7.1 Overview

The LM2734 device is a constant frequency PWM buck regulator IC that delivers a 1-A load current. The regulator has a preset switching frequency of either 550 kHz (LM2734Y) or 1.6 MHz (LM2734X). These high frequencies allow the LM2734 device to operate with small surface-mount capacitors and inductors, resulting in DC/DC converters that require a minimum amount of board space. The LM2734 device is internally compensated, so it is simple to use, and requires few external components. The LM2734 device uses current-mode control to regulate the output voltage.

The following operating description of theLM2734 device will refer to the Simplified Block Diagram () and to the waveforms in Figure 12. The LM2734 device supplies a regulated output voltage by switching the internal NMOS control switch at constant frequency and variable duty cycle. A switching cycle begins at the falling edge of the reset pulse generated by the internal oscillator. When this pulse goes low, the output control logic turns on the internal NMOS control switch. During this on-time, the SW pin voltage (V_SW) swings up to approximately V_IN, and the inductor current (I_L) increases with a linear slope. I_L is measured by the current-sense amplifier, which generates an output proportional to the switch current. The sense signal is summed with the regulator’s corrective ramp and compared to the error amplifier’s output, which is proportional to the difference between the feedback voltage and V_REF. When the PWM comparator output goes high, the output switch turns off until the next switching cycle begins. During the switch off-time, inductor current discharges through Schottky diode D1, which forces the SW pin to swing below ground by the forward voltage (V_D) of the catch diode. The regulator loop adjusts the duty cycle (D) to maintain a constant output voltage.

Figure 12. LM2734 Waveforms of SW Pin Voltage and Inductor Current