Datasheet MAX1836, MAX1837 (Maxim) - 9

link to page 12 link to page 9 MAX1836/MAX1837 24V Internal Switch, 100% Duty Cycle, Step-Down Converters
Input-Output (Dropout) Voltage
the Selector Guide. For example, the MAX1836EUT33 A step-down converter’s minimum input-to-output volt- has a preset 3.3V output voltage. age differential (dropout voltage) determines the lowest The MAX1836/MAX1837 output voltage may be adjusted useable input supply voltage. In battery-powered sys- by connecting a voltage divider from the output to FB tems, this limits the useful end-of-life battery voltage. To (Figure 5). When externally adjusting the output voltage, maximize battery life, the MAX1836/MAX1837 operate connect OUT to GND. Select R2 in the 10kΩ to 100kΩ with duty cycles up to 100%, which minimizes the input- range. Calculate R1 with the following equation: to-output voltage differential. When the supply voltage approaches the output voltage, the P-channel MOSFET  VOUT   remains on continuously to supply the load. R1= R2   − 1   FB V   Dropout voltage is defined as the difference between the input and output voltages when the input is low enough for where VFB = 1.25V, and VOUT may range from 1.25V to the output to drop out of regulation. For a step-down con- VIN. When setting output voltages above 5.5V, the shut- verter with 100% duty cycle, the dropout voltage depends down feature cannot be used, so SHDN must be perma- on the MOSFET drain-to-source on-resistance (R nently connected to IN. DS(ON)) and inductor series resistance; therefore, it is proportional
Inductor Selection
to the load current: When selecting the inductor, consider these four param- eters: inductance value, saturation current rating, series DRO V POUT = IOUT × (RDS(ON) + RINDUCTOR) resistance, and size. The MAX1836/MAX1837 operate with a wide range of inductance values. For most applica-
Shutdown (SHDN)
tions, values between 10μH and 100μH work best with A logic-level low voltage on SHDN shuts down the the controller’s switching frequency. Calculate the mini- MAX1836/MAX1837. When shut down, the supply cur- mum inductance value as follows: rent drops to 3μA to maximize battery life, and the internal P-channel MOSFET turns off to isolate the output from the ( IVN(MAX) − VOUT)tON(MIN) input. The output capacitance and load current determine L(MIN) = I the rate at which the output voltage decays. A logic-level LIM high voltage on SHDN activates the MAX1836/MAX1837. where tON(MIN) = 1.0μs. Inductor values up to six times Do not leave SHDN floating. If unused, connect SHDN to L(MIN) are acceptable. Low-value inductors may be small- IN. When setting output voltages above 5.5V, the shut- er in physical size and less expensive, but they result in down feature cannot be used, so SHDN must be perma- higher peak-current overshoot due to current-sense com- nently connected to IN. The SHDN input voltage slew rate parator propagation delay (300ns). Peak-current over- must be greater than 10V/ms. shoot reduces efficiency and could exceed the current ratings of the internal switching MOSFET and external
Thermal-Overload Protection
components. Thermal-overload protection limits total power dissipa- tion in the MAX1836/MAX1837. When the junction tem- perature exceeds T INPUT OUTPUT J = +160°C, a thermal sensor turns off 4.5V OR 24V L1 1.25V TO V the pass transistor, allowing the IC to cool. The thermal IN IN LX sensor turns the pass transistor on again after the IC’s CIN D1 COUT junction temperature cools by 10°C, resulting in a pulsed SHDN R1 output during continuous thermal-overload conditions. FB
Design Information MAX1836 MAX1837
R2
Output Voltage Selection
GND OUT The feedback input features dual-mode operation. Connect the output to OUT and FB to GND for the preset output voltage. The MAX1836/MAX1837 are supplied
NOTE:
HIGH-CURRENT PATHS SHOWN WITH BOLD LINES. with factory-set output voltages of 3.3V or 5V. The two- digit part number suffix identifies the output voltage. See Figure 5. Adjustable Output Voltage www.maximintegrated.com Maxim Integrated │

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