TPS62020
TPS62021
TPS62026
SLVS076C – JUNE 2003 – REVISED DECEMBER 2004
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DETAILED DESCRIPTION
OPERATION
The TPS6202x is a synchronous step-down converter that typically operates at a 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-requency 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.
POWER SAVE MODE OPERATION
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:
V
I
I
+
transition
18.66
W
(1)
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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