LM2771SDEV

User's Guide SNVA195A – November 2006 – Revised April 2013 AN-1547 LM2771 Evaluation Board 1 Board Operation 1.1 Basic Connections To operate the LM2771 evaluation board, connect a supply voltage (2.7V to 5.5V) between board connectors Vin and GND. Connecting the EN header's center pin to ON position (VIN) enables the device, and connecting it to OFF position (GND) disables the device. 1.2 Circuit Description The core of the LM2771 is a two-phase charge pump controlled by an internally generated nonoverlapping clock. The charge pump operates by using an external flying capacitor, C1, to transfer charge from the input to the output. At input voltages below 3.5V (typ.) the LM2771 operates in a "pass mode", with the input current being equal to the load current. At input voltages above 3.5V (typ.) the part utilizes a gain of ½, resulting in an input current equal to half the load current. The two phases of the switched capacitor switching cycle will be referred to as the "charge phase" and the "discharge phase". During the charge phase, the flying capacitor is charged by the input supply. After half of the switching cycle [ t = 1/(2 × FSW) ], the LM2771 switches to the discharge phase. In this configuration, the charge that was stored on the flying capacitor in the charge phase is transferred to the output. The LM2771 uses fixed frequency pre-regulation to regulate the output voltage to 1.5V during moderate to high load currents. The input and output connections of the flying capacitor is made with internal MOS switches. Pre-regulation limits the gate drive of the MOS switch connected between the voltage input and the flying capacitor. Controlling the on resistance of this switch limits the amount of charge transferred into and out of the flying capacitor during the charge and discharge phases, and in turn helps to keep the output ripple very low. When output currents are low (