MC34704 FREESCALE [Freescale Semiconductor, Inc], MC34704 Datasheet - Page 39

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

Compensating for boost operation:
• L: A boost power stage can be designed to operate in
stage is when the input voltage is equal to one half of the
output voltage, which results in the Maximum
• COUT: The three elements of output capacitor that
• R1 and RB:
desire value using a Vref=0.6V, select R1 between 10k and
100K and then calculate RB as follows:
Analog Integrated Circuit Device Data
Freescale Semiconductor
C
1. Define I
L
RB
L
CCM for load currents above a certain level usually 5 to
15% of full load. The minimum value of inductor to
maintain CCM can be determined by using the following
procedure:
However the worst case condition for the boost power
contribute to its impedance and output voltage ripple are
the ESR, the ESL and the capacitance C. The minimum
capacitor value is approximately:
• Where
• Now calculate the maximum allowed ESR to reach the
These two resistors help to set the output voltage to the
ESR
OUT
min
min
15% of full load:
desired
=
---------------------- -
------------- 1
V
Vo D 1 D
----------------------------------------- -
Vo T
--------------- -
16I
------------------------------------------ -
⎛
⎝
Io
--------------------------- -
Vo
Fsw Vo
REF
--------------------- -
1 D
R1
max
Io
–
OB
OB
VO
max
2I
–
VO
D
max
as the minimum current to maintain CCM as
OB
Vo
max
r
–
r
is the desired output voltage ripple.
:
r
r
+
I
OB
2
T
⎞
⎠
[ ]
[H]
[H]
[F]
I
L
, then:
[ ]
• Compensation network. (C1,C2,C3, R2, R3)
frequencies referring to the plant are:
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:
1. Output LC filter cutoff frequency (F
2. Cutoff frequency due to capacitor ESR:
F
3. The right plane zero frequency:
4. Crossover frequency (or bandwidth): select this
F
F
D
RHP
F
For compensating a buck converter, 4 important
• Where D’
The Type 3 external compensation network will be in
F
ESR
PO
P1
BW
LC
min
by:
frequency as far away form the RHP
possible:
=
Z
=
«
=
=
=
RHP
-------------- -
F
=
F
------------------------- -
2
ESR
------------------------------------- -
2
BW
6
--------------------- -
Vout
--------------------------------------- -
D
Vin
D
LC
Z
C
min
min
min
OUT
OUT
min
max
1
2 L
is the minimum off time percentage given
2
F
F
R
ESR
Z1
2P
LOAD
=
=
FUNCTIONAL DEVICE OPERATION
0.9F
F
----------
SW
2
LC
COMPONENT CALCULATION
F
LC
22
Z
[Hz]
):
=
as much as
[Hz]
[Hz]
[Hz]
1.1F
LC
34704
39