TI UC2577T-ADJ

UC2577-ADJ
Simple Step-Up Voltage Regulator
FEATURES
DESCRIPTION
•
Requires Few External Components
•
NPN Output Switches 3.0A, 65V(max)
•
Extended Input Voltage Range: 3.0V to 40V
•
Current Mode Operation for Improved
Transient Response, Line Regulation, and
Current Limiting
The UC2577-ADJ device provides all the active functions necessary to implement step-up (boost), flyback, and forward converter
switching regulators. Requiring only a few components, these simple regulators efficiently provide up to 60V as a step-up regulator,
and even higher voltages as a flyback or forward converter regulator.
•
Soft Start Function Provides Controlled
Startup
•
52kHz Internal Oscillator
•
Output Switch Protected by Current Limit,
Undervoltage Lockout and Thermal
Shutdown
•
Improved Replacement for LM2577-ADJ
Series
TYPICAL APPLICATIONS
•
Simple Boost and Flyback Converters
•
SEPIC Topology Permits Input Voltage to
be Higher or Lower than Output Voltage
•
Transformer Coupled Forward Regulators
•
Multiple Output Designs
BLOCK DIAGRAM
The UC2577-ADJ features a wide input voltage range of 3.0V to
40V and an adjustable output voltage. An on-chip 3.0A NPN switch
is included with undervoltage lockout, thermal protection circuitry,
and current limiting, as well as soft start mode operation to reduce
current during startup. Other features include a 52kHz fixed frequency on-chip oscillator with no external components and current
mode control for better line and load regulation.
A standard series of inductors and capacitors are available from
several manufacturers optimized for use with these regulators and
are listed in this data sheet.
CONNECTION DIAGRAM
5-Pin TO-220 (Top View)
T Package
Also available in TO-263 Package (TD).
UDG-94034
3/97
UC2577-ADJ
ABSOLUTE MAXIMUM RATINGS (Note 1)
RECOMMENDED OPERATING RANGE
Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45V
Output Switch Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65V
Output Switch Current (Note 2) . . . . . . . . . . . . . . . . . . . . . 6.0A
Power Dissipation . . . . . . . . . . . . . . . . . . . . . . Internally Limited
Storage Temperature Range . . . . . . . . . . . . . −65°C to +150°C
Lead Temperature (Soldering, 10 sec.) . . . . . . . . . . . . . . 260°C
Maximum Junction Temperature . . . . . . . . . . . . . . . . . . . 150°C
Minimum ESD Rating (C = 100pF, R = 15kΩ) . . . . . . . . . . . 2kV
Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . 3.0V ≤ VIN ≤ 40V
Output Switch Voltage . . . . . . . . . . . . . . . 0V ≤ VSWITCH ≤ 60V
Output Switch Current . . . . . . . . . . . . . . . . . . . . ISWITCH ≤ 3.0A
Junction Temperature Range . . . . . . . . . . −40°C ≤ TJ ≤ +125°C
ELECTRICAL CHARACTERISTICS Unless otherwise stated, these specifications apply for TA = −40°C to +125°C, VIN =
5V, VFB = VREF, ISWITCH = 0, and TA =TJ.
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNITS
12.0
12.60
V
System Parameters Circuit Figure 1 (Note 3)
Output Voltage
VIN = 5V to 10V, ILOAD = 100mA to 800mA
11.40
TJ = 25°C
11.60
Line Regulation
VIN = 3.0V to 10V, ILOAD = 300mA
20
TJ = 25°C
Load Regulation
VIN = 5V, ILOAD = 100mA to 800mA
20
TJ = 25°C
Efficiency
VIN = 5V, ILOAD = 800mA
80
VFB = 1.5V (Switch Off)
7.5
12.40
V
100
mV
50
mV
100
mV
50
mV
%
Device Parameters
Input Supply Current
TJ = 25°C
ISWITCH = 2.0A, VCOMP = 2.0V (Max Duty Cycle)
45
TJ = 25°C
Input Supply UVLO
ISWITCH = 100mA
2.70
TJ = 25°C
Oscillator Frequency
Reference Voltage
Measured at SWITCH Pin, ISWITCH = 100mA
42
TJ = 25°C
48
Measured at FB Pin, VIN = 3.0V to 40V, VCOMP = 1.0V
1.206
TJ = 25°C
1.214
52
1.230
Reference Voltage Line Regulation
VIN = 3.0V to 40V
0.5
Error Amp Input Bias Current
VCOMP = 1.0V
100
TJ = 25°C
Error Amp Transconductance
Error Amp Voltage Gain
Error Amplifier Output Swing
ICOMP = −30µA to +30µA, VCOMP = 1.0V
1600
TJ = 25°C
2400
VCOMP = 0.8V to 1.6V, RCOMP = 1.0MW (Note 4)
250
TJ = 25°C
500
Upper Limit VFB = 1.0V
2.0
TJ = 25°C
2.2
Lower Limit VFB = 1.5V
3700
VFB = 1.0V to 1.5V, VCOMP = 1.0V
±90
TJ = 25°C
±130
Soft Start Current
VFB = 1.0V, VCOMP = 0.5V
1.5
TJ = 25°C
2.5
Maximum Duty Cycle
VCOMP = 1.5V, ISWITCH = 100mA
90
TJ = 25°C
93
2
mA
10
mA
85
mA
70
mA
2.95
V
2.85
V
62
kHz
56
kHz
1.254
V
1.246
V
mV
800
nA
300
nA
5800
µmho
4800
µmho
800
V/V
V/V
2.4
V
V
0.3
0.55
0.40
V
±200
±400
µA
±300
µA
9.5
µA
7.5
µA
TJ = 25°C
Error Amp Output Current
14
5.0
95
V
%
%
UC2577-ADJ
ELECTRICAL CHARACTERISTICS Unless otherwise stated, these specifications apply for TA = −40°C to +125°C, VIN =
5V, VFB = VREF, ISWITCH = 0, and TA =TJ.
