34701_07 Datasheet, PDF (30/38 Pages) Freescale Semiconductor, Inc – 1.5 A Switch-Mode Power Supply with Linear Regulator

TYPICAL APPLICATIONS
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.
L
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:
f_LC
=
----------1------------
2π C_OL
f_z(c) = _2----π---C-----2---₍---R-1---1----+-----R-----3----₎
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.
f_p(FF)
=
----------------------------V----^I--^N-----------------------------
---1----
f_sw
×
-⁽--V----^I--^N-----–-----V----^R----^e--^f--⁾-
R4C_F
+
V_m1
×
----------1----------
2πR4C_F
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.
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.).
34701
30
Analog Integrated Circuit Device Data
Freescale Semiconductor