1. Overview

️· Device: AS1337 (two variants: AS1337A and AS1337B)
️· Type: Synchronous Buck-Boost DC-DC Converter. It can step-up (boost) or step-down (buck) the input voltage.
️· Function: Provides a regulated output voltage from an unstable input voltage. Ideal for battery-powered applications.
️· Switching Frequency: Fixed at 1.2 MHz (important for component selection – smaller components can be used).

2. Operation

️· Buck-Boost Mode: The core operating mode. Allows output voltage to be higher or lower than the input voltage.
️· Powersave Mode: For efficiency at light loads. The converter intermittently turns on, recharging a capacitor to maintain the output voltage. Frequency of the PWM changes with the load.
️· Shutdown Mode: The converter is turned off, drawing very little current (less than 1µA). This is where the AS1337A and AS1337B differ significantly (see "Features").
️· Buck Mode: Specific operation allowing V_IN - V_OUT operation.

3. Features & Key Technical Details

️· Input Voltage Range: Not explicitly stated, but designed for low-voltage operation (implied from the application examples and the description of "low-voltage start-up").
️· Output Voltage: Programmable from 2.5V to 5V using a voltage divider (VOUT = 1.23V * (1 + R1/R2)).
️· Low-Voltage Start-Up: Requires a minimum input voltage of only 0.85V (typical) to start up.
️· Soft Start: Limits inductor current during startup to reduce overshoot.
️· Current Mode PWM Control: Provides good line and load regulation.
️· Peak Current Limiting: Approximately 850mA, independent of input and output voltages.
️· Zero Current Comparator: Prevents inductor current from reversing polarity at light loads, improving efficiency.
️· Anti-Ringing Control: Damps oscillations on the LX pin.
️· POK (Power-On Indicator) Function: Indicates when the output voltage is within 88% of the nominal voltage. Pulled low when voltage drops below that level.
️· Shutdown Modes (Critical Difference between A & B):
- AS1337A: During shutdown, the input battery is connected to the output through the inductor and a P-channel MOSFET, providing backup power. (Battery Feedthrough)
- AS1337B: During shutdown, the output is disconnected from the input and is discharged through an NMOS transistor to GND. (Battery Disconnect)
️· CFF Capacitor: An optional capacitor between VOUT and FB, may reduce VOUT ripple and quiescent current in Powersave mode. Typical values range from 15pF to 220pF.

4. Applications

️· LED matrix displays
️· Bar-graph displays
️· Instrument-panel meters
️· Dot matrix displays
️· Set-top boxes
️· White goods
️· Professional audio equipment
️· Medical equipment
️· Industrial controllers
️· Single AA or Lithium cell powered devices.

5. Key Takeaways

️· Flexibility: The AS1337 excels in applications where the input voltage can vary above and below the desired output voltage.
️· Efficiency: The synchronous design, combined with the powersave mode, provides excellent energy efficiency.
️· Low-Voltage Operation: It's designed to work well in low-voltage environments.
️· AS1337A vs. AS1337B: The *critical* difference is the shutdown behavior. Choose A for backup power during shutdown, and B for complete disconnection.
️· Component Selection: The fixed 1.2 MHz switching frequency simplifies component selection.
️· Feedback Network: Correct selection of the feedback resistor values is critical to the desired output voltage.