Application Information

THEORY OF OPERATION

The LM2734Z is a constant frequency PWM buck regulator

IC that delivers a 1A load current. The regulator has a preset

switching frequency of 3MHz. This high frequency allows the

LM2734Z to operate with small surface mount capacitors

and inductors, resulting in a DC/DC converter that requires a

minimum amount of board space. The LM2734Z is internally

compensated, so it is simple to use, and requires few exter-

nal components. The LM2734Z uses current-mode control to

regulate the output voltage.

The following operating description of the LM2734Z will refer

to the Simplified Block Diagram (Figure 1) and to the wave-

forms in Figure 2. The LM2734Z 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

up to approximately V

creases with a linear slope. I

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 am-

plifier’s output, which is proportional to the difference be-

tween the feedback voltage and V

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

adjusts the duty cycle (D) to maintain a constant output

voltage.

BOOST FUNCTION

Capacitor C

generate a voltage V

voltage to the internal NMOS control switch. To properly

drive the internal NMOS switch during its on-time, V

BOOST

needs to be at least 1.6V greater than V

LM2734Z will operate with this minimum voltage, it may not

have sufficient gate drive to supply large values of output

Block Diagram

20130306

FIGURE 1.

20130307

FIGURE 2. LM2734Z Waveforms of SW Pin Voltage and

Inductor Current

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