Simple Step-Up Voltage Regulator

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

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

The UC2577-ADJ device provides all the active functions neces-
sary to implement step-up (boost), flyback, and forward converter
switching regulators. Requiring only a few components, these sim-
ple regulators efficiently provide up to 60V as a step-up regulator,
and even higher voltages as a flyback or forward converter regula-
tor.

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 fre-
quency 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.

UC2577-ADJ

3/97

FEATURES

DESCRIPTION

CONNECTION DIAGRAM

BLOCK DIAGRAM

UDG-94034

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

TYPICAL APPLICATIONS

5-Pin TO-220 (Top View)
T Package

Also available in TO-263 Package (TD).

PARAMETER

TEST CONDITIONS

MIN

TYP

MAX

UNITS

System Parameters

Circuit Figure 1 (Note 3)

Output Voltage

VIN = 5V to 10V, I

LOAD

= 100mA to 800mA

11.40

12.0

12.60

= 25

11.60

12.40

Line Regulation

VIN = 3.0V to 10V, I

LOAD

= 300mA

100

= 25

Load Regulation

VIN = 5V, I

LOAD

= 100mA to 800mA

100

= 25

Efficiency

VIN = 5V, I

LOAD

= 800mA

Device Parameters

Input Supply Current

= 1.5V (Switch Off)

7.5

= 25

SWITCH

= 2.0A, V

COMP

= 2.0V (Max Duty Cycle)

= 25

Input Supply UVLO

SWITCH

= 100mA

2.70

2.95

= 25

2.85

Oscillator Frequency

Measured at SWITCH Pin, I

SWITCH

= 100mA

kHz

= 25

kHz

Reference Voltage

Measured at FB Pin, VIN = 3.0V to 40V, V

COMP

= 1.0V

1.206

1.230

1.254

= 25

1.214

1.246

Reference Voltage Line Regulation

VIN = 3.0V to 40V

0.5

Error Amp Input Bias Current

COMP

= 1.0V

100

800

= 25

300

Error Amp Transconductance

COMP

A to +30

A, V

COMP

= 1.0V

1600

3700

5800

mho

= 25

2400

4800

mho

Error Amp Voltage Gain

COMP

= 0.8V to 1.6V, R

COMP

= 1.0MW (Note 4)

250

800

V/V

= 25

500

V/V

Error Amplifier Output Swing

Upper Limit V

= 1.0V

2.0

2.4

= 25

2.2

Lower Limit V

= 1.5V

0.3

0.55

= 25

0.40

Error Amp Output Current

= 1.0V to 1.5V, V

COMP

= 1.0V

200

400

= 25

130

300

Soft Start Current

= 1.0V, V

COMP

= 0.5V

1.5

5.0

9.5

= 25

2.5

7.5

Maximum Duty Cycle

COMP

= 1.5V, I

SWITCH

= 100mA

= 25

Unless otherwise stated, these specifications apply for T

+125

C, VIN =

5V, V

= V

REF

, I

SWITCH

= 0, and T

ELECTRICAL CHARACTERISTICS

RECOMMENDED OPERATING RANGE

Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45V
Output Switch Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65V
Output Switch Current (Note 2) . . . . . . . . . . . . . . . . . . . . . 6.0A
Power Dissipation . . . . . . . . . . . . . . . . . . . . . . Internally Limited
Storage Temperature Range . . . . . . . . . . . . .

C to +150

Lead Temperature (Soldering, 10 sec.) . . . . . . . . . . . . . . 260

Maximum Junction Temperature . . . . . . . . . . . . . . . . . . . 150

Minimum ESD Rating (C = 100pF, R = 15k

) . . . . . . . . . . . 2kV

ABSOLUTE MAXIMUM RATINGS (Note 1)

UC2577-ADJ

Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . 3.0V

VIN

40V

Output Switch Voltage . . . . . . . . . . . . . . . 0V

SWITCH

60V

Output Switch Current . . . . . . . . . . . . . . . . . . . . I

SWITCH

3.0A

Junction Temperature Range . . . . . . . . . .

+125

UC2577-ADJ

PARAMETER

TEST CONDITIONS

MIN

TYP

MAX

UNITS

Device Parameters (cont.)

Switch Transconductance

12.5

A/V

Switch Leakage Current

SWITCH

= 65V, V

= 1.5V (Switch Off)

600

= 25

300

Switch Saturation Voltage

SWITCH

= 2.0A, V

COMP

= 2.0V (Max Duty Cycle)

0.5

0.9

J = 25

0.7

NPN Switch Current Limit

COMP

= 2.0V

3.0

4.3

6.0

Thermal Resistance

Junction to Ambient

C/W

Junction to Case

C/W

COMP Pin Current

COMP

= 0

= 25

Unless otherwise stated, these specifications apply for T

+125

C, VIN =

5V, V

= V

REF

, I

SWITCH

= 0, and T

ELECTRICAL CHARACTERISTICS

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 1.0M

resistor is connected to the compensation pin (which is the error amplifier's output) to ensure accuracy in

measuring A

VOL.

