lm27241 National Semiconductor Corporation, lm27241 Datasheet - Page 10

• Manufacturer Part Number: lm27241
• Description: Synchronous Buck Regulator Controller For Mobile Systems
• Manufacturer: National Semiconductor Corporation

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Operation Descriptions
GENERAL
The LM27241 is a synchronous buck controller using a
voltage-mode control topology. This 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 LM27241, 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), automati-
cally 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 LM27241, this advantage of current-mode control
can in fact be completely replicated. The technique is called
line feedforward. In addition, the LM27241 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 LM27241 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.
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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
LM27241. 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.
CH1: HDRV, CH2: LDRV, CH3: SW, CH4: I
Output 1V
FIGURE 2. Normal FPWM Mode Operation at Light
@
0.04A, VIN = 10V, FPWM, L = 10µH, f = 300kHz
FIGURE 1. Voltage Feedforward
Loads
L
(0.2A/div)
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