The UCC28704 Off-line flyback controller is a highly integrated, 6-pin primary-side regulated PWM controller for designing high efficiency AC-to-DC power supplies with low standby power consumption to comply with global efficiency standards. The controller has ultra-low current consumption at start-up to enable designs with <30-mW no-load input power and save standby mode energy consumption. Intelligent primary-side sensing and control enables 5% output voltage and current control without using an opto coupler or secondary-side feedback circuits.
UCC28704 incorporates an enhanced load transient response technique which helps minimize the output capacitor, reducing the overall system size and cost. The controller also eliminates the need for loop compensation components which simplify the design and debugging process for power supply designers. The converter output voltage and current is regulated to prevent overstress conditions that may damage the load or connectors. Similarly, the CCUV shutdown feature monitors for an output under voltage fault to protect connectors from over heat or burn out from soft shorts, which greatly enhance overall system reliability. Board or component over temperature protection is facilitated with an NTC interface pin.
UCC28704 can be readily used with TI secondary-side synchronous rectifier (SR) controllers at 2-A or higher output current levels for higher conversion efficiency or more compact designs.
PART NUMBER | PACKAGE | BODY SIZE (NOM) |
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UCC28704 | SOT23-6 | 2.90 mm × 1.60 mm |
DATE | REVISION | NOTES |
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February 2016 | A | Initial release. |
PIN | I/O(1) | DESCRIPTION | ||
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NAME | NO. | |||
NTC/SU | 1 | I/O | NTC/SU is a multi-function pin. First, it provides an interface to an external NTC (negative temperature coefficient) resistor for remote temperature sensing. Pulling this pin low shuts down PWM action. Additionally, when used with an external depletion-mode FET and a low-voltage NPN transistor this pin provides high-voltage start up control. A maximum 100-pF noise filter capacitance may be added to this pin. The pin should be left floating if not used. | |
VDD | 2 | P | VDD is the bias supply input pin to the device. | |
DRV | 3 | O | DRV is an output pin used to drive the gate of an external high voltage MOSFET switching transistor. | |
CS | 4 | I | CS input connects to a ground referenced current sense resistor in series with the power switch. The resulting voltage is used to monitor and control the peak-primary current. A series resistor is added to this pin to compensate the peak-primary current levels as the AC mains input varies. A small capacitance, up to 30 pF, can be added to this pin to filter the current sense signal. | |
GND | 5 | G | GND pin is both the reference pin for the controller and the low-side return for the drive output. Special care should be taken to return all AC decoupling capacitors as close as possible to this pin and avoid any common-mode signal trace length with analog signal return paths. | |
VS | 6 | I | VS is an input used to provide voltage and timing feedback to the controller. This pin is connected to a voltage divider between an auxiliary winding and GND. The value of the upper resistor of this divider is used to program the AC-mains run-and-stop thresholds and line compensation at the CS pin. Avoid placing a filter capacitor on this input which would interfere with accurate sensing of this waveform. |