In actual applications, this pin's load resistance should be

10M

, resulting in A

VOL

that is typically twice

the guaranteed minimum limit.

Figure 1. Circuit Used to Specify System Parameters

UDG-94035

L = 415-0930 (AIE)

D = any manufacturer

OUT

= Sprague Type 673D

Electrolytic 680

F, 20V

R1 = 48.7k in series with 511

(1%)

R2 = 5.62k (1%)

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 termi-
nal of the inductor rises above VIN, discharging the
stored current through the output diode (D) into the out-
put capacitor (C

OUT

) at a rate of (V

OUT

- VIN)/L. The en-

ergy stored in the inductor is thus transferred to the
output.

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 ampli-
fier which amplifies the difference between the feedback
voltage and an internal 1.23V precision reference volt-
age. The output of the error amplifier is then compared to
a voltage proportional to the switch current, or the induc-
tor current, during the switch on time. A comparator ter-
minates the switch on time when the two voltages are
equal and thus controls the peak switch current to main-
tain a constant output voltage. Figure 2 shows voltage
and current waveforms for the circuit. Formulas for calcu-
lation are shown in Figure 3.

STEP-UP REGULATOR DESIGN PROCEDURE

Refer to the Block Diagram
Given:

INmin

= Minimum input supply voltage

OUT

= Regulated output voltage

UC2577-ADJ

Step-up (Boost) Regulator

Duty Cycle

OUT

SAT

OUT

Avg. Inductor
Current

IND(AVG)

LOAD

Inductor
Current Ripple

IND

SAT

52,000

Peak Inductor
Current

IND(PK)

LOAD

IND

Peak Switch
Current

SW(PK)

LOAD

IND

Switch Voltage
when Off

SW(OFF)

OUT

+ V

Diode Reverse
Voltage

OUT

- V

SAT

Avg. Diode
Current

D(AVG)

LOAD

Peak Diode
Current

D(PK)

LOAD

IND

Power
Dissipation

0.25

LOAD

(

)

= Forward Biased Diode Voltage, I

LOAD

= Output Load

Figure 2. Step-up Regulator Waveforms

First, determine if the UC2577 can provide these values
of V

OUT

and I

LOADmax

when operating with the minimum

value of V

. The upper limits for V

OUT

and I

LOADmax

are

given by the following equations.

OUT

60V and

OUT

INmin

LOADmax

2.1A

INmin

OUT

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 out-
put voltage. These resistors form a voltage divider and
present a portion of the output voltage to the error ampli-
fier which compares it to an internal 1.23V reference. Se-
lect R1 and R2 such that:

R1
R2

OUT

1.23V

Figure 3. Step-up Regulator Formulas

APPLICATIONS INFORMATION

2. Inductor Selection (L)

A. Preliminary Calculations

To select the inductor, the calculation of the following
three parameters is necessary:

Dmax, the maximum switch duty cycle (0

0.9):

max

OUT

INmin

OUT

0.6V

where typically V

= 0.5V for Schottky diodes and V

0.8V for fast recovery diodes.
E

T, the product of volts

time that charges the induc-

tor:

max

(

INmin

0.6V

)

52,000Hz

(

)

IND, DC

, the average inductor current under full load:

IND,

1.05

LOADmax

max

B. Identify Inductor Value:

1. From Figure 4, identify the inductor code for the region
indicated by the intersection of E

T and I

IND, DC

. This

code gives the inductor value in microhenries. The L or H
prefix signifies whether the inductor is rated for a maxi-
mum E

T of 90V

s (L) or 250V

s (H).

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:

0.3

0.4 0.45

0.35

0.5 0.6 0.7 0.8 0.9 1.0

1.5

2.0

2.5

3.0

100

200

150

L47

L68

L100

L150

L220

L330

H2200

L680

H1500

H1000

H680

H470

H330

H220

H150

L470

E·T (V·

IND, DC

(A)

Figure 4. Inductor Selection Graph

If L

min

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:

Find the lowest value inductor that is greater than
L

min

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.

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:

AIE

(ferrite, pot-core inductors): Benefits of this type

are low etectromagnetic interference (EMI), small
physical size, and very low power dissipation (core
loss).

Pulse

(powdered iron, toroid core inductors): Bene-

fits 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 in-
ductors generate more EMI than the other types, and
this may interfere with signals sensitive to noise.

UC2577-ADJ

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 volt-
age. 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.

APPLICATIONS INFORMATION (cont.)

Part Number UC2577

Document Outline