2 Powering Up The Board

Since the EN pins are directly tied to the input voltage, starting up the board is a single-step procedure. Simply connect a voltage rail between 5V and 20V to the VIN and GND terminals and there should be 1.2V and 2.5V output at the corresponding terminals. Certain bench-top power supplies upon powering up may shoot up to their maximum output voltages momentarily before settling to the programmed value. If their maximum voltage is above 22V, it can damage the LM26400Y demonstration board. In this case, either connect the board after the input power supply is powered up, or use the current limit knob of the power supply to bring up the input voltage.

The linear soft-start ramps for the two output voltages should last about 1ms and 2ms. Load can be applied prior to power-up. If no load is applied, the two channels will operate in pulse skipping mode or discontinuous conduction mode. If an output is shorted either before or after start-up, removal of the short-circuit condition should bring the corresponding output back to normal voltage.

If additional output capacitors are desired, C8 and C9 on the back of the board are reserved for that purpose.

If it is desired to control the start-up and shutdown timing, connect the logic signals to the EN1 and/or EN2 pads on the back of the board. Make sure the voltages on the EN pads are never higher than VIN. If only a soft-start slope needs to be adjusted, simply change the corresponding SS capacitor (C5 or C6).

To operate between 3.3V and 5V of input voltage, populate D3 and D4 (on the back of the board) each with a SOT-23 Schottky diode such as the BAT54 and apply a 5V supply to the "5V_Bias" pad on the back. Do not exceed 6V on the 5V bias. The LM26400Y device itself can work with an input voltage as low as 3V. The demonstration board when using an external bootstrap bias can operate down to 3.3V under room temperature. This extra 0.3V requirement is due to the large duty cycle in the 2.5V channel being too close to the maximum allowed.