TYPICAL APPLICATIONS
COMPONENTS CALCULATION
COMPONENTS CALCULATION
The following formulas are intended for the calculation of
all external components related with the Boost converter and
Network compensation.
CSG = ACSA × RSense
In order to calculate a Duty Cycle, the internal losses of the
MOSFET and Diode should be taken into consideration.
RComp × 5 × G × Iout × L
(1 – D) × Vout × CSG
-------------------------------------M------------------------------
Cout =
The output voltage ripple (ΔVout) depends on the ESR of
the Output capacitor, for a low output voltage ripple it is
recommended to use Ceramic capacitors that usually have
very low ESR. Since ceramic capacitor are expensive,
Electrolytic or Tantalum capacitors can be mixed with
ceramic capacitors to have a cheaper solution.
Vout + VD – Vin
--------------------------------------------
D =
Vout + VD – VSW
The average input current depends directly to the output
current when the internal switch is off.
Iout
1 – D
------------
=
Iinavg
Vout × ΔVout × FSW × L
--------------------------------------------------------------
=
ESRCout
Vout × (1 – D)
Inductor
The output capacitor should handle at least the following
RMS current.
For calculating the Inductor we should consider the losses
of the internal switch and winding resistance of the inductor.
Network Compensation
Since this Boost converter is current controlled, Type II
compensation is needed.
(Vin – VSW – (Iinavg × rw)) × D
---------------------------------------------------------------------------------
L =
Iinavg × r × FSW
It is important to look for an inductor rated at least for the
maximum input current.
D
1 – D
IrmsCout = Iout × ------------
I order to calculate the Network Compensation, first we
need to calculate all Boost Converter components.
Vin × (Vout – Vin)
-------------------------------------------------
+
Iinmax = Iinavg
Input Capacitor
2 × L × FSW × Vout
For this type of compensations we need to push out the
Right Half Plane Zero to higher frequencies where it can’t
affect the overall loop significantly.
The input capacitor should handle at least the following
RMS current.
Vout × (1 – D)2
---------------------------------------
=
fRHPZ
Iout × 2 × π × L
Vin × (Vout – Vin)
⎛
⎝
⎞
⎠
-------------------------------------------------
IrmsCin
=
× 0.3
The Crossover frequency must be set much lower than the
location of the Right half plane zero
2 × L × FSW × Vout
Output Capacitor
For the output capacitor selection the internal current
sense gain (CSG) and the Transconductance should be
taken in consideration.
fRHPZ
--------------
=
fCross
5
The CSG is the internal RSense times the current sense
amplifier gain (ACSA).
Since our system has a fixed Slope compensation set by
RSLOPE, RComp should be fixed for all configurations.
RComp = 5.6Kohm
CComp1 and CComp2 should be calculated as follows:
34844
Analog Integrated Circuit Device Data
Freescale Semiconductor
30
FREESCALE [ Freescale ]
