MC34704 FREESCALE [Freescale Semiconductor, Inc], MC34704 Datasheet - Page 42

• Manufacturer Part Number: MC34704
• Description: Multiple Channel DC-DC Power Management IC
• Manufacturer: FREESCALE [Freescale Semiconductor, Inc]

VREF7 pin (VREF7=1.5V) which provide a positive voltage to
assure a positive voltage at the FB7 pin.
• Compensation network. (C1,C2,C3, R2, R3)
frequencies referring to the plant are:
42
34704
FUNCTIONAL DEVICE OPERATION
COMPONENT CALCULATION
F
D
RB
LC
NOTE: RB is not grounded, instead is connected to
For compensating a buck converter, 4 important
• Output LC filter cutoff frequency (F
Where D’
The passive components associated to these frequencies are calculated with the following formulas.
On the 34704 V
min
C1
C2
R2
R3
C3
=
=
=
--------------------------------- - R1
1.5 Vo
------------------------- -
2
=
=
=
=
=
------------------------ -
D
Vout
⎛
⎝
⎛
⎝
⎛
⎝
⎛
⎝
------------------
V
LC
Vin
–
Vin
--------------------------- -
2
--------------------------- -
2
--------------------------- -
2
--------------------------- -
2
min
RAMP
min
0.9
OUT
min
min
F
F
F
F
max
is the minimum off time percentage given by:
–
1
1
1
1
Z2
Z1
P1
P2
0.9
⎛
⎜
⎝
C2
R2
R1
C1
RAMP
----------------
D
min
1
⎞
⎠
⎞
⎠
⎞
⎠
⎞
⎠
2
is half of 1.2V since each operation mode spends only half the ramp.
⎞
⎟
⎠
⎛
⎝
---------------------------- -
2
F
1
PO
R1
⎞
⎠
LC
):
[ ]
[Hz]
charge of canceling some of these poles and zeros to
achieve stability in the system. The following poles and
zeroes frequencies are provided by the type 3 compensation:
F
RHP
F
F
F
ESR
BW
PO
P1
• Cutoff frequency due to capacitor ESR:
• The right plane zero frequency:
• Crossover frequency (or bandwidth): select this
The Type 3 external compensation network will be in
=
Z
frequency as far away form the RHP
possible:
«
=
=
RHP
-------------- -
=
F
F
------------------------------------- -
2
ESR
BW
6
--------------------------------------- -
D
Z
C
min
D 2 L
OUT
1
2
F
F
R
ESR
Z1
2P
LOAD
=
=
Analog Integrated Circuit Device Data
0.9F
F
----------
SW
2
LC
Freescale Semiconductor
F
22
=
Z
as much as
1.1F
LC
[Hz]
[Hz]
[Hz]