Datasheet

   
   
SLUS419C − AUGUST 1999 − REVISED NOVEMBER 2001
22
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TYPICAL APPLICATION
output capacitor
The following equations can be used to estimate the minimum output capacitance and the capacitor’s maximum
allowable equivalent series resistance (ESR), where C
OUT
is the minimum output capacitance and t
S
is the
period of the switching frequency. ∆V
OUT
is the maximum allowable output ripple voltage, selected as
approximately 1% of the output voltage. For this design, the minimum calculated output capacitance is 170 µF
and the maximum allowable ESR is 96 mΩ. A Panasonic HFQ 1800-µF electrolytic capacitor with an ESR of
0.048 Ω is used.
C
OUT
+
1
8
ǒ
V
OUT
) V
F
Ǔ
ǒ
D
MAX
ǒ
t
S
Ǔ
2
Ǔ
L DV
OUT
ESR +
DV
OUT
DI
L
R
SENSE2
The dc-to-dc power converter is designed for peak current mode control. R
SENSE2
is the resistor that senses
the current in the forward converter. The sense resistor in Figure 9 is referred to as R4. The following equations
can be used to calculate R
SENSE2
. Where I
M
is the magnetizing current of the transformer used in the step-down
converter and V
BOOST
is the output voltage of the boost stage. D is the typical duty ratio of the forward converter.
V
ISENSE2_peak
is the peak current sense comparator voltage that is typically 1.15 V. For this design example L
M
is approximately 8 mH and the R
SENSE2
is approximately 1 Ω.
I
M
+
V
BOOST
L
M
D
f
S
R
SENSE2
+
V
ISENSE2_peak
I
M
)
N
S
N
P
ǒ
DI
L
2
) I
OUT(max)
1.3
Ǔ
soft-start
The UCC38500 has soft-start circuitry to allow for a controlled ramp of the second stage’s duty cycle during
start-up. This is accomplished through the SS2 circuitry described earlier in this data sheet. Equation (37)
calculates the approximate capacitance needed based on the designer’s soft-start requirements. Where I
SS2
is the soft-start charging current, which is typically 10 µA. ∆t is the desired soft start time, which was selected
to be approximately 5 ms for this example. The calculated soft-start capacitor (C
SS
) for this example is
approximately 10 nF.
C
SS
+
I
ISS2
D t
4.5
slope compensation
When designing with peak current-mode control, slope compensation may be necessary to prevent instability.
In this design, the magnetizing current provided more than enough slope compensation. If slope compensation
is needed with external components, please refer to Unitrode/Texas Instruments Application Note, Practical
Considerations in Current Mode Power Supplies, TI Literature No. SLUA110.
(33)
(34)
(35)
(36)
(37)