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNITS
Device Parameters (cont.)
Switch Transconductance
Switch Leakage Current
12.5
VSWITCH = 65V, VFB = 1.5V (Switch Off)
10
TJ = 25°C
Switch Saturation Voltage
ISWITCH = 2.0A, VCOMP = 2.0V (Max Duty Cycle)
0.5
TJ = 25°C
NPN Switch Current Limit
VCOMP = 2.0V
Thermal Resistance
Junction to Ambient
65
Junction to Case
2
VCOMP = 0
25
COMP Pin Current
3.0
TJ = 25°C
4.3
A/V
600
µA
300
µA
0.9
V
0.7
V
6.0
A
°C/W
°C/W
50
µA
40
µA
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating ratings
indicate conditions during which the device is intended to be functional, but device parameter specifications may not be
guaranteed under these conditions. For guaranteed specifications and test conditions, see the Electrical Characteristics.
Note 2: Output current cannot be internally limited when the UC2577 is used as a step-up regulator. To prevent damage to
the switch, its current must be externally limited to 6.0A. However, output current is internally limited when the UC2577 is
used as a flyback or forward converter regulator.
Note 3. External components such as the diode, inductor, input and output capacitors can affect switching regulator
performance. When the UC2577 is used as shown in the Test Circuit, system performance will be as specified by the
system parameters.
Note 4: A 1.0MΩ resistor is connected to the compensation pin (which is the error amplifier’s output) to ensure accuracy in
measuring AVOL. In actual applications, this pin’s load resistance should be ≥ 10MΩ, resulting in AVOL that is typically twice
the guaranteed minimum limit.
UDG-94035
L = 415-0930 (AIE)
D = any manufacturer
COUT = Sprague Type 673D
Electrolytic 680µF, 20V
R1 = 48.7k in series with 511Ω (1%)
R2 = 5.62k (1%)
Figure 1. Circuit Used to Specify System Parameters
3
UC2577-ADJ
APPLICATIONS INFORMATION
Step-up (Boost) Regulator
The Block Diagram shows a step-up switching regulator
utilizing the UC2577. The regulator produces an output
voltage higher than the input voltage. The UC2577 turns
its switch on and off at a fixed frequency of 52kHz, thus
storing energy in the inductor (L). When the NPN switch
is on, the inductor current is charged at a rate of VIN/L.
When the switch is off, the voltage at the SWITCH terminal of the inductor rises above VIN, discharging the
stored current through the output diode (D) into the output capacitor (COUT) at a rate of (VOUT - VIN)/L. The energy stored in the inductor is thus transferred to the
output.
D
VOUT + VF − VIN VOUT − VIN
≈
VOUT
VOUT + VF − VSAT
Avg. Inductor
Current
IIND(AVG)
ILOAD
1−D
Inductor
Current Ripple
∆IIND
Peak Inductor
Current
IIND(PK)
ILOAD ∆IIND
+
2
1−D
Peak Switch
Current
ISW(PK)
ILOAD ∆IIND
+
2
1−D
Duty Cycle
The output voltage is controlled by the amount of energy
transferred, which is controlled by modulating the peak
inductor current. This modulation is accomplished by
feeding a portion of the output voltage to an error amplifier which amplifies the difference between the feedback
voltage and an internal 1.23V precision reference voltage. The output of the error amplifier is then compared to
a voltage proportional to the switch current, or the inductor current, during the switch on time. A comparator terminates the switch on time when the two voltages are
equal and thus controls the peak switch current to maintain a constant output voltage. Figure 2 shows voltage
and current waveforms for the circuit. Formulas for calculation are shown in Figure 3.
