UCC1884 Unitrode Semiconductor, UCC1884 Datasheet

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UCC1884

Manufacturer Part Number
UCC1884
Description
Frequency Foldback Current Mode PWM Controller
Manufacturer
Unitrode Semiconductor
Datasheet
Frequency Foldback Current Mode PWM Controller
FEATURES
BLOCK DIAGRAM
12/96
Frequency Foldback Reduces Operating
Frequency Under Fault Conditions
Accurate Programmable Volt-Second
Clamp
Programmable Maximum Duty Cycle
Clamp
Oscillator Synchronization
Overcurrent Protection
Shutdown with Full Soft Start
Wide Gain Bandwidth Amplifier
(GBW > 2.5MHz)
Current Mode Operation
Precision 5V Reference
DESCRIPTION
The UCC3884 is a high performance current mode PWM controller
intended for single ended switch mode power supplies. The chip im-
plements a frequency foldback scheme that decreases the oscillator
frequency as the output voltage falls below a programmed value.
This technique decreases the average output current sourced into a
low impedance load which can occur during an output short circuit or
overload condition. Excessive short circuit current is more prevalent
in high frequency converters where the propagation delay and switch
turn-off time forces a minimum attainable duty cycle. An accurate
volt-second clamp limits the duty cycle during line or load transient
conditions which could otherwise saturate the transformer. The volt-
second clamp may also be used with an external overvoltage protec-
tion circuit to handle fault conditions such as current sense
disconnect or current transformer saturation. The frequency fold-
back, volt-second clamp, cycle-by-cycle current limit, and overcur-
rent shutdown provide a rich set of protection features for use in
peak current mode pulse width modulators.
PRELIMINARY
UCC1884
UCC2884
UCC3884
UDG-96026-1

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UCC1884 Summary of contents

Page 1

... The frequency fold- back, volt-second clamp, cycle-by-cycle current limit, and overcur- rent shutdown provide a rich set of protection features for use in peak current mode pulse width modulators. UCC1884 UCC2884 UCC3884 PRELIMINARY UDG-96026-1 ...

Page 2

... Section of Databook for thermal limitations and considerations of packages. CONNECTION DIAGRAMS DIL-16, SOIC-16 (Top View Packages ELECTRICAL CHARACTERISTICS: UCC1884, –40°C to 85°C for the UCC2884, and 0°C to 70°C for the UCC3884 150pF, R VREF, VVS = 0V, CSS = 2.5nF, VDD = 11V, Output no load, T PARAMETER 5V Reference Section VREF ...

Page 3

... ELECTRICAL CHARACTERISTICS (cont.): for UCC1884, –40°C to 85°C for the UCC2884, and 0°C to 70°C for the UCC3884 150pF, R VREF, VVS = 0V, CSS = 2.5nF, VDD = 11V, Output no load, T PARAMETER Error Amplifier Section (cont PWM Section Minimum Duty Cycle Maximum Duty Cycle ...

Page 4

... When VOUT increases, the frequency increases by OFF increasing the discharge current. The maximum operat- ing frequency occurs when VOUT = 3.5V. The C time is constant to guarantee a maximum output duty cy- cle. This pin must be above 3.0V to allow synchronization to occur. 4 UCC1884 UCC2884 UCC3884 . This is a variable dis VOUT/R . The OFF ...

Page 5

... IC is not currently in a soft start cycle bias at the PWM comparator’s non-inverting input and a reset domi- nant PWM latch permit zero duty cycle operation. technique The error amplifier has a wide gain-bandwidth product and its non-inverting input is internally set to 2.5VDC. 5 UCC1884 UCC2884 UCC3884 ...

Page 6

... CLKSYNC signal as shown on the Oscillator Synchronization Diagram (Figure 2). The fastest (master) CLKSYNC pin will couple a negative pulse into the slower (slave) CLKSYNC pins forcing the slaves’ CT pins to quickly discharge as shown on the Oscillator Wave- form diagram (Figure 3). 6 UCC1884 UCC2884 UCC3884 UDG-96029-1 UDG-96028-1 ...

Page 7

... This guarantees that if a slave unit tries to synchronize the master, the master frequency will still be faster than the slave fre- quency and the master will synchronize all the re- maining units. 7 UCC1884 UCC2884 UCC3884 resistor which pro- resis- OFF ...

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