General Description
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 ✕ 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.

Features
♦ 4.5V to 28V Input Voltage Range
♦ Master DC-DC Step-Down Converter
   Preset 3.3V or Adjustable (1.236V to 0.8 ✕ 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

Applications
   xDSL, Cable, and ISDN Modems
   Set-Top Boxes
   Wireless Local Loop