FUNCTIONAL DEVICE OPERATION
COMPONENT CALCULATION
The passive components associated to these frequencies are calculated with the following formulas
Vinmin
⎛
⎞
1
1
⎛
⎞
⎠
------------------ ---------------- ------------------------------
C1 =
C2 =
R2 =
R3 =
C3 =
⎜
⎝
⎟
2 ⎝
⎠
VRAMP
2π(FPOR1)
D′min
1
⎛
⎝
⎞
⎠
-----------------------------
2π(FZ2R1)
1
⎛
⎝
⎞
⎠
-----------------------------
2π(FZ1C1)
1
⎛
⎝
⎞
⎠
-----------------------------
2π(FP1C2)
1
⎛
⎝
⎞
⎠
-----------------------------
2π(FP2R2)
On the 34704 VRAMP is half of 1.2 V since each operation mode spends only half the ramp.
Regulator 3 (Synchronous Buck - internally
compensated)
• Then the output capacitor can be chosen as follow:
• L: A buck power stage can be designed to operate in 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:
[F]
IomaxdtI
----------------------
ΔVomax
COUT
≤
Where ΔVOmax is the maximum allowed transient
overshoot expressed as a percentage of the output voltage,
typically from 3 to 5% of Vo.
1. Define IOB as the minimum current to maintain CCM as
15% of full load.
• Finally find the maximum allowed ESR to allow the
desired transient response:
(Vo + Iomax (RDSONLSFET + RL)(D′min)T
-------------------------------------------------------------------------------------------------------------
≥
Lmin
2IOB
Vo
2IOB
[H]
------------
ΔVor(Fsw)(L)
--------------------------------------
=
[Ω]
Lmin ≥ D′T
ESRmax
Vo(1 – Dmin
)
• COUT: The three elements of output capacitor that
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.
NOTE: do not use the parameters ΔVOR and ΔVOmax
indistinctly, the first one indicates the output voltage rip-
ple, while the second one is the maximum output volt-
age overshoot (transient response).
• R1 and RB: These two resistors help to set the output
voltage to the desire value using a VREF=0.6 V, select R1
between 10 k and 100 K and then calculate RB as follows:
• First calculate the dt_I (inductor current rising time)
given by:
[Ω]
R1
Io
T
[s]
---------------------
RB =
-------m-----a---x------
dtI =
Vo
ΔIostep
-----------
– 1
Vref
Where the parameter ΔIO_step is the maximum current
step during the current rising time and is define as:
Regulator 8 (Synchronous Boost - internally
compensated -Voltage or current feedback)
Dmax Vinmin – Vo
⎞ ⎛
[A]
REG8 is a Synchronous Boost converter set to 15V with a
maximum current of 30 mA and can be used with voltage
⎛
⎝
⎞
⎠
------------- -------------------------------
ΔIostep =
⎠ ⎝
Fsw
L
34704
Analog Integrated Circuit Device Data
Freescale Semiconductor
44
FREESCALE [ Freescale ]
