UCC3954

· Converts Lithium-Ion Cell to +3.3V at 700mA Load Current
· Load Disconnect in Shutdown
· High Efficiency Flyback Operation
· Internal 0.15W Switch
· Low Battery LED Driver
· Internal 2A Current Limit
· Internal 200kHz Oscillator
· 8 Pin D, N, 14 Pin PW Packages

Input Supply Voltage (BAT+ to BAT) . . . . . . . . . . . . . . . . 4.5V
VOUT
Maximum Forced Voltage (ref. to BAT+) . . . . . . .  . . . . . . . 5.5V
SWITCH
Maximum Forced Voltage (ref. to BAT) . . . . . .  . . . . . . . 10.2V
Maximum Forced Current . . . . . . . . . . .  .. . . Internally Limited
SD
Maximum Forced Voltage (ref. to BAT+) . . . . . .  . .. . . . . . 5.5V
Maximum Forced Current. . . . . . . . . . . . . . . . . . . . .  . . . 10mA
COMP
Maximum Forced Voltage (ref. to BAT) . . . . . . . . . . . . . . 4.5V
Maximum Forced Current. . . . . . . . . . . . . . .  . . . . Self Limiting
Storage Temperature . . . . . . . . . . . . . . .  . . 65°C to +150°C
Junction Temperature. . . . . . . . . . . . . . . . . . 55°C to +150°C
Lead Temperature (Soldering, 10 sec.) . . . . . . . . . . . . +300°C

The UCC3954, along with a few external components, develops a regulated +3.3V from a single lithium-ion battery whose terminal voltage can vary between 2.5V and 4.2V. The UCC3954 employs a simple flyback (Buck-Boost) technique to convert the battery energy to +3.3V. This is accomplished by referencing the lithium-ion cell's positive terminal to system ground. The negative terminal of the battery is the return point for the UCC3954. This approach enables the converter to maintain constant frequency operation whether the cell voltage is above or below the output voltage. An additional benefit of this technique is its inherent ability to disconnect the battery from the load in shutdown mode.

The UCC3954 operates as a fixed 200kHz switching frequency voltage mode flyback converter. The oscillator time base and ramp are internally generated by the UCC3954 and require no external components. A 2A current limit for the internal 0.15 power switch provides protection in the case of an output short circuit. When left open, an internal 100k resistor pulls the SD pin to BAT, which puts the UCC3954 in shutdown mode, and thereby reduces power consumption to sub-mA levels. A low battery detect function will drive the LOWBAT pin low (minimum of 5mA sink current) when the battery has been discharged to within 200mV of the predefined lockout voltage. The LOWBAT pin is intended for use with an external LED to provide visual warning that the battery is nearly exhausted. The lockout mode is activated when the battery is discharged to 2.55V. In lockout mode, the part consumes 15mA. Once the UCC3954 has entered lockout mode, the user must insert a fresh battery whose open circuit voltage is greater than 3.1V. This prevents a system-level oscillation of the lockout function due to the lithium-ion battery's large equivalent series resistance.

Additional features of the UCC3954 include a trimmed 1.1V reference and internal feedback scaling resistors, a precision error amplifier, low quiescent current drain in shutdown mode, and a softstart function. The UCC3954 is offered in the 8 pin D, 14 pin PW (surface mount), and N (through hole) packages.