RAMP

PWM

Error Amp O/P

VIN = Low

VIN = High

RAMP

PWM

Error Amp O/P

LM2647

www.ti.com

SNVS210F – JUNE 2003 – REVISED APRIL 2013

OPERATION DESCRIPTIONS

GENERAL

The LM2647 provides two identical synchronously switched buck regulator channels that operate 180° out of

phase. A voltage-mode control topology was selected to provide fixed-frequency PWM regulation at very low

duty cycles, in preference to current-mode control, because the latter has inherent limitations in being able to

achieve low pulse widths due to blanking time requirements. Because of a minimum pulse width of about 30ns

for the LM2647, very low duty cycles (low output, high input) are possible. The main advantage of current-mode

control is the fact that the slope of its ramp (derived from the switch current), automatically increases with

increase in input voltage. This leads to improved line rejection and fast response to line variations. In typical

voltage-mode control, the ramp is derived from the clock, not from the switch current. But by using the input

voltage together with the clock signal to generate the ramp as in the LM2647, this advantage of current-mode

control can in fact be completely replicated. The technique is called line feedforward. In addition, the LM2647

features a user-selectable Pulse-skip mode that significantly improves efficiency at light loads by reducing

switching losses, and driver consumption, both of which are proportional to switching frequency.

INPUT VOLTAGE FEEDFORWARD

The feedforward circuit of the LM2647 adjusts the slope of the internal PWM ramp in proportion to the regulator

input voltage. See Figure 9 for an illustration of how the duty cycle changes as a result of the change in the slope

of the ramp, even though the error amplifier output has not had time to react to the line disturbance. The almost

instantaneous duty cycle correction provided by the feedforward circuit significantly improves line transient

rejection.

Figure 9. Voltage Feedforward

FORCED-PWM MODE AND PULSE-SKIP MODE

Forced-PWM mode (FPWM) leads to Continuous Conduction Mode (CCM) even at very light loads. It is one of

two user-selectable modes of operation provided by the LM2647. When FPWM is chosen (FPWM pin high), the

bottom FET will always be turned ON whenever the top FET is OFF. See Figure 10 for a typical FPWM plot.

Copyright © 2003–2013, Texas Instruments Incorporated

Submit Documentation Feedback

11

Product Folder Links: LM2647