UCC3954

PRELIMINARY INFORMATION

6/98

FEATURES

• Converts Lithium-Ion Cell to

+3.3V at 700mA Load

Current

• Load Disconnect in

Shutdown

• High Efficiency Flyback

Operation

• Internal 0.15Ω Switch

• Low Battery LED Driver

• Internal 2A Current Limit

• Internal 200kHz Oscillator

• 8 Pin D, N, 14 Pin PW

Packages

Single Cell Lithium-Ion to +3.3V Converter

BLOCK DIAGRAM

UDG-96137-1

DESCRIPTION

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 fre-

quency 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

Ω power switch provides protection in the case of an output short circuit. When

left open, an internal 100k

Ω resistor pulls the SD pin to BAT–, which puts the

UCC3954 in shutdown mode, and thereby reduces power consumption to sub-

µAlev-

els. 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 prede-

fined 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 acti-

vated when the battery is discharged to 2.55V. In lockout mode, the part consumes

15

µA. 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 se-

ries 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.