURRENT TA = +50 °C TJ(MAX) = +150 °C TA = +70 °C TA = +25 °C Figure 4. Power Operating Region: Maximum Output Current vs. Input-Output Differential Voltage |
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