PWM

COMP

OVP

COMP

ERROR

AMP

UVP

COMP

LOGIC

Drive

UVP

Reset

OVP

Set

Reset

Thermal

SD

Oscillator

Duty

Cycle Limit

Load Current

Measurement

Driver

Shutdown

Comparator

Thermal

Shutdown

Bandgap Voltage

Reference

+

-

-

+

BG

-

+

BG

+

BG

-

SHDN

GND

FSLCT

¦

SW

FB

SS

LM3224

www.ti.com

SNVS277C – DECEMBER 2004 – REVISED MARCH 2013

Block Diagram

General Description

The LM3224 utilizes a PWM control scheme to regulate the output voltage over all load conditions. The operation

can best be understood referring to the block diagram and Figure 21 of the Operation section. At the start of

each cycle, the oscillator sets the driver logic and turns on the NMOS power device conducting current through

the summation of the SW voltage and the ramp compensation. The ramp compensation is used in PWM

architectures to eliminate the sub-harmonic oscillations that occur during duty cycles greater than 50%. Once the

driver logic turning off the NMOS power device. The inductor current then flows through the schottky diode to the

load and output capacitor, cycle 2 of Figure 21 (b). The NMOS power device is then set by the oscillator at the

end of the period and current flows through the NMOS power device once again.

The LM3224 has dedicated protection circuitry running during normal operation to protect the IC. The Thermal

Shutdown circuitry turns off the NMOS power device when the die temperature reaches excessive levels. The

UVP comparator protects the NMOS power device during supply power startup and shutdown to prevent

operation at voltages less than the minimum input voltage. The OVP comparator is used to prevent the output

voltage from rising at no loads allowing full PWM operation over all load conditions. The LM3224 also features a

shutdown mode decreasing the supply current to 0.1µA (typ.).

These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam

during storage or handling to prevent electrostatic damage to the MOS gates.

Copyright © 2004–2013, Texas Instruments Incorporated

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