FAN53168MTC FAIRCHILD [Fairchild Semiconductor], FAN53168MTC Datasheet - Page 22

• Manufacturer Part Number: FAN53168MTC
• Description: 6-Bit VID Controlled 2-4 Phase DC-DC Controller
• Manufacturer: FAIRCHILD [Fairchild Semiconductor]

FAN53168
With the multimode feedback structure of the FAN53168,
one needs to set the feedback compensation to make the
converter’s output impedance working in parallel with the
output decoupling meet this goal. There are several poles and
zeros created by the output inductor and decoupling capaci-
tors (output filter) that need to be compensated for.
A type-three compensator on the voltage feedback is
adequate for proper compensation of the output filter. The
expressions given in Equations 25–29 are intended to yield
an optimal starting point for the design; some adjustments
may be necessary to account for PCB and component para-
sitic effects (see the Tuning Procedure for the FAN53168
section).
The first step is to compute the time constants for all of the
poles and zeros in the system:
22
T
T
T
R
R
T
T
T
T
T
A
B
C
D
E
A
B
C
D
E
=
=
=
=
=
=
=
=
=
=
--------------------------------------------------------------------------------------------------------------------------- -
6.56mF
3 1.3mΩ
2 650nH
---------------------------------------------------------------------------------------- -
n R
6.56mF
1.3mΩ 0.6mΩ
------------------------------------------ -
(
C
(
0.974V
-------------------------------------------------------------------------------------- -
----------------------------------------------------------------- -
C
V
-------------------------------------------------------- -
×
×
×
1.0mΩ
R
3 6.56mF
X
RT
X
X
×
×
×
O
+
1.0mΩ
×
(
C
(
+
V
R
R' R
R


×
A
X
6.56mF 220µF
–
L
VID
O
×
O
×
+
D
(
–
1.5V 55.3mΩ
×
–
1.3mΩ 0.6mΩ
+
×


–
–
×
(
0.6mΩ 1.3mΩ
C
650nH
1.3Ω 0.6mΩ
A
------------------------ -
×
5 5.95mΩ
(
R'
R'
R
O
1 0.375
2 f
Z
D
R
DS
×
×
)
) C
)
×
×
–
×
×
E
+
×
+
1.3mΩ
R
×
+
R
–
SW
------- -
R
C
–
L
=
2
R
–
----------------------
DS
–
X
O
O
X
Z
L
V
4.79µs
5 6.95mΩ
------------------------------ -
2 228kHz
×
×


×
VID
)
×
×
R
R
------------------ -
V
×
×
O
+
O
RT
1.5V
R
×
0.974V
)
1.6mΩ 0.974V
------------------------------------------- -
)
–
X
) 6.56mF
(
+
+
+
1.3mΩ
R'
×
2 L
375pH
----------------- - ×
1.3mΩ
--------------------------------------------------------
220µF 1.3mΩ
n C
×
1.5V


×
×
=
×
=
)
(
2
X
55.3mΩ
1 n D
6.86µs
×
×
–
=
R
×
O
1.97µs
+
×
) V
V
×
=
VID
500ns
RT
The compensation values can then be solved for using the
following:
Choosing the closest standard values for these components
C
In continuous inductor-current mode, the source current of
the high-side MOSFET is approximately a square wave with
a duty ratio equal to n (V
nth of the maximum output current. To prevent large voltage
transients, a low ESR input capacitor sized for the maximum
rms current must be used. The maximum rms capacitor
current is given by:
Figure 6. Typical Transient Response for Design Example
C
C
R
C
C
A
A
A
B
IN
FB
=
=
=
=
Selection and Input Current di/dt Reduction
=
n R
------------------------------ -
3 1.3mΩ
---------------------------------------------------- -
T
------ -
C
------ -
R
T
55.3mΩ 1.33kΩ
T
------ -
R
C
B
A
B
×
R
×
D
A
E
=
=
×
O
=
6.86µs
------------------
-------------------
1.33kΩ
253pF
1.97µs
R
×
-------------------
27.1kΩ
500ns
B
T
×
A
×
4.79µs
Ω
=
=
=
27.1kΩ
1.48nF
OUT
18.5pF
=
/V
253pF
Ω
IN
) and an amplitude of one-
PRODUCT SPECIFICATION
REV. 1.0.0 6/9/03