Datasheet

LTC4081
15
4081f
Zero Current Comparator
The zero or reverse current comparator monitors the induc-
tor current to the output and shuts off the synchronous
rectifi er when this current reduces to a predetermined
value (I
ZERO
). In fi xed frequency mode, this is set to nega-
tive 15mA meaning that the regulator allows the inductor
current to fl ow in the reverse direction (from the output to
ground through the synchronous rectifi er) to a maximum
value of 15mA. This is done to ensure that the regulator
is able to regulate at very light loads without skipping any
cycles thereby keeping output voltage ripple and noise low
at the cost of effi ciency.
However, in Burst Mode operation, I
ZERO
is set to positive
35mA meaning that the synchronous switch is turned off
as soon as the current through the inductor to the output
decreases to 35mA in the discharge cycle. This preserves
the charge on the output capacitor and increases the overall
effi ciency at light loads.
Soft-Start
The LTC4081 switching regulator provides soft-start in
both modes of operation by slowly charging an internal
capacitor. The voltage on this capacitor, in turn, slowly
ramps the current limits of both switches from a low value
to their respective maximum values over a period of about
400
μ s. The soft-start capacitor is discharged completely
whenever the regulator is disabled.
Short-Circuit Protection
In the event of a short circuit at the output or during
start-up, V
OUT
will be near zero volts. Since the downward
slope of the inductor current is ~V
OUT
/L, the inductor
current may not get a chance to discharge enough to
avoid a runaway situation. Because the current sensing
OPERATIO
U
is blanked for ~12ns at the beginning of each clock cycle,
inductor current can build up to a dangerously high level
over a number of cycles even if there is a hard current
limit on the main PMOS switch. This is why the switching
regulator in the LTC4081 also monitors current through
the synchronous NMOS switch and imposes a hard limit
on it. If the inductor current through the NMOS switch at
the end of a discharge cycle is not below this limit, the
regulator skips the next charging cycle thereby preventing
inductor current runaway.
Switching Regulator Undervoltage Lockout
Whenever V
BAT
is less than 2.7V, an undervoltage lock-
out circuit keeps the regulator off, preventing unreliable
operation. However, if the regulator is already running
and the battery voltage is dropping, the undervoltage
comparator does not shut down the regulator until V
BAT
drops below 2.5V.
Dropout Operation
When the BAT pin voltage approaches V
OUT
, the duty cycle
of the switching regulator approaches 100%. When V
BAT
is approximately equal to V
OUT
, the regulator is said to be
in dropout. In dropout, the main switch (MP2) stays on
continuously with the output voltage being equal to the
battery voltage minus the voltage drops across the main
switch and the inductor.
Global Thermal Shutdown
The LTC4081 includes a global thermal shutdown which
shuts off the entire device (battery charger and switch-
ing regulator) if the
die temperature exceeds 160°C. The
LTC4081 resumes normal
operation once the temperature
drops approximately 14°C.