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DETAILED DESCRIPTION

OPERATION

POWER SAVE MODE OPERATION

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transition +

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18.66 W

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TPS62020, TPS62021

SLVS076B – JUNE 2003 – REVISED APRIL 2004

The TPS62020 is a synchronous step-down converter operating with typically 1.25-MHz fixed frequency. At

moderate to heavy load currents the device operates in pulse width modulation (PWM), and at light load currents

the device enters power save mode operation using pulse frequency modulation (PFM). When operating in PWM

mode, the typical switching frequency is 1.25 MHz with a minimum switching frequency of 1 MHz. This makes

the device suitable for xDSL applications minimizing RF (radio frequency) interference.

During PWM operation the converter uses a unique fast response voltage mode controller scheme with input

voltage feed-forward to achieve good line and load regulation allowing the use of small ceramic input and output

capacitors. At the beginning of each clock cycle initiated by the clock signal (S) the P-channel MOSFET switch

turns on and the inductor current ramps up until the comparator trips and the control logic turns off the switch.

The current limit comparator also turns off the switch in case the current limit of the P-channel switch is

exceeded. After the dead time preventing current shoot through, the N-channel MOSFET rectifier is turned on

and the inductor current ramps down. The next cycle is initiated by the clock signal, again turning off the

N-channel rectifier and turning on the P-channel switch.

The Gm amplifier as well as the input voltage determines the rise time of the saw tooth generator, and therefore,

any change in input voltage or output voltage directly controls the duty cycle of the converter, giving a very good

line and load transient regulation.

As the load current decreases, the converter enters power save mode operation. During power save mode the

converter operates with reduced switching frequency in PFM mode and with a minimum quiescent current

maintaining high efficiency.

The converter monitors the average inductor current and the device enters power save mode when the average

inductor current is below the threshold. The transition point between PWM and power save mode is given by the

transition current with the following equation:

During power save mode the output voltage is monitored with the comparator by the threshold's comp low and

comp high. As the output voltage falls below the comp low threshold set to typically 0.8% above the nominal

output voltage, the P-channel switch turns on. The P-channel switch remains on until the transition current

Equation 1 is reached. Then the N-channel switch turns on completing the first cycle. The converter continues to

switch with its normal duty cycle determined by the input and output voltage but with half the nominal switching

frequency of 625-kHz typ. Thus the output voltage rises and, as soon as the output voltage reaches the comp

high threshold of 1.6%, the converter stops switching. Depending on the load current, the converter switches for

a longer or shorter period of time in order to deliver the energy to the output. If the load current increases and the

output voltage can not be maintained with the transition current Equation 1, the converter enters PWM again.

See Figure 11 and Figure 12 under the typical graphs section and Figure 14 for power save mode operation.

Among other techniques this advanced power save mode method allows high efficiency over the entire load

current range and a small output ripple of typically 1% of the nominal output voltage.

Setting the power save mode thresholds to typically 0.8% and 1.6% above the nominal output voltage at light

load current results in a dynamic voltage positioning achieving lower absolute voltage drops during heavy load

transient changes. This allows the converter to operate with small output capacitors like 10 µF or 22 µF and still

having a low absolute voltage drop during heavy load transient. Refer to Figure 14 as well for detailed operation

of the power save mode.

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