LM2657 datasheet(10/26 Pages) NSC

10 / 26 page

Operation Descriptions

GENERAL

The LM2657 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 pref-

erence to current-mode control, because the latter has in-

herent 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 LM2657, very low duty

cycles (low output, high input) are possible. The main ad-

vantage of current-mode control is the fact that the slope of

its ramp (derived from the switch current), automatically

increases with an 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 LM2657, this advantage of current-mode control

can in fact be completely replicated. The technique is called

line feedforward. In addition, the LM2657 features a user-

selectable Pulse-skip mode that significantly improves effi-

ciency 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 LM2657 adjusts the slope of

the internal PWM ramp in proportion to the regulator input

voltage. See Figure 1 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 signifi-

cantly improves line transient rejection.

FORCED-PWM MODE AND PULSE-SKIP MODE

Forced-PWM mode (FPWM) leads to Continuous Conduc-

tion Mode (CCM) even at very light loads. It is one of two

user-selectable modes of operation provided by the

LM2657. When FPWM is chosen (FPWM pin high), the

bottom FET will always be turned ON whenever the top FET

is OFF. See Figure 2 for a typical FPWM plot.

20134709

FIGURE 1. Voltage Feedforward

20134710

CH1: HDRV, CH2: LDRV, CH3: SW, CH4: IL (0.2A/div)

Output 1V @ 0.04A, VIN = 10V, FPWM, L = 10µH, f = 300kHz

FIGURE 2. Normal FPWM Mode Operation at Light

Loads

LM2657 Datasheet(PDF) 10 Page - National Semiconductor (TI)

LM2657 Datasheet(HTML) 10 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Datasheet Download

Link URL