Datasheet

TLE 6389
Datasheet Rev. 2.1 30 2007-08-13
Figure 4 Reset timing
7.7 100% duty-cycle operation and dropout
The TLE 6389 operates with a duty cycle up to 100%. This feature allows to operate with
the lowest possible drop voltage at low battery voltage as it occurs at cold cranking. The
MOSFET is turned on continuously when the supply voltage approaches the output
voltage level, conventional switching regulators with less than 100% duty cycle would fail
in that case.
The drop- or dropout voltage is defined as the difference between the input and output
voltage levels when the input is low enough to drop the output out of regulation. Dropout
depends on the MOSFET drain-to-source on-resistance, the current-sense resistor and
the inductor series resistance. It is proportional to the load current:
t
t
V
VOUT, RT
V
FB,RT
t
rr
< t
rr
t
t
rd
thermal
shutdown
under
voltage
over
load
t
rd
t
rd
t
rd
V
IN
V
VOUT
V
FB
V
RO
V
drop
I
LOAD
R
DS ON()PMOS
R
SENSE
R
INDUCTANCE
++()⋅=