Datasheet

TPS40055-EP
SGLS310D –JULY 2005–REVISED FEBRUARY 2012 www.ti.com
APPLICATION INFORMATION
The TPS40055 allows the user to optimize the PWM controller to the specific application.
The TPS40055 is the controller of choice for synchronous buck designs, which includes most applications. It has
two quadrant operations and will source or sink output current. This provides the best transient response.
SETTING THE SWITCHING FREQUENCY (PROGRAMMING THE CLOCK OSCILLATOR)
The TPS40055 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
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 3.
(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 2 ).
Figure 2. Voltage Feed-Forward Effect on PWM Duty Cycle
The PWM ramp must be faster than the master clock frequency or the PWM is prevented from starting. The
PWM ramp time is programmed via a single resistor (R
KFF
) pulled up to VIN. R
KFF
is related to RT and the
minimum input voltage (V
IN(min)
) through the following:
(2)
where:
V
IN(min)
is the ensured minimum start-up voltage. The actual start-up voltage is nominally about 10% lower at
25°C.
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