LM2736 datasheet(8/27 Pages) TI1

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VIN

TON

Inductor

Current

D = TON/TSW

VSW

TOFF

TSW

IPK

Voltage

LM2736

SNVS316E – SEPTEMBER 2004 – REVISED SEPTEMBER 2011

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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 minimum amount of board space. The LM2736 is internally compensated, so it is simple to use,

and requires few external 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 3) and to the

waveforms in Figure 4. 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 (VSW) swings up to approximately VIN, and the

inductor current (IL) increases with a linear slope. IL 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 VREF. 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 (VD) of the catch diode. The regulator loop adjusts the duty

cycle (D) to maintain a constant output voltage.

Figure 4. LM2736 Waveforms of SW Pin Voltage and Inductor Current

BOOST FUNCTION

Capacitor CBOOST and diode D2 in Figure 5 are used to generate a voltage VBOOST. VBOOST - VSW is the gate drive

voltage to the internal NMOS control switch. To properly drive the internal NMOS switch during its on-time,

VBOOST needs to be at least 1.6V greater than VSW. Although the LM2736 will operate with this minimum voltage,

it may not have sufficient gate drive to supply large values of output current. Therefore, it is recommended that

VBOOST be greater than 2.5V above VSW for best efficiency. VBOOST – VSW should not exceed the maximum

operating limit of 5.5V.

5.5V > VBOOST – VSW > 2.5V for best performance.

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