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UC2577T-ADJ Fiches technique(PDF) 6 Page - Texas Instruments

No de pièce UC2577T-ADJ
Description  Simple Step-Up Voltage Regulator
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Fabricant  TI [Texas Instruments]
Site Internet  http://www.ti.com
Logo TI - Texas Instruments

UC2577T-ADJ Fiches technique(HTML) 6 Page - Texas Instruments

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UC2577-ADJ
3. Compensation Network (RC, CC) and Output
Capacitor (COUT) Selection
The compensation network consists of resistor RC and
capacitor CC which form a simple pole-zero network and
stabilize the regulator. The values of RC and CC depend
upon the voltage gain of the regulator, ILOADmax, the in-
ductor L, and output capacitance COUT. A procedure to
calculate and select the values for RC, CC, and COUT
which ensures stability is described below. It should be
noted, however, that this may not result in optimum com-
pensation. To guarantee optimum compensation a stand-
ard procedure for testing loop stability is recommended,
such as measuring VOUT transient responses to pulsing
ILOAD.
A. Calculate the maximum value for RC.
RC
750
• ILOADmax • VOUT2
VINmin
2
Select a resistor less than or equal to this value, not to
exceed 3k
Ω.
B. Calculate the minimum value for COUT using the fol-
lowing two equations.
COUT
0.19
L • RC ILOADmax
VINmin
• VOUT
and
COUT
VINmin
• RC • (VINmin + (3.74 • 105 • L))
487,800
• VOUT3
The larger of these two values is the minimum value that
ensures stability.
C. Calculate the minimum value of CC.
CC
58.5
• VOUT2 • COUT
RC
2 • VINmin
The compensation capacitor is also used in the soft start
function of the regulator. When the input voltage is ap-
plied to the part, the switch duty cycle is increased slowly
at a rate defined by the compensation capacitor and the
soft start current, thus eliminating high input currents.
Without the soft start circuitry, the switch duty cycle would
instantly rise to about 90% and draw large currents from
the input supply. For proper soft starting, the value for CC
should be equal or greater than 0.22
µF.
Figure 6 lists several types of aluminum electrolytic ca-
pacitors which could be used for the output filter. Use the
following parameters to select the capacitor.
Working Voltage (WVDC): Choose a capacitor with a
working voltage at least 20% higher than the regulator
output voltage.
Ripple Current: This is the maximum RMS value of cur-
rent that charges the capacitor during each switching cy-
cle. For step-up and flyback regulators, the formula for
ripple current is:
IRIPPLErms
=
ILOADmax
Dmax
1
− Dmax
Choose a capacitor that is rated at least 50% higher than
this value at 52kHz.
Equivalent Series Resistance (ESR): This is the primary
cause of output ripple voltage, and it also affects the val-
ues of RC and CC needed to stabilize the regulator. As a
result, the preceding calculations for CC and RC are only
valid if the ESR does not exceed the maximum value
specified by the following equations.
ESR
0.01
• 15V
IRIPPLE(PP)
and
8.7
• 10−3 • VIN
ILOADmax
where
IRIPPLE(P−P
)
=
1.15
• ILOADmax
1
Dmax
Select a capacitor with an ESR, at 52kHz, that is less
than or equal to the lower value calculated. Most electro-
lytic capacitors specify ESR at 120kHz which is 15% to
30% higher than at 52kHz. Also, note that ESR increases
by a factor of 2 when operating at
−20°C.
In general, low values of ESR are achieved by using
large value capacitors (C
≥ 470µF), and capacitors with
high WVDC, or by paralleling smaller value capacitors.
Inductor
Code
Manufacturer’s Part Number
AIE
Pulse
Renco
L47
415 - 0932
PE - 53112
RL2442
L68
415 - 0931
PE - 92114
RL2443
L100
415 - 0930
PE - 92108
RL2444
L150
415 - 0953
PE - 53113
RL1954
L220
415 - 0922
PE - 52626
RL1953
L330
415 - 0926
PE - 52627
RL1952
L470
415 - 0927
PE - 53114
RL1951
L680
415 - 0928
PE - 52629
RL1950
H150
415 - 0936
PE - 53115
RL2445
H220
430 - 0636
PE - 53116
RL2446
H330
430 - 0635
PE - 53117
RL2447
H470
430 - 0634
PE - 53118
RL1961
H680
415 - 0935
PE - 53119
RL1960
H1000
415 - 0934
PE - 53120
RL1959
H1500
415 - 0933
PE - 53121
RL1958
H2200
415 - 0945
PE - 53122
RL2448
AIE Magnetics, Div. Vernitron Corp., (813)347-2181
2801 72nd Street North, St. Petersburg, FL 33710
Pulse Engineering, (619)674-8100
12220 World Trade Drive, San Diego, CA 92128
Renco Electronics, Inc., (516)586-5566
60 Jeffryn Blvd. East, Deer Park, NY 11729
Figure 5. Table of Standardized Inductors and
Manufacturer’s Part Numbers
APPLICATIONS INFORMATION (cont.)
6


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