TPS62000, TPS62001, TPS62002, TPS62003

TPS62004, TPS62005, TPS62006, TPS62007, TPS62008

SLVS294F – SEPTEMBER 2000 – REVISED AUGUST 2015

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8 Detailed Description

8.1 Overview

The TPS6200x is a step down converter operating in a current mode PFM/PWM scheme with a typical switching

frequency of 750 kHz.

At moderate to heavy loads, the converter operates in the pulse width modulation (PWM) and at light loads the

converter enters a power save mode (pulse frequency modulation, PFM) to keep the efficiency high.

In the PWM mode operation, the part operates at a fixed frequency of 750 kHz. At the beginning of each clock

cycle, the high side P-channel MOSFET is turned on. The current in the inductor ramps up and is sensed via an

internal circuit. The high side switch is turned off when the sensed current causes the PFM/PWM comparator to

trip when the output voltage is in regulation or when the inductor current reaches the current limit (set by ILIM).

After a minimum dead time preventing shoot through current, the low side N-channel MOSFET is turned on and

the current ramps down again. As the clock cycle is completed, the low side switch is turned off and the next

clock cycle starts.

In discontinuous conduction mode (DCM), the inductor current ramps to zero before the end of each clock cycle.

In order to increase the efficiency the load comparator turns off the low side MOSFET before the inductor current

becomes negative. This prevents reverse current flowing from the output capacitor through the inductor and low

side MOSFET to ground that would cause additional losses.

As the load current decreases and the peak inductor current does not reach the power save mode threshold of

typically 120 mA for more than 15 clock cycles, the converter enters a pulse frequency modulation (PFM) mode.

In the PFM mode, the converter operates with:

•

Variable frequency

•

Constant peak current that reduces switching losses

•

Quiescent current at a minimum

Thus maintaining the highest efficiency at light load currents. In this mode, the output voltage is monitored with

the error amplifier. As soon as the output voltage falls below the nominal value, the high side switch is turned on

and the inductor current ramps up. When the inductor current reaches the peak current of typical: 150 mA +

ramps down, the low side switch is turned off before the inductor current becomes negative which completes the

cycle. When the output voltage falls below the nominal voltage again, the next cycle is started.

The converter enters the PWM mode again as soon as the output voltage can not be maintained with the typical

peak inductor current in the PFM mode.

The control loop is internally compensated reducing the amount of external components.

The switch current is internally sensed and the maximum current limit can be set to typical 600 mA by connecting

8

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