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PFM Mode at Light Load

Comparator High

Comparator Low

Comparator Low 2

PWM Mode at Medium to Full Load

1.6%

0.8%

DYNAMIC VOLTAGE POSITIONING

DIGITAL SELF-CALIBRATION

SOFT START

LOW DROPOUT OPERATION 100% DUTY CYCLE

TPS62220, TPS62221, TPS62222

TPS62223, TPS62224

TPS62228, TPS62229

SLVS491C – SEPTEMBER 2003 – REVISED SEPTEMBER 2004

DETAILED DESCRIPTION (continued)

Figure 14. Power-Save Mode Thresholds and Dynamic Voltage Positioning

The converter enters the fixed frequency PWM mode again as soon as the output voltage falls below the comp

low 2 threshold.

As described in the power-save mode operation sections and as detailed in Figure 14, the output voltage is

typically 0.8% above the nominal output voltage at light load currents, as the device is in power-save mode. This

gives additional headroom for the voltage drop during a load transient from light load to full load. During a load

transient from full load to light load, the voltage overshoot is also minimized due to active regulation by turning on

the N-channel rectifier switch.

In addition to the control circuit as shown in the block diagram, the TPS6222x series uses an internal digital

self-calibration of the output voltage to minimize DC load and line regulation. This method of self-calibration

allows simple internal loop compensation without the use of external components. The device monitors the

output voltage and as soon as the output voltage drops below typically 1.6% or exceeds typically 1.6% of Vout

the duty cycle will be adjusted in digital steps. As a result, the output voltage changes in digital steps either up or

down where one step is typically 1% of Vout. This results in virtually zero line and load regulation and keeps the

output voltage tolerance within

±3% overload and line variations.

The TPS6222x has an internal soft-start circuit that limits the inrush current during start-up. This prevents

possible voltage drops of the input voltage in case a battery or a high impedance power source is connected to

the input of the TPS6222x. The soft start is implemented as a digital circuit increasing the switch current in steps

of typically 83 mA,167 mA, 335 mA and then the typical switch current limit of 670 mA. Therefore, the start-up

time mainly depends on the output capacitor and load current.

The TPS6222x offers a low input to output voltage difference, while still maintaining operation with the 100% duty

cycle mode. In this mode, the P-channel switch is constantly turned on. This is particularly useful in

battery-powered applications to achieve longest operation time by taking full advantage of the whole battery

voltage range. The minimum input voltage to maintain regulation, depending on the load current and output

voltage, can be calculated as

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