FUNCTIONAL DEVICE OPERATION
COMPONENT CALCULATION
• Output LC filter cutoff frequency (FLC):
VRRM ≥ Vin – Vo
• L: The minimum value of inductor to maintain CCM can be
D′min
[Hz]
determined by using the following procedure:
--------------------------
=
FLC
2π
LCOUT
2
Vinmin
------------------ -------------------------------
≥
–VoT
2Iomax Vo – Vinmin
⎛
⎞
⎠
Where D’min is the minimum off time percentage given by:
[H]
Lmin
⎝
• 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. The minimum
capacitor value is approximately:
Vinmin
-------------------------
=
D′min
Voutmax
• Cutoff frequency due to capacitor ESR:
1
IomaxDmax
----------------------------
FSWΔVor
[F]
COUT
≥
[Hz]
----------------------------------------
=
FESR
2π(COUT)ESR
• Where ΔVOr is the desired output voltage ripple.
• Now calculate the maximum allowed ESR to reach the
desired.
• The right plane zero frequency:
(D′min)2RLOAD
[Hz]
----------------------------------------
=
RHPZ
D • 2πL
ΔVor
[Ω]
-------------------------------------------------
ESR ≤
• Crossover frequency (or bandwidth): select this
frequency as far away form the RHPZ as much as
possible:
Iomax
IOB
⎛
⎞
----------------------- -------------
+
⎝
⎠
1 – Dmax 1 – D
RHPZ
[Hz]
---------------
«
FBW
• R1 and RB: These two resistors help to set the output
voltage to the desire value using a VFB7=0.6V, select R1
between 10 k and 150 K and then calculate RB as follows:
6
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:
0.9
------------------------------------
RB =
R1
[Ω]
1.5 – Vo – 0.9
NOTE: RB is not grounded, instead is connected to
VREF7 pin (VREF7=1.5 V) which provide a positive voltage
to assure a positive voltage at the FB7 pin.
FPO = FBW
FP1 = FESR
FZ1 = 0.9FLC
FSW
F22 = 1.1FLC
-----------
=
F2P
2
• Compensation network. (C1,C2,C3, R2, R3)
For compensating a buck converter, 4 important
frequencies referring to the plant are:
34704
Analog Integrated Circuit Device Data
Freescale Semiconductor
46
FREESCALE [ Freescale ]
