LM3203

2.7V to 5.5V

0.8V to 3.6V

L1

3.3 PH

C1*

10 PF

C5*

0.1 PF

C2

4.7 PF

SW

FB

PGND

SGND

10 pF

267k

BYPOUT

R1

R2

133k

C3

BYP

EN

SYSTEM

CONTROLLER

DAC = 0.267V-1.2V

ON/OFF

LM3203

www.ti.com

SNVS355C – MAY 2005 – REVISED JULY 2005

Figure 3. Typical Operating System Circuit where baseband controls the output voltage using a DAC

Referring to Figure 3, the LM3203 operates as follows. During the first part of each switching cycle, the control

block in the LM3203 turns on the internal PFET (P-channel MOSFET) switch. This allows current to flow from the

input through the inductor to the output filter capacitor and load. The inductor limits the current to a ramp with a

controller turns the PFET switch off, blocking current flow from the input, and then turns the NFET (N-channel

MOSFET) synchronous rectifier on. In response, the inductor’s magnetic field collapses, generating a voltage

that forces current from ground through the synchronous rectifier to the output filter capacitor and load. As the

stored energy is transferred back into the circuit and depleted, the inductor current ramps down with a slope

when inductor current is going low, smoothing the voltage across the load.

The output voltage is regulated by modulating the PFET switch on time to control the average current sent to the

load. The effect is identical to sending a duty-cycle modulated rectangular wave formed by the switch and

synchronous rectifier at SW to a low-pass filter formed by the inductor and output filter capacitor. The output

voltage is equal to the average voltage at the SW pin.

While in PWM (Pulse Width Modulation) mode, the output voltage is regulated by switching at a constant

frequency and then modulating the energy per cycle to control power to the load. Energy per cycle is set by

modulating the PFET switch on-time pulse width to control the peak inductor current. This is done by comparing

the PFET drain current to a slope-compensated reference current generated by the error amplifier. At the

beginning of each cycle, the clock turns on the PFET switch, causing the inductor current to ramp up. When the

Copyright © 2005, Texas Instruments Incorporated

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