UC1572
UC2572
UC3572
DESCRIPTION
The UC3572 is a negative output flyback pulse width modulator which con-
verts a positive input voltage to a regulated negative output voltage. The
chip is optimized for use in a single inductor negative flyback switching con-
verter employing an external PMOS switch. The block diagram consists of
a precision reference, an error amplifier configured for voltage mode opera-
tion, an oscillator, a PWM comparator with latching logic, and a 0.5A peak
gate driver. The UC3572 includes an undervoltage lockout circuit to insure
sufficient input supply voltage is present before any switching activity can
occur, and a pulse-by-pulse current limit. Output current can be sensed and
limited to a user determined maximum value. The UVLO circuit turns the
chip off when the input voltage is below the UVLO threshold. In addition, a
sleep comparator interfaces to the UVLO circuit to turn the chip off. This re-
duces the supply current to only 50
A, making the UC3572 ideal for battery
powered applications.
Negative Output Flyback Pulse Width Modulator
FEATURES
Simple Single Inductor Flyback PWM
for Negative Voltage Generation
Drives External PMOS Switch
Contains UVLO Circuit
Includes Pulse-by-Pulse Current Limit
Low 50
A Sleep Mode Current
SLUS275A - MARCH 1999 - REVISED AUGUST 2001
BLOCK DIAGRAM
UDG-94094-2
2
UC1572
UC2572
UC3572
ABSOLUTE MAXIMUM RATINGS
VCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35V
EAINV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.6V to VCC
I
EAOUT
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25mA
RAMP. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to 4V
CS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to VCC
Iout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.7A to 0.7A
I
3VREF
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -15mA
Storage Temperature . . . . . . . . . . . . . . . . . . . -65C to +150C
Junction Temperature . . . . . . . . . . . . . . . . . . . -65C to +150C
Lead Temperature (Soldering, 10 sec.) . . . . . . . . . . . . . +300C
Currents are positive into, negative out of the specified
terminal. Consult Packaging Section of Databook for thermal
limitations and considerations of packages.
DIL-8, SOIC-8 (TOP VIEW)
D, N or J Packages
CONNECTION DIAGRAM
TEMPERATURE RANGE
PACKAGE
UC1572
55C to +125C
J
UC2572
40C to +85C
D, N or J
UC3572
0C to +70C
D or N
ORDERING INFORMATION
ELECTRICAL CHARACTERISTICS:
Unless otherwise specified, VCC = 5V, CT = 680pF, T
A
= T
J.
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNITS
Reference Section
3VREF
2.94
3
3.06
V
Line Regulation
VCC = 4.75 to 30V
1
10
mV
Load Regulation
I
3VREF
= 0V to 5mA
1
10
mV
Oscillator Section
Frequency
VCC = 5V to 30V
85
100
115
kHz
Error Amp Section
EAINV
EAOUT = 2V
10
0
10
mV
I
EANV
= 1mA
0.2
0.9
V
I
EAINV
EAOUT = 2V
0.2
1.0
A
AVOL
EAOUT = 0.5V to 3V
65
90
dB
EAOUT High
EAINV = 100mV
3.6
4
4.4
V
EAOUT Low
EAINV = 100mV
0.1
0.2
V
I
EAOUT
EAINV = 100mV, EAOUT = 2V
350
500
A
EAINV = 100mV, EAOUT = 2V
7
20
mA
Unity Gain Bandwidth
T
J
= 25C, F = 10kHz
0.6
1
MHz
Current Sense Comparator Section
Threshold
0.185
0.205
0.225
V
Input Bias Current
CS = 0
0.4
1
A
CS Propogation Delay
300
nS
3
UC1572
UC2572
UC3572
ELECTRICAL CHARACTERISTICS:
Unless otherwise specified, VCC = 5V, CT = 680pF, T
A
= T
J.
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNITS
Gate Drive Output Section
OUT High Saturation
I
OUT
= 0
0
0.3
V
I
OUT
= 10mA
0.7
1.5
V
I
OUT
= 100mA
1.5
2.5
V
OUT Low Saturation
I
OUT
= 10mA
0.1
0.4
V
I
OUT
= 100mA
1.5
2.2
V
Rise Time
T
J
= 25C, C
LOAD
= 1nF + 3.3 Ohms
30
80
nS
Fall Time
T
J
= 25C, C
LOAD
= 1nF + 3.3 Ohms
30
80
nS
Pulse Width Modulator Section
Maximum Duty Cycle
EAINV = +100mV, VCC = 5V to 30V
92
96
%
Minimum Duty Cycle
EAINV = 100mV, VCC = 5V to 30V
0
%
Modulator Gain
EAOUT = 1.5V to 2.5V
45
55
65
%/V
Undervoltage Lockout Section
Start Threshold
3.5
4.2
4.5
V
Hysteresis
100
200
300
mV
Sleep Mode Section
Threshold
1.8
2.2
2.6
V
Supply Current Section
IVCC
VCC = 5V, 30V
9
15
mA
VCC = 30, CS = 3V
50
150
A
Figure 1. Typical waveforms.
