LM2736 datasheet(7/31 Pages) TI

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BOOST

Output

Control

Logic

Current

Limit

Thermal

Shutdown

Under

Voltage

Lockout

Corrective Ramp

Reset

Pulse

PWM

Comparator

Current-Sense Amplifier

RSENSE

Internal

Regulator

and

Enable

Circuit

Oscillator

Driver

0.3:

Switch

Internal

Compensation

GND

Error Amplifier

VREF

1.25V

COUT

OFF

VBOOST

VSW

CBOOST

VOUT

CIN

VIN

ISENSE

1.375V

OVP

Comparator

Error

Signal

LM2736

www.ti.com

SNVS316F – SEPTEMBER 2004 – REVISED APRIL 2013

Block Diagram

Figure 19.

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 19) and to

the waveforms in Figure 20. 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.

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