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DETAILED DESCRIPTION

PWM OPERATION

LIGHT-LOAD CONDITION

I

OUT(LL) [

1

2

L

ƒ

V

In )

V

OUT

V

OUT

V

IN

(1)

TPS51124

SLVS616A – NOVEMBER 2005 – REVISED NOVEMBER 2005

The main control loop of the switching mode power supply (SMPS) is designed as an adaptive on-time pulse

width modulation (PWM) controller. It supports a proprietary D-CAP Mode. D-CAP Mode uses an internal

compensation circuit and is suitable for low external component-count configuration, with appropriate amount of

ESR at the output capacitor(s). The output voltage is monitored at a feedback point voltage. The reference

voltage at the feedback point is a combination of a fixed 0.750-V precision reference and a synchronized,

precision 15-mV ramp signal. Lower output voltages in notebook systems (e.g., 1.05 V, 1.5 V) require extremely

low output ripple. By providing a ramp signal, the TPS51124 is easier to use in low-output ripple systems. The

combination of the precision ramp and reference yield an effective target reference of 0.758 V. The accuracy of

this effective reference remains 1.3% over line and temperature.

At the beginning of each cycle, the synchronous high-side MOSFET is turned on, or becomes ON state. This

MOSFET is turned off, or becomes OFF state, after the internal one-shot timer expires. This one shot is

determined by the converter’s input voltage, VIN, and the output voltage, VOUT, to keep the frequency fairly

constant over the input voltage range; hence, it is called adaptive on-time control (see PWM Frequency and

Adaptive On-time Control). The high-side MOSFET is turned on again when feedback information indicates

insufficient output voltage, and inductor current information indicates a below-the-over-current limit condition.

Repeating operation in this manner, the controller regulates the output voltage. The synchronous low-side

MOSFET is turned on each OFF state to keep the conduction loss at a minimum. The low-side MOSFET is

turned off when the inductor current information detects zero level. This enables seamless transition to the

reduced frequency operation at light-load conditions so that high efficiency is kept over a broad range of load

current.

TPS51124 automatically reduces switching frequency at light-load conditions to maintain high efficiency. This

reduction of frequency is achieved smoothly and without increase of Vout ripple or load regulation. Detail

operation is described as follows. As the output current decreases from heavy-load condition, the inductor

current is also reduced, and eventually comes to the point that its valley touches zero current, which is the

boundary between continuous conduction and discontinuous conduction modes. The low-side MOSFET is turned

off when this zero inductor current is detected. As the load current is further decreased, the converter runs in

discontinuous conduction mode and it takes longer and longer to discharge the output capacitor to the level that

requires the next ON cycle. The ON time is kept the same as that in the heavy-load condition. In reverse, when

the output current increases from light load to heavy load, the switching frequency increases to the preset value

as the inductor current reaches the continuous conduction. The transition load point to the light-load operation,

where f is the PWM switching frequency.

Switching frequency versus output current in the light-load condition is a function of L, f, Vin, and Vout, but it

It should be noted that in the PWM control path is a small ramp . This ramp is transparent in normal, continuous

conduction mode and does not measurably affect the regulation voltage. However, in discontinuous, light-load

mode, an upward shift in regulation voltage of about 0.75% will be observed. The variation of this shift minimally

affects the reference tolerance. Therefore, the reference value in skip mode is 0.764 V

±1.3% over line and

temperature.

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