Operation Down to an Input
Voltage of 1.8 V
High Efficiency Boost, SEPIC or
Flyback (Buck-Boost) Topologies
Drives External FETs for
High-Current Applications
Up to 2-MHz Oscillator
Synchronizable Fixed Frequency
Operation
High-Efficiency Low-Power Mode
High-Efficiency at Very Low-Power
with Programmable Variable
Frequency Mode
Pulse-by-Pulse Current Limit
5-μA Supply Current in Shutdown
150-μA Supply Current in Sleep
Mode
Selectable NMOS or PMOS
Rectification
Built-In Power-On Reset
(UCC39422 Only)
Built-In Low-Voltage Detect
(UCC39422 Only)
simplified schematic block diagram and
application circuit

description
The UCC39421 family of synchronous
PWM controllers is optimized to operate
from dual alkaline/NiCd cells or a single
Lithium-Ion (Li-Ion) cell, and convert to
adjustable output voltages from 2.5 V to
8 V. For applications where the input
voltage does not exceed the output, a
standard boost configuration is used.
For other applications where the input voltage can swing above and below the output, a 1:1 coupled inductor
(Flyback or SEPIC) is used in place of the single inductor. Fixed frequency operation can be programmed, or
synchronized to an external clock source. In applications where (at light loads) variable frequency mode is
acceptable, the IC can be programmed to automatically enter PFM (pulse frequency modulation) mode for an
additional efficiency benefit.
Synchronous rectification provides excellent efficiency at high power levels, where N- or P- type MOSFETs can
be used. At lower power levels (between 10% and 20% of full load) where fixed frequency operation is required,
low power mode is entered. This mode optimizes efficiency by cutting back on the gate drive of the charging
FET. At very low power levels, the IC enters a variable frequency mode (PFM). PFM can be disabled by the user.