+

-

+

-

+

-

VOS

Vramp

P1b_PWM

P2b_PWM

Vcenter

Vc

PWM

Generator

LM3668

SNVS449O – JUNE 2007 – REVISED APRIL 2015

www.ti.com

8.4 Device Functional Modes

8.4.1 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.

Figure 43. PWM Generator Block Diagram

8.4.2 PFM Operation

At very light loads, the converter enters PFM mode and operates with reduced switching frequency and supply

current to maintain high efficiency. The part automatically transitions into PFM mode when either of two following

conditions occur for a duration of 128 or more clock cycles:

A. The inductor current reaches zero.

In PFM operation, the compensation circuit in the error amplifier is turned off. The error amplifier works as a

hysteretic comparator. The PFM comparator senses the output voltage via the feedback pin and controls the

switching of the output FETs such that the output voltage ramps between ~0.8% and ~1.6% of the nominal PWM

output voltage (Figure 44). If the output voltage is below the ‘high’ PFM comparator threshold, the P1 & P2 (Buck

mode) or N2 & P1 (Boost mode) power switches are turned on. It remains on until the output voltage reaches the

Once the P1 (Buck mode) or N2 (Boost mode) power switch is turned off, the N1 & P2 (Buck mode) or P1 & P2

(Boost mode) power switches are turned on until the inductor current ramps to zero. When the zero inductor

current condition is detected, the N1(Buck mode) or P2 (Boost mode) power switches are turned off. If the output

voltage is below the ‘high’ PFM comparator threshold, the P1 & P2 (Buck mode) or N2 & P1 (Boost mode)

switches are again turned on and the cycle is repeated until the output reaches the desired level. Once the

output reaches the ‘high’ PFM threshold, the N1 & P2 (Buck mode) or P1 & P2 (Boost mode) switches are turned

on briefly to ramp the inductor current to zero, then both output switches are turned off and the part enters an

extremely low power mode. Quiescent supply current during this ‘sleep’ mode is 45 µA (typ), which allows the

part to achieve high efficiency under extremely light load conditions.

16

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