LM2736 datasheet(7/22 Pages) NSC

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Application Information

THEORY OF OPERATION

The LM2736 is a constant frequency PWM buck regulator IC

that delivers a 750mA load current. The regulator has a

preset switching frequency of either 550kHz (LM2736Y) or

1.6MHz (LM2736X). These high frequencies allow the

LM2736 to operate with small surface mount capacitors and

inductors, resulting in DC/DC converters that require a mini-

mum amount of board space. The LM2736 is internally

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

nal components. The LM2736 uses current-mode control to

regulate the output voltage.

The following operating description of the LM2736 will refer

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

forms in Figure 2. The LM2736 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 (IL) in-

creases 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 am-

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

tween 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.

BOOST FUNCTION

Capacitor C

BOOST and diode D2 in Figure 3 are used to

generate a voltage V

BOOST.VBOOST -VSW is the gate drive

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

SW. Although the

LM2736 will operate with this minimum voltage, it may not

have sufficient gate drive to supply large values of output

Block Diagram

20124206

FIGURE 1.

20124207

FIGURE 2. LM2736 Waveforms of SW Pin Voltage and

Inductor Current

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