MAX1864 xDSL/Cable Modem Triple Output Power Supply


MAX1864 xDSL/Cable Modem Triple Output Power Supply RSS Sample
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Maxim Integrated
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Create a design and simulate using EE-Sim® tools: The MAX1864/MAX1865 power-supply controllers are designed to address cost-conscious applications such as cable modem Consumer Premise Equipment (CPE), xDSL CPE, and set-top boxes. Operating off a low-cost, unregulated DC supply (such as a wall adapter output), the MAX1864 generates three positive outputs, and the MAX1865 generates four positive outputs and one negative output to provide a cost-effective system power supply.

The MAX1864 includes a current-mode synchronous step-down controller and two positive regulator gain blocks. The MAX1865 has one additional positive gain block and one negative regulator gain block. The main synchronous step-down controller generates a high-current output that is preset to 3.3V or adjustable from 1.236V to 0.8 x VIN with an external resistive-divider. The 100kHz/200kHz operating frequency allows the use of low-cost aluminum-electrolytic capacitors and low-cost power magnetics. Additionally, the MAX1864/MAX1865 step-down controllers sense the voltage across the low-side MOSFET's on-resistance to efficiently provide the current-limit signal, eliminating the need for costly current-sense resistors.

The MAX1864/MAX1865 generate additional supply rails at low cost. The positive regulator gain blocks use an external PNP pass transistor to generate low-voltage rails directly from the main step-down converter (such as 2.5V or 1.8V from the main 3.3V output) or higher voltages using coupled windings from the step-down converter (such as 5V, 12V, or 15V). The MAX1865's negative gain block uses an external NPN pass transistor in conjunction with a coupled winding to generate -5V, -12V, or -15V.

All output voltages are externally adjustable, providing maximum flexibility. Additionally, the MAX1864/MAX1865 feature soft-start for the step-down converter and all the positive linear regulators, and have a power-good output that monitors all of the output voltages.

* 4.5V to 28V Input Voltage Range * Master DC-DC Step-Down Converter * Preset 3.3V or Adjustable (1.236V to 0.8 x VIN) Output Voltage * Fixed-Frequency (100kHz/200kHz) PWM Controller * No Current-Sense Resistor * Adjustable Current Limit * 95% Efficient * Two (MAX1864)/Four (MAX1865) Analog Gain Blocks * Positive Analog Blocks Drive Low-Cost PNP Pass Transistors to Build Positive Linear Regulators * Negative Analog Block (MAX1865) Drives a Low-Cost NPN Pass Transistor to Build a Negative Linear Regulator * Power-Good Indicator * Soft-Start Ramp for All Positive Regulators

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