ꢀꢁ ꢂ ꢃꢄ ꢄꢅ ꢆ
SLUS563B − AUGUST 2003 − REVISED APRIL 2004
APPLICATION INFORMATION
The TPS40052 allows the user to optimize the PWM controller to the specific application.
The TPS40052 is the controller of choice for synchronous buck designs, the output of which is required to track
another voltage. It has two quadrant operation and can source or sink output current, providing the best transient
response.
SW NODE RESISTOR AND DIODE
The SW node of the converter will be 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 ( 3.3 Ω to 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 10. 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 components are
shown in Figure 10 as Rsw and D2.
SETTING THE SWITCHING FREQUENCY (PROGRAMMING THE CLOCK OSCILLATOR)
The TPS40052 has independent clock oscillator and ramp generator circuits. The clock oscillator serves as the
master clock to the ramp generator circuit. The switching frequency, f
in kHz, of the clock oscillator is set by
SW
a single resistor (R ) to ground. The clock frequency is related to R , in kΩ by equation (1).
T
T
1
R +
ǒ
* 23
Ǔ
kW
T
*6
f
17.82 10
SW
(1)
UVLO OPERATION
The TPS40052 uses fixed UVLO protection. The fixed UVLO monitors the input voltage. The UVLO circuit holds
the soft-start low until the input voltage has exceeded the undervoltage threshold.
7
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