UDG-94095
4
UC1572
UC2572
UC3572
4
8
7
1
2
6
5
3
R
SLEEP1
56k
R
SLEEP2
33k
R
COMP
R
REF
RV
SENSE
40k
RSLEEP3
1MEG
R
CS
C
IN
10
F
COUT
100
F
VOUT
GND
12V OUT
D
FLYBACK
L
FLYBACK
M
SWITCH
C
RAMP
680pF
CV
CC
10
F
M
SLEEP
GND
SLEEP
VIN
VCC
3VREF
RAMP
EAINV
EAOUT
GND
CS
OUT
UC1572
C3V
REF
100nF
C
COMP
Figure 2. Typical application: +5V to 12V flyback converter.
UDG-99057
PIN DESCRIPTIONS
3VREF: Precision 3V reference. Bypass with 100nF ca-
pacitor to GND.
CS: Current limit sense pin. Connect to a ground refer-
enced current sense resistor in series with the flyback in-
ductor. OUT will be held high (PMOS switch off) if CS
exceeds 0.2V.
EAINV: Inverting input to error amplifier. Summing junc-
tion for 3VREF and VOUT sense. The non-inverting input
of the error amplifier is internally connected to GND. This
pin will source a maximum of 1mA.
EAOUT: Output of error amplifier. Use EAOUT and EAINV
for loop compensation components.
GND: Circuit Ground.
OUT: Gate drive for external PMOS switch connected
between V
CC
and the flyback inductor. OUT drives the
gate of the PMOS switch between V
CC
and GND.
RAMP: Oscillator and ramp for pulse width modulator.
Frequency is set by a capacitor to GND by the equation
F
k C
RAMP
=
1
15
Recommended operating frequency range is 10kHz to
200kHz.
VCC: Input voltage supply to chip. Range is 4.75 to 30V.
Bypass with a 1
F capacitor.
IMPORTANT NOTICE
Texas Instruments and its subsidiaries (TI) reserve the right to make changes to their products or to discontinue
any product or service without notice, and advise customers to obtain the latest version of relevant information
to verify, before placing orders, that information being relied on is current and complete. All products are sold
subject to the terms and conditions of sale supplied at the time of order acknowledgment, including those
pertaining to warranty, patent infringement, and limitation of liability.
TI warrants performance of its products to the specifications applicable at the time of sale in accordance with
TI's standard warranty. Testing and other quality control techniques are utilized to the extent TI deems necessary
to support this warranty. Specific testing of all parameters of each device is not necessarily performed, except
those mandated by government requirements.
Customers are responsible for their applications using TI components.
In order to minimize risks associated with the customer's applications, adequate design and operating
safeguards must be provided by the customer to minimize inherent or procedural hazards.
TI assumes no liability for applications assistance or customer product design. TI does not warrant or represent
that any license, either express or implied, is granted under any patent right, copyright, mask work right, or other
intellectual property right of TI covering or relating to any combination, machine, or process in which such
products or services might be or are used. TI's publication of information regarding any third party's products
or services does not constitute TI's approval, license, warranty or endorsement thereof.
Reproduction of information in TI data books or data sheets is permissible only if reproduction is without
alteration and is accompanied by all associated warranties, conditions, limitations and notices. Representation
or reproduction of this information with alteration voids all warranties provided for an associated TI product or
service, is an unfair and deceptive business practice, and TI is not responsible nor liable for any such use.
Resale of TI's products or services with
statements different from or beyond the parameters stated by TI for
that product or service voids all express and any implied warranties for the associated TI product or service,
is an unfair and deceptive business practice, and TI is not responsible nor liable for any such use.
Also see: Standard Terms and Conditions of Sale for Semiconductor Products. www.ti.com/sc/docs/stdterms.htm
Mailing Address:
Texas Instruments
Post Office Box 655303
Dallas, Texas 75265
Copyright
2001, Texas Instruments Incorporated
PDF 
ZIP