Features: ` Primary Side Voltage Feedforward Control of Isolated Power Supplies
` Accurate DC Control of Secondary Side Short Circuit Current using Primary Side Average Current Mode Control
` Accurate Programmable Maximum Duty Cycle Clamp
` Maximum Volt-Second Product Clamp to Prevent Core Saturation
` Practical Operation Up to 1MHz
` High Current (2A Pk) Totem Pole Output Driver
` Wide Bandwidth (8MHz) Current Error Amplifier
` Undervoltage Lockout Monitors VCC, VIN and VREF
` Output Active Low During UVLO
` Low Startup Current (500A)Pinout
SpecificationsSupply Voltage (Pin 15) . . . . . . . . . . . . . . . . . . . . . . . . . . .. . 22V
Output Current, Source or Sink (Pin 14)
DC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........ 0.5A
Pulse (0.5s) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........ . 2.2A
Power Ground to Ground (Pin 1 to Pin 13) . . . . . . . . . ...... ±0.2V
Analog Input Voltages
(Pins 3, 4, 7, 8, 12, 16) . . . . . . . . . . . . . . . . . . . . ..........-0.3 to 7V
Analog Input Currents, Source or Sink
(Pins 3, 4, 7, 8, 11, 12, 16) . . . . . . . . . . . . . . . . . . . . . .......... 1mA
Analog Output Currents, Source or Sink (Pins 5 & 10) . . ....... 5mA
Power Dissipation at TA = 60°C . . . . . . . . . . . . . . . . . . . . . . 1W
Storage Temperature Range. . . . . . . . . . . . . . . -65°C to +150°C
Lead Temperature (Soldering 10 seconds) . . . . . . . . . .... +300°C
Notes: All voltages are with respect to ground (DIL and SOIC pin 1). Currents are positive into the specified terminal.
Pin numbers refer to the 16 pin DIL and SOIC packages. Consult Packaging Section of Databook for thermal limitations and considerations of packages.
DescriptionThe UC3548 family of PWM control ICs uses voltage feedforward control to regulate the output voltage of isolated power supplies. The UC3548 resides on the primary side and has the necessary features to accurately control secondary side short circuit current with average current mode control techniques. The UC3548 can be used to control a wide variety of converter topologies.
In addition to the basic functions required for pulse width modulation, the UC3548 implements a patented technique of sensing secondary current from the primary side in an isolated buck derived converter. A current waveform synthesizer monitors switch current and simulates the inductor current downslope so that the complete current waveform can be constructed on the primary side without actual secondary side measurement. This information on the primary side is used by an average current mode control circuit to accurately limit maximum output current.
The UC3548 circuitry includes a precision reference, a wide bandwidth error amplifier for average current control, an oscillator to generate the system clock, latching PWM comparator and logic circuits, and a high current output driver. The current error amplifier easily interfaces with an optoisolator from a secondary side voltage sensing circuit.
A full featured undervoltage lockout (UVLO) circuit is contained in the UC3548. UVLO monitors the supply voltage to the controller (VCC), the reference voltage (VREF), and the input line voltage (VIN). All three must be good before soft start commences. If either VCC or VIN is low, the supply current required by the chip is only 500mA and the output is actively held low.
Two on board protection features set controlled limits to prevent transformer core saturation. Input voltage is monitored and pulse width is constrained to limit the maximum volt-second product applied to the transformer. A unique patented technique limits maximum duty cycle within 3% of a user programmed value.
These two features allow for more optimal use of transformers and switches, resulting in reduced system size and cost.
Both patents embodied in the UC3548 belong to Lambda Electronics Incorporated and are licensed for use in applications employing these devices.
UC1548 Data Sheet
