Simplified Block Diagram
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- 4.5V to 28V Input Voltage Range
- Master DC-DC Step-Down Converter:
- Preset 3.3V or Adjustable (1.236V to 0.75 x VIN) Output Voltage
- Fixed Frequency (200kHz) PWM Controller
- No Current-Sense Resistor
- Adjustable Current Limit
- 95% Efficient
- Two (MAX1964)/Four (MAX1965) Analog Gain Blocks:
- Positive Analog Blocks Drive Low-Cost PNP Pass Transistors to Build Positive Linear Regulators
- Negative Analog Block (MAX1965) Drives a Low-Cost NPN Pass Transistor to Build a Negative Linear Regulator
- Power-Good Indicator
- Voltage Sequencing (MAX1964) or Tracking (MAX1965)
- Set-Top Boxes
- Wireless Local Loop (WLL)
- xDSL, Cable, and ISDN Modems
The MAX1964 includes a current-mode synchronous step-down controller and two positive regulator gain blocks. The MAX1965 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 0.75 x VIN with an external resistive-divider. The 200kHz operating frequency allows the use of low-cost aluminum-electrolytic capacitors and low-cost power magnetics. Additionally, the MAX1964/MAX1965 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 MAX1964/MAX1965 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 MAX1965'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. During startup, the MAX1964 features voltage sequencing and the MAX1965 features voltage tracking. Both controllers provide a power-good output that monitors all of the output voltages.