Datasheet

VOS

Vramp

P1b_PWM

P2b_PWM

Vcenter

PWM

Generator

VIN

Load

SW1 SW2

LM3668

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SNVS449M –JUNE 2007–REVISED MAY 2013

Boost Operation

When the input voltage is smaller than the output voltage, the device enters boost mode operation where P1 is

always ON, while switches N2 & P2 control the output. Figure 48 shows the simplified circuit for boost mode

operation.

Figure 48. Simplified Circuit for Boost Operation

PWM Operation

In PWM operation, the output voltage is regulated by switching at a constant frequency and then modulating the

energy per cycle to control power to the load. In Normal operation, the internal error amplifier provides an error

signal, Vc, from the feedback voltage and Vref. The error amplifier signal, Vc, is compared with a voltage,

Vcenter, and used to generate the PWM signals for both Buck & Boost modes. Signal Vcenter is a DC signal

which sets the transition point of the buck and boost modes. Below are three regions of operation:

• Region I: If Vc is less than Vcenter, Buck mode.

• Region II: If Vc and Vcenter are equal, both PMOS switches (P1, P2) are on and both NMOS switches (N1,

N2) are off. The power passes directly from input to output via P1 & P2

• Region III: If Vc is greater than Vcenter, Boost mode.

The Buck-Boost operation is avoided, to improve the efficiency across V

and load range.

Figure 49. PWM Generator Block Diagram

Internal Synchronous Rectification

While in PWM mode, the LM3668 uses an internal MOSFET as a synchronous rectifier to reduce rectifier forward

voltage drop and associated power loss. Synchronous rectification provides a significant improvement in

efficiency whenever the output voltage is relatively low compare to the voltage drop across an ordinary rectifier

diode.

Product Folder Links: LM3668