參數(shù)資料
型號(hào)： KIT33394DWBEVB
廠商： Freescale Semiconductor
文件頁(yè)數(shù)： 22/44頁(yè)
文件大?。?/td> 0K
描述： KIT EVAL FOR MC33394 8X PWR SPLY
標(biāo)準(zhǔn)包裝： 1
33394
29
MOTOROLA ANALOG INTEGRATED CIRCUIT DEVICE DATA
After substitution, the calculated inductance value is L = 45
H, which gives 47 H standard component value.
The peak–to peak ripple current value is:
IL = 0.345 A.
The peak inductor current is given by:
ILpk = 0.5IL + Io = 0.5x0.345 + 1.2 = 1.37[A]
The inductor saturation current is given by the upper value
of the 33394 internal switch current limit Ilim(max) = 3.0 A.
Considering also the inductor serial resistance, these
requirements are met, for example by the PO250.473T
inductor from Pulse Engineering, Inc.
5.2.1.2. Selecting the Catch Diode D1
The rectifier D1 current capability has to be greater than
calculated average current value.
The maximum reverse voltage stress placed upon this
rectifier D1 is given by maximum input voltage (maximum
transient battery voltage). These requirements are met, for
example by the HSM350 (3 A, 50 V) schottky diode from
Microsemi, Inc.
5.2.1.3. Selecting the Output Capacitor
The output capacitor Co should be a low ESR part,
therefore the 100
F tantalum capacitor with 80 m ESR was
chosen.
From the formula for calculating the ripple voltage:
VppESR = IL x RESR = 0.345 x 0.08 = 28 [mV]
One device that meets both, the low ESR, and the
temperature stability requirements is, for example, the
TPSV107K020R0085 tantalum capacitor from AVX Corp.
Boost Converter Power Capability
The boost converter with selected components has to be
able to deliver the required power.
Due to the nature of this non–compensated PFM control
technique, the Boost converter output ripple voltage is higher
than if it utilized a typical PWM control method. Therefore the
switcher output voltage level is set higher than in the Buck
mode (in the Boost mode VPRE = +6 V), in order to maintain
a sufficient dropout voltage for the 5–volt linear regulators
(VDDH, VREFs) and to avoid unwanted Resets to the
microcontroller.
The most stringent conditions for the 33394 boost
converter occur with the lowest input voltage:
Vin(min) = 3.5 V
Io = 0.8 A
Vpre = +6 V
f = 200 kHz
Vfwd1 = Vfwd2 = 0.5 V
d = 0.75, duty cycle is fixed at 75% in boost mode
Figure 16. 33394 Switcher Topology – Boost Mode
IL
VRES
L
Vin
RLOAD
VO
+
–
+
CO
RD
Q2
ESR
D2
Vfwd2
IQ
IL
ILIM
I01
I02
DIL1 < DIL2
Tt
IL1
IL2
L1 > L2
IO1 > IO2
The input voltage drop associated with the resistance of
the internal switch Q1 and inductor series resistance can be
estimated as:
VD [ Ipk(min)
RD + 2.5 A
0.35
W + 0.875 V
Where:
VD is the voltage dissipated on the major parasitic
resistances, RDSon of the internal power switch and inductor
series resistance RL.
For the worst case conditions:
RD = RDSon(max) + RL = 0.25 + 0.1 = 0.35[]
Ipk(min) is the minimum internal power switch current limit
value.
Then the equation for calculating
IL can be modified as
follows:
DIL +
Vin * VD
L
[(Vo ) Vfwd2) * (Vin * VD)]
d
(Vo ) Vfwd2)
f
+
+ 3.5 * 0.875
47
10*6
[(6
) 0.5) * (3.5 * 0.875)]
0.75
(6
) 0.5)
0.2
106
+ 125[mA]
Then the maximum average input current can be
calculated as:
IinAve + Ipk(min) *
1
2
DIL + 2.5 * 0.125
2
+ 2.43[A]
Finally, the boost converter power capability has to be
higher than the required output power or:
Pin(max)
h u Pout
Where Pin(max) is the boost converter maximum input power:
h is the boost converter efficiency, in our case h is
estimated to be h = 85%, and includes switching losses of the
external power switch Q2 (MOSFET) inductor and capacitors
AC losses, and output rectifier D2 (schottky) switching
losses.
Pout is the boost converter output power, which includes
power loss of the output rectifier D2:
Pout + (Vo ) Vfwd2)
Io + (6 ) 0.5)
0.8
+ 5.2[W]
Pin + (Vin * VD)
IinAve
h +
+ (3.5 * 0.875)
2.43
0.85
+ 5.42[W]
As can be seen, the boost converter input power capability
meets the required criteria.
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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