FUNCTIONAL DEVICE OPERATION
COMPONENT CALCULATION
1. First calculate the dt_I (inductor current rising time)
• 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:
given by:
Iomax
T
[s]
---------------------
dtI =
R1
ΔIostep
---------------------
RB =
[Ω]
Vo
-----------
– 1
Where the parameter ΔIo_step is the maximum current
Vref
step during the current rising time and is define as:
• Compensation network. (C1,C2,C3, R2, R3): For
compensating a buck converter, 3 important frequencies
referring to the plant are:
[A]
Dmax Vinmin – Vo
⎛
⎝
⎞ ⎛
⎞
⎠
------------- -------------------------------
ΔIostep =
⎠ ⎝
1. Output LC filter cutoff frequency (FLC):
Fsw
L
2. Then the output capacitor can be chosen as follow:
1
[Hz]
--------------------------
2π
FLC
=
LCOUT
IomaxdtI
----------------------
ΔVomax
[A]
COUT
≤
2. Cutoff frequency due to capacitor ESR:
• Where ΔVOmax is the maximum allowed transient
overshoot expressed as a percentage of the output
voltage, typically from 3 to 5% of Vo.
1
[Hz]
[Hz]
----------------------------------------
=
FESR
2π(COUT)ESR
3. Crossover frequency (or bandwidth):
3. Finally find the maximum allowed ESR to allow the
desired transient response:
FSW
-----------
=
FBW
10
ΔVor(Fsw)(L)
--------------------------------------
=
ESRmax
[Ω]
The Type 3 external compensation network will be in
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.
Vo(1 – Dmin
)
NOTE: Do not use the parameters ΔVOr and ΔVOmax
indistinctly, the first one indicates the output voltage ripple,
while the second one is the maximum output voltage
overshoot (transient response).
34704
Analog Integrated Circuit Device Data
Freescale Semiconductor
41
FREESCALE [ Freescale ]
