34701 datasheet(30/38 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

34701

TYPICAL APPLICATIONS

BUCK REGULATOR CONTROL CIRCUIT

The 34701 buck regulator utilizes a PWM Voltage Mode

topology with Feed-Forward to achieve an excellent line and

load regulation. The control circuit block diagram is shown in

Figure 29.

Figure 29. Buck Regulator Control Circuit

The integrated 40 pF capacitor CF charged through the

external resistor R4 provides the feed-forward ramp

waveform, the amplitude of which is proportional to the input

voltage, thus providing the feed-forward function.

Figure 30 shows the Bode plot of the 34701 buck regulator

control loop gain and phase versus frequency.

The first double pole on the Bode plot is created by the

buck regulator output L-C filter, and its frequency can be

calculated as:

Where CO is the value of the buck output capacitor and L

is the inductance value of the output filter inductor L.

The frequency of the compensating zero can be calculated

as follows.

The Feed-Forward implemented by resistor R4 and

integrated capacitor CF creates a pole in the overall loop

transfer function, the frequency of which can be calculated

from the following formula.

Where VRef is the buck regulator reference voltage

(VRef = 0.8 V typ.) at the INV pin,

VIN is the buck regulator input voltage,

Vm1 is the ramp generated by the internal ramp

generator (Vm1 = 0.5 V typ.).

π C

----------------------

()

πC2 R1 R3

()

----------------------------------------

pFF

()

-------

Ref

–

()

R4C

--------------------------------

----------------------------------------------------------------

πR4C

---------------------

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