VIN − VSAT
L
Switch Voltage
VSW(OFF)
when Off
Diode Reverse
Voltage
•
D
52,000
VOUT + VF
VR
VOUT - VSAT
Avg. Diode
Current
ID(AVG)
ILOAD
Peak Diode
Current
ID(PK)
ILOAD ∆IIND
+
.
2
1−D
STEP-UP REGULATOR DESIGN PROCEDURE
Power
Dissipation
PD
Refer to the Block Diagram
Given:
VINmin = Minimum input supply voltage
VOUT = Regulated output voltage
ILOAD • D • VIN
 ILOAD 
D+
0.25Ω 

50 (1−D)
 1−D 
VF = Forward Biased Diode Voltage, ILOAD = Output Load
2
Figure 3. Step-up Regulator Formulas
First, determine if the UC2577 can provide these values
of VOUT and ILOADmax when operating with the minimum
value of VIN. The upper limits for VOUT and ILOADmax are
given by the following equations.
VOUT ≤ 60V and
VOUT ≤ 10 • VINmin
2.1A • VINmin
ILOADmax ≤
VOUT
These limits must be greater than or equal to the values
specified in this application.
1. Output Voltage Section
Resistors R1 and R2 are used to select the desired output voltage. These resistors form a voltage divider and
present a portion of the output voltage to the error amplifier which compares it to an internal 1.23V reference. Select R1 and R2 such that:
R1 VOUT
=
−1
R2 1.23V
Figure 2. Step-up Regulator Waveforms
4
UC2577-ADJ
APPLICATIONS INFORMATION (cont.)
2. Inductor Selection (L)
A. Preliminary Calculations
To select the inductor, the calculation of the following
three parameters is necessary:
If Lmin is smaller than the inductor values found in step
B1, go on to step C. Otherwise, the inductor value found
in step B1 is too low; an appropriate inductor code
should be obtained from the graph as follows:
Dmax, the maximum switch duty cycle (0 ≤ D ≤ 0.9):
1. Find the lowest value inductor that is greater than
Lmin .
2. Find where E • T intersects this inductor value to
determine if it has an L or H prefix. If E • T intersects
both the L and H regions, select the inductor with an
H prefix.
Dmax =
VOUT + VF − VINmin
VOUT + VF − 0.6V
where typically VF = 0.5V for Schottky diodes and VF =
0.8V for fast recovery diodes.
E • T, the product of volts • time that charges the inductor:
E•T=
C. Inductor Selection
Select an inductor from the table of Figure 5 which cross
references the inductor codes to the part numbers of the
three different manufacturers. The inductors listed in this
table have the following characteristics:
Dmax • (VINmin − 0.6V)106
(V• µs)
52,000Hz
IIND, DC, the average inductor current under full load:
IIND, DC =
AIE (ferrite, pot-core inductors): Benefits of this type
are low etectromagnetic interference (EMI), small
physical size, and very low power dissipation (core
loss).
1.05 • ILOADmax
1 − Dmax
B. Identify Inductor Value:
1. From Figure 4, identify the inductor code for the region
indicated by the intersection of E • T and IIND, DC. This
code gives the inductor value in microhenries. The L or H
prefix signifies whether the inductor is rated for a maximum E • T of 90Vµs (L) or 250Vµs (H).
Pulse (powdered iron, toroid core inductors): Benefits are low EMI and ability to withstand E • T and
peak current above rated value better than ferrite
cores.
Renco (ferrite, bobbin-core inductors): Benefits are
low cost and best ability to withstand E • T and peak
current above rated value. Be aware that these inductors generate more EMI than the other types, and
this may interfere with signals sensitive to noise.
2. If D < 0.85, go to step C. If D ≥ 0.85, calculate the
minimum inductance needed to ensure the switching
regulator’s stability:
200
H2200
150
H1500
H1000
H680
H470
H330
H220
100
90
H150
E·T (V·µs)
80
70
L680
60
50
45
40
L470
L330
L220
L150
L100
L68
35
30
L47
25
20
0.3 0.35 0.4 0.45 0.5 0.6 0.7 0.8 0.9 1.0
1.5
2.0
2.5
3.0
IIND, DC (A)
Note: This chart assumes that the inductor ripple current inductor is approximately 20% to 30% of the average inductor current
(when the regulator is under full load). Greater ripple current causes higher peak switch currents and greater output ripple voltage. Lower ripple current is achieved with larger value inductors. The factor of 20% to 30% is chosen as a convenient balance
between the two extremes.
Figure 4. Inductor Selection Graph
5
UC2577-ADJ
APPLICATIONS INFORMATION (cont.)
