- 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
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.