TPS40050

TPS40051

TPS40053

SLUS540F − DECEMBER 2002 − REVISED JUNE 2004

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APPLICATION INFORMATION

The TPS40050/51/53 family of parts allows the user to optimize the PWM controller to the specific application.

The TPS40051 will be the controller of choice for synchronous buck designs which will include most

applications. It has two quadrant operation and will source or sink output current. This provides the best

transient response.

The TPS40050 operates in one quadrant and sources output current only, allowing for paralleling of converters

and ensures that one converter does not sink current from another converter. This controller also emulates a

standard buck converter at light loads where the inductor current goes discontinuous. At continuous output

inductor currents the controller operates as a synchronous buck converter to optimize efficiency.

The TPS40053 operates in one quadrant as a standard buck converter during start up. After the output has

reached the regulation point, the controller operates in two quadrant mode and is put in a synchronous buck

configuration. This is useful for applications that have the output voltage ’pre-biased’ at some voltage before

the controller is enabled. When the TPS40053 controller is enabled it does not sink current during start up which

would pull current from the pre-biased voltage supply.

SW NODE RESISTOR AND DIODE

The SW node of the converter is negative during the dead time when both the upper and lower MOSFETs are

OFF. The magnitude of this negative voltage is dependent on the lower MOSFET body diode and the output

current which flows during this dead time. This negative voltage could affect the operation of the controller,

especially at low input voltages.

Therefore, a resistor (between 3.3

Ω and 4.7 Ω ) and Schottky diode must be placed between the lower

MOSFET drain and pin 12, SW, of the controller as shown in Figure 15. The Schottky diode must have a voltage

rating to accommodate the input voltage and ringing on the SW node of the converter. A 30-V Schottky such

as a BAT54 or a 40-V Schottky such as a Zetex ZHCS400 or Vishay SD103AWS are adequate. These

SETTING THE SWITCHING FREQUENCY (PROGRAMMING THE CLOCK OSCILLATOR)

The TPS4005x has independent clock oscillator and ramp generator circuits. The clock oscillator serves as the

is charted in Figure 2.

R

1

f

SW

17.82

* 23 kW

(1)