TPS40055MPWPREPG4 datasheet(8/36 Pages) TI1

No de pièce	TPS40055MPWPREPG4

Description	WIDE-INPUT SYNCHRONOUS BUCK CONTROLLER

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Fabricant	TI1 [Texas Instruments]

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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, fSW in kHz, of the clock oscillator is set by a

single resistor (RT) to ground. The clock frequency is related to RT, 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 (RKFF) pulled up to VIN. RKFF is related to RT and the

minimum input voltage (VIN(min)) through the following:

(2)

where:

VIN(min) is the ensured minimum start-up voltage. The actual start-up voltage is nominally about 10% lower at

25°C.

8

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Product Folder Link(s): TPS40055-EP

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