MC34704 FREESCALE [Freescale Semiconductor, Inc], MC34704 Datasheet - Page 40

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

Regulator 3 (Synchronous Buck - internally
compensated)
• L: A buck power stage can be designed to operate in CCM
• C
step during the current rising time and is define as:
40
34704
FUNCTIONAL DEVICE OPERATION
COMPONENT CALCULATION
L
1. Define I
L
The passive components associated to these frequencies are calculated with the following formulas
On the 34704 V
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:
contribute to its impedance and output voltage ripple are
the ESR, the ESL and the capacitance C. A good
approach to calculate the minimum real capacitance
needed is to include the transient response analysis to
control the maximum overshoot as desired.
• First calculate the dt_I (inductor current rising time)
dtI
Where the parameter
• Then the output capacitor can be chosen as follow:
C1
C2
R2
R3
C3
min
Iostep
min
OUT
15% of full load.
given by:
=
=
=
=
=
=
: The three elements of output capacitor that
---------------------------------------------------------------------------------------------------------- -
------------------- -
Io
D T
------------------
V
⎛
⎝
⎛
⎝
⎛
⎝
⎛
⎝
Vin
Vo
Iostep
--------------------------- -
2
--------------------------- -
2
--------------------------- -
2
--------------------------- -
2
RAMP
=
max
OB
+
min
----------- -
2I
⎛
⎝
F
F
F
F
Vo
D
------------ -
T
Io
Fsw
1
Z2
1
Z1
1
P1
1
P2
OB
as the minimum current to maintain CCM as
max
⎛
⎜
⎝
max
C2
R2
R1
C1
----------------
D
RAMP
⎞ Vin
⎠
min
1
⎛
⎝
⎞
⎠
⎞
⎠
⎞
⎠
⎞
⎠
R
------------------------------ -
2
DSONLSFET
⎞
⎟
⎠
is half of 1.2V since each operation mode spends only half the ramp.
⎛
⎝
min
IO
---------------------------- -
2
2I
L
OB
_
–
step is the maximum current
F
Vo
PO
1
⎞
⎠
R1
+
R
⎞
⎠
L
D
min
[H]
[s]
T
[A]
overshoot expressed as a percentage of the output voltage,
typically from 3 to 5% of Vo.
• R1 and RB: These two resistors help to set the output
Regulator 8 (Synchronous Boost - internally
compensated -Voltage or current feedback)
maximum current of 30mA and can be used with voltage
feedback using the standard voltage divider configuration, or
can be programmed to work with a current feedback
configuration to control the current flowing through a LED
ESR
RB
C
Where
• Finally find the maximum allowed ESR to allow the
voltage to the desire value using a V
between 10k and 100K and then calculate RB as follows:
REG8 is a Synchronous Boost converter set to 15V with a
OUT
NOTE: do not use the parameters
indistinctly, the first one indicates the output voltage rip-
ple, while the second one is the maximum output volt-
age overshoot (transient response).
desired transient response:
=
max
-------------------- -
----------- - 1
Vref
Vo
Io
--------------------- -
=
VO
R1
Vo
max
-------------------------------------
–
Vo 1 D
max
max
Vo
dtI
r
is the maximum allowed transient
Fsw L
–
Analog Integrated Circuit Device Data
min
Freescale Semiconductor
REF
VO
=0.6V, select R1
R
and
[ ]
[ ]
[F]
VO
max