Inductor
Code
L47
L68
L100
L150
L220
L330
L470
L680
H150
H220
H330
H470
H680
H1000
H1500
H2200
Manufacturer’s Part Number
AIE
Pulse
Renco
415 - 0932
PE - 53112
RL2442
415 - 0931
PE - 92114
RL2443
415 - 0930
PE - 92108
RL2444
415 - 0953
PE - 53113
RL1954
415 - 0922
PE - 52626
RL1953
415 - 0926
PE - 52627
RL1952
415 - 0927
PE - 53114
RL1951
415 - 0928
PE - 52629
RL1950
415 - 0936
PE - 53115
RL2445
430 - 0636
PE - 53116
RL2446
430 - 0635
PE - 53117
RL2447
430 - 0634
PE - 53118
RL1961
415 - 0935
PE - 53119
RL1960
415 - 0934
PE - 53120
RL1959
415 - 0933
PE - 53121
RL1958
415 - 0945
PE - 53122
RL2448
COUT ≥
C. Calculate the minimum value of CC.
CC ≥
58.5 • VOUT2 • COUT
RC2 • VINmin
The compensation capacitor is also used in the soft start
function of the regulator. When the input voltage is applied 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 C C
should be equal or greater than 0.22µF.
Figure 6 lists several types of aluminum electrolytic capacitors 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.
Figure 5. Table of Standardized Inductors and
Manufacturer’s Part Numbers
Ripple Current: This is the maximum RMS value of current that charges the capacitor during each switching cycle. For step-up and flyback regulators, the formula for
ripple current is:
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 inductor 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 compensation. To guarantee optimum compensation a standard procedure for testing loop stability is recommended,
such as measuring VOUT transient responses to pulsing
ILOAD.
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 values 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 ≤
A. Calculate the maximum value for R C.
750 • ILOADmax • VOUT2
8.7 • 10−3 • VIN
0.01 • 15V
and ≤
where
IRIPPLE(P−P)
ILOADmax
IRIPPLE(P−P) =
VINmin2
Select a resistor less than or equal to this value, not to
exceed 3kΩ.
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 electrolytic 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.
B. Calculate the minimum value for COUT using the following two equations.
COUT ≥
487,800 • VOUT3
The larger of these two values is the minimum value that
ensures stability.
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
RC ≤
VINmin • RC • (VINmin + (3.74 • 105 • L))
0.19 • L • RC • ILOADmax
and
VINmin • VOUT
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.
6
UC2577-ADJ
APPLICATIONS INFORMATION (cont.)
4. Input Capacitor Selection (CIN)
To reduce noise on the supply voltage caused by the
switching action of a step-up regulator (ripple current
noise), VIN should be bypassed to ground. A good quality 0.1µF capacitor with low ESR should provide sufficient decoupling. If the UC2577 is located far from the
supply source filter capacitors, an additional electrolytic
(47µF, for example) is required.
VOUTmax
20V
30V
40V
Nichicon - Types PF, PX, or PZ
927 East StateParkway, Schaumburg, IL 60173
(708)843-7500
50V
United Chemi-CON - Types LX, SXF, or SXJ
9801 West Higgens, Rosemont, IL 60018
(708)696-2000
Schottky
1A
3A
1N5817
1N5820
MBR120P MBR320P
1N5818
1N5821
MBR130P MBR330P
11DQ03
31DQ03
1N5819
1N5822
MBR140P MBR340P
11DQ04
31DQ04
MBR150 MBR350
11DQ05
31DQ05
100V
Figure 6. Aluminum Electrolytic Capacitors Recommended
for Switching Regulators
Fast Recovery
1A
3A
1N4933
MUR105
1N4934
MUR110
10DL1
MR851
30DL1
MR831
MBRxxx and MURxxx are manufactured by Motorola.
1DDxxx, 11Cxx and 31Dxx are manufactured by
International Rectifier
5. Output Diode Selection (D)
In the step-up regulator, the switching diode must withstand a reverse voltage and be able to conduct the peak
output current of the UC2577. Therefore a suitable diode
must have a minimum reverse breakdown voltage
greater than the circuit output voltage, and should also
be rated for average and peak current greater than
ILOADmax and IDpk. Because of their low forward voltage
drop (and thus higher regulator efficiencies), Schottky
barrier diodes are often used in switching regulators. Refer to Figure 7 for recommended part numbers and voltage ratings of 1A and 3A diodes.
Figure 7. Diode Selection Chart
ORDERING INFORMATION
Unitrode Type Number
UC2577T-ADJ
5 Pin TO-220 Plastic Package
UC2577TD-ADJ 5 Pin TO-263 Plastic Package
UNITRODE CORPORATION
7 CONTINENTAL BLVD. • MERRIMACK, NH 03054
TEL. (603) 424-2410 • FAX (603) 424-3460
7
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