Datasheet


SLVS612 − APRIL 2006
15
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APPLICATION INFORMATION
The poles and zeros for a type III network are described in equations (20).
f
Z1
+
1
2p R2 C1
(Hertz) f
Z2
+
1
2p R1 C3
(Hertz)
f
P1
+
1
2p R2 C2
(Hertz) f
P2
+
1
2p R3 C3
(Hertz)
The value of R1 is somewhat arbitraty, but influences other component values. A value between 50 kΩ and
100 kΩ usually yields reasonable values.
The unity gain frequency is described in equation (21)
f
C
+
1
2p R1 C2 G
(Hertz)
where G is the reciprocal of the modulator gain at f
C
.
The modulator gain as a function of frequency at f
C
, is described in equation (22).
AMOD(f) + AMOD
ǒ
f
LC
f
C
Ǔ
2
and G +
1
AMOD(f)
Minimum Load Resistance
Care must be taken not to load down the output of the error amplifier with the feedback resistor, R2, that is too
small. The error amplifier has a finite output source and sink current which must be considered when sizing R2.
Too small a value does not allow the output to swing over its full range.
R2
(MIN)
+
V
C(max)
I
SOURCE (min)
+
3.45 V
2mA
+ 1725 W
CALCULATING THE BOOST AN BP10 BYPASS CAPACITOR
The BOOST capacitance provides a local, low impedance source for the high-side driver. The BOOST capacitor
should be a good quality, high-frequency capacitor. The size of the bypass capacitor depends on the total gate
charge of the MOSFET and the amount of droop allowed on the bypass capacitor. The BOOST capacitance
is described in equation (24).
C
BOOST
+
Q
g
DV
(Farads)
The 10-V reference pin, BP10V needs to provide energy for both the synchronous MOSFET and the high-side
MOSFET via the BOOST capacitor. Neglecting any efficiency penalty, the BP10V capacitance is described in
equation (25).
C
BP10
+
ǒ
Q
gHS
) Q
gSR
Ǔ
DV
(Farads)
(20)
(21)
(22)
(23)
(24)
(25)