TPS40060 Datasheet, PDF (7/28 Pages) Texas Instruments – WIDE-INPUT SYNCHRONOUS BUCK CONTROLLER

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TPS40060
TPS40061
SLUS543D – DECEMBER 2002 – REVISED SEPTEMBER 2004
APPLICATION INFORMATION
The TPS40060/61 family of parts allows the user to optimize the PWM controller to the specific application.
The TPS40061 is 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 TPS40060 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.
SW NODE RESISTOR
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 10-Ω resistor must be placed between the lower MOSFET drain and pin 12 (SW) of the controller as
shown in Figure 13 as R_SW.
SETTING THE SWITCHING FREQUENCY (PROGRAMMING THE CLOCK OSCILLATOR)
The TPS40060 and TPS40061 have independent clock oscillator and ramp generator circuits. The clock
oscillator serves as the master clock to the ramp generator circuit. The switching frequency, f_SW in kHz, of the
clock oscillator is set by a single resistor (R_T) to ground. The clock frequency is related to R_T, in kΩ by
Equation 1 and the relationship is charted in Figure 2.
ǒ Ǔ R_T + _fSW
1
17.82
10^{*6 * 23 kW}
(1)
PROGRAMMING THE RAMP GENERATOR CIRCUIT
The ramp generator circuit provides the actual ramp used by the PWM comparator. The ramp generator provides
voltage feed-forward control by varying the PWM ramp slope with line voltage, while maintaining a constant ramp
magnitude. Varying the PWM ramp directly with line voltage provides excellent response to line variations since
the PWM does not have to wait for loop delays before changing the duty cycle. (See Figure 1).
VIN
VIN
SW
V_PEAK
COMP
RAMP
T₁
t_ON1
d
+
t_ON
T
t_ON2
T₂
t_ON1 > t_ON2 and d₁ > d₂
Figure 1. Voltage Feed-Forward Effect on PWM Duty Cycle
SW
RAMP
COMP
V_VALLEY
UDG-02131
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