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MAX1836-MAX1837 Datasheet(PDF) 9 Page - Maxim Integrated Products |
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MAX1836-MAX1837 Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 13 page 24V Internal Switch, 100% Duty Cycle, Step-Down Converters _______________________________________________________________________________________ 9 Input-Output (Dropout) Voltage A step-down converter’s minimum input-to-output volt- age differential (dropout voltage) determines the lowest useable input supply voltage. In battery-powered sys- tems, this limits the useful end-of-life battery voltage. To maximize battery life, the MAX1836/MAX1837 operate with duty cycles up to 100%, which minimizes the input- to-output voltage differential. When the supply voltage approaches the output voltage, the P-channel MOSFET remains on continuously to supply the load. Dropout voltage is defined as the difference between the input and output voltages when the input is low enough for the output to drop out of regulation. For a step-down converter with 100% duty cycle, the dropout voltage depends on the MOSFET drain-to-source on- resistance (RDS(ON)) and inductor series resistance; therefore, it is proportional to the load current: Shutdown (SHDN) A logic-level low voltage on SHDN shuts down the MAX1836/MAX1837. When shut down, the supply cur- rent drops to 3µA to maximize battery life, and the inter- nal P-channel MOSFET turns off to isolate the output from the input. The output capacitance and load cur- rent determine the rate at which the output voltage decays. A logic-level high voltage on SHDN activates the MAX1836/MAX1837. Do not leave SHDN floating. If unused, connect SHDN to IN. When setting output volt- ages above 5.5V, the shutdown feature cannot be used, so SHDN must be permanently connected to IN. The SHDN input voltage slew rate must be greater than 10V/ms. Thermal-Overload Protection Thermal-overload protection limits total power dissipa- tion in the MAX1836/MAX1837. When the junction tem- perature exceeds TJ = +160°C, a thermal sensor turns off the pass transistor, allowing the IC to cool. The ther- mal sensor turns the pass transistor on again after the IC’s junction temperature cools by 10°C, resulting in a pulsed output during continuous thermal-overload conditions. Design Information Output Voltage Selection The feedback input features dual-mode operation. Connect the output to OUT and FB to GND for the pre- set output voltage. The MAX1836/MAX1837 are sup- plied with factory-set output voltages of 3.3V or 5V. The two-digit part number suffix identifies the output voltage (see the Selector Guide). For example, the MAX1836EUT33 has a preset 3.3V output voltage. The MAX1836/MAX1837 output voltage may be adjust- ed by connecting a voltage divider from the output to FB (Figure 5). When externally adjusting the output volt- age, connect OUT to GND. Select R2 in the 10k Ω to 100k Ω range. Calculate R1 with the following equation: where VFB = 1.25V, and VOUT may range from 1.25V to VIN. When setting output voltages above 5.5V, the shut- down feature cannot be used, so SHDN must be per- manently connected to IN. Inductor Selection When selecting the inductor, consider these four para- meters: inductance value, saturation current rating, series resistance, and size. The MAX1836/MAX1837 operate with a wide range of inductance values. For most applications, values between 10µH and 100µH work best with the controller’s switching frequency. Calculate the minimum inductance value as follows: where tON(MIN) = 1.0µs. Inductor values up to six times L(MIN) are acceptable. Low-value inductors may be smaller in physical size and less expensive, but they result in higher peak-current overshoot due to current- sense comparator propagation delay (300ns). Peak- current overshoot reduces efficiency and could exceed the current ratings of the internal switching MOSFET and external components. L (V - V ) I (MIN) IN(MAX) OUT ON(MIN) LIM = t R1 R2 V V - OUT FB = 1 VDROPOUT OUT DS(ON) INDUCTOR =× + () IR R MAX1836 MAX1837 IN GND LX SHDN FB OUT NOTE: HIGH-CURRENT PATHS SHOWN WITH BOLD LINES. OUTPUT 1.25V TO VIN INPUT 4.5V OR 24V CIN D1 L1 COUT R1 R2 Figure 5. Adjustable Output Voltage |
Similar Part No. - MAX1836-MAX1837 |
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Similar Description - MAX1836-MAX1837 |
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