MAX756ESA+

The MAX756/MAX757 are CMOS step-up DC-DC switching regulators for small, low input voltage or battery-powered systems. The MAX756 accepts a positive input voltage down to 0.7V and converts it to a higher pin-selectable output voltage of 3.3V or 5V. The MAX757 is an adjustable version that accepts an input voltage down to 0.7V and generates a higher adjustable output voltage in the range from 2.7V to 5.5V. Typical full-load efficiencies for the MAX756/MAX757 are greater than 87%. The MAX756/MAX757 combine a switch-mode regulator with an N-channel MOSFET, precision voltage reference, and power-fail detector in a single monolithic device. The MOSFET is a “sense-FET” type for best efficiency, and has a very low gate threshold voltage to ensure start-up under low-battery voltage conditions (1.1V typ). A unique minimum off time, current-limited, pulse-frequency modulation (PFM) control scheme is a key feature of the MAX756/MAX757. This PFM scheme combines the advantages of pulse-width modulation (PWM) (high output power and efficiency) with those of a traditional PFM pulse-skipper (ultra-low quiescent currents). There is no oscillator; at heavy loads, switching is accomplished through a constant peak-current limit in the switch, which allows the inductor current to self-oscillate between this peak limit and some lesser value. At light loads, switching frequency is governed by a pair of one-shots, which set a minimum off-time (1µs) and a maximum on-time (4µs). The switching frequency depends on the load and the input voltage, and can range as high as 500kHz. The peak switch current of the internal MOSFET power switch is fixed at 1A ±0.2A. The switch's on resistance is typically 0.5Ω, resulting in a switch voltage drop (VSW) of about 500mV under high output loads. The value of VSW decreases with light current loads. Conventional PWM converters generate constant-frequency switching noise, whereas this architecture produces variable-frequency switching noise. However, the noise does not exceed the switch current limit times the filter-capacitor equivalent series resistance (ESR).

Applications

• 3.3V to 5V Step-Up Conversion
• Palmtop Computers
• Portable Data-Collection Equipment
• Personal Data Communicators/Computers
• Medical Instrumentation
• 2-Cell & 3-Cell Battery-Operated Equipment
• Glucose Meters

Key features

• Operates Down to 0.7V Input Supply Voltage
• 87% Efficiency at 200mA
• 60µA Quiescent Current
• 20µA Shutdown Mode with Active Reference and LBI Detector
• 500kHz Maximum Switching Frequency
• +1.5% Reference Tolerance Over Temperature
• Low-Battery Detector (LBI/LBO)
• 8-Pin DIP and SO Packages