UC1573
UC2573
UC3573
DESCRIPTION
The UC3573 is a Buck pulse width modulator which steps down and regulates a positive input voltage. The chip is optimized for use in a single inductor buck switching converter employing an external PMOS switch. The
block diagram consists of a precision reference, an error amplifier configured for voltage mode operation, an oscillator, a PWM comparator with
latching logic, and a 0.5A peak gate driver. The UC3573 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. Input current can be sensed and limited to a user determined maximum value. In addition, a sleep comparator interfaces to the UVLO circuit
which turns the chip off when the input voltage is below the UVLO threshold. This reduces the supply current to only 50µA, making the UC3573
ideal for battery powered applications.
Buck Pulse Width Modulator Stepdown Voltage Regulator
FEATURES
• Simple Single Inductor Buck PWM
Stepdown Voltage Regulation
• Drives External PMOS Switch
• Contains UVLO Circuit
• Includes Pulse-by-Pulse Current Limit
• Low 50µA Sleep Mode Current
SLUS346 - APRIL 1999
BLOCK DIAGRAM
UDG-94106-1
2
UC1573
UC2573
UC3573
DIL-8, SOIC-8 (TOP VIEW)
J or N, D Packages
ABSOLUTE MAXIMUM RATINGS
VCC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35V
EAINV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –0.6V to VCC
I
EAOUT
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25mA
RAMP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –0.3V to 4V
CS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –0.3V to VCC
I
OUT
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –0.7A to 0.7A
I
3VREF
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –15mA
Storage Temperature . . . . . . . . . . . . . . . . . . . –65°C to +150°C
Junction Temperature. . . . . . . . . . . . . . . . . . . –65°C to +150°C
Lead Temperature (Soldering, 10 sec.). . . . . . . . . . . . . +300°C
Currents are positive into, negative out of the specified terminal. Consult Packaging Section of Databook for thermal limitations and considerations of packages.
CONNECTION DIAGRAMS
ELECTRICAL CHARACTERISTICS:
Unless otherwise specified, these parameters apply for TA= –55°C to +125°C for
the UC1573, –40°C to +85°C for the UC2573, and 0°C to +70°C for the UC3573, VCC = 5V, CT = 680pF, TA= TJ.
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
= 0 to –5mA 1 10 mV
Oscillator Section
Frequency VCC= 5V, 30V 85 100 115 kHz
Error Amp Section
EAINV EAOUT = 2V 1.45 1.5 1.55 V
I
EAINV
EAOUT = 2V –0.2 –1 µA
AVOL EAOUT = 0.5V to 3V 65 90 dB
EAOUT High EAINV = 1.4V 3.6 4 4.4 V
EAOUT Low EAINV = 1.6V 0.1 0.2 V
I
EAOUT
EAINV = 1.4V, EAOUT = 2V –350 –500 µA
EAINV = 1.6V, EAOUT = 2V 7 20 mA
Unity Gain Bandwidth T
J
= 25°C, F = 10kHz 0.6 1 MHz
Current Sense Comparator Section
Threshold (referred to VCC) –0.39 –0.43 –0.47 V
Input Bias Current CS = VCC 150 800 nA
CS Propagation Delay 400 ns
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
= 25°C, C
LOAD
= 1nF + 3.3 Ohms 30 80 ns
Fall Time T
J
= 25°C, C
LOAD
= 1nF + 3.3 Ohms 30 80 ns
Pulse Width Modulator Section
Maximum Duty Cycle EAINV = 1.4V 92 96 %
Minimum Duty Cycle EAINV = 1.6V 0 %
Modulator Gain EAOUT = 1.5V to 2.5V 25 35 45 %/V
Undervoltage Lockout Section
Start Threshold 3.5 4.2 4.5 V
Hysteresis 100 200 300 mV