LTC3643
11
3643fb
For more information www.linear.com/LTC3643
OPERATION
Boost Mode Startup
The LTC3643 will begin to operate if the RUN voltage is
raised above 1.2V and V
IN
is greater than 3V. In order
to prevent conduction from the V
IN
to CAP through the
body diode of the top switch, an input disconnect switch
is placed between V
IN
and INDIS. The body diode of the
disconnect switch and that of the top switch
are opposite
in polarity in order to prevent conduction of current when
CAP is less than V
IN
. The gate of the disconnect switch
remains grounded until the regulator begins to operate.
Once started, an internal charge pump will slowly charge
up the gate of the disconnect switch and regulate that
gate voltage such that only 100mA of current is flowing
through the switch. That 100mA will thus
conduct through
the inductor between INDIS and SW, and the body diode
of the top switch to charge up the capacitor at the CAP
node. The 100mA limit is set as a way to limit the power
dissipation through the disconnect switch and the top
switch, and protect those switches from overheating.
During this period, the voltage at INDIS and SW will be
clamped at a
diode above CAP.
Once the INDIS voltage approaches V
IN
, and it is sensed
that the internal disconnect switch is fully enhanced,
an internal signal is issued to allow the top and bottom
switches to operate as needed to charge the CAP voltage
up to its programmed value.
Boost Output Over-Current/Short Operation
The current through the disconnect switch is constantly
being monitored to resolve overcurrent situations.
Once
the switch is fully enhanced, if the current through the
disconnect switch ever exceeds 4.8A, a signal is sent to
the control circuitry to keep the top switch on indefinitely
until that current level subsides.
During an output short situation, if the CAP voltage were
to ever collapse below the V
IN
voltage, then simply leaving
the top switch on to discharge the inductor current would
not
work. In these situations, if the current through the
disconnect switch ever exceeds 8A, the gate of the discon-
nect switch is immediately pulled down, thus effectively
shutting off the disconnect switch. At the same time, both
the top and bottom switches are shut off. The current in
the inductor will then conduct through the body diode of
the top switch and decrease down to 0A. From
there, the
internal charge pump will slowly charge back up the gate
of the disconnect switch and regulate the current through
the switch to 100mA, much like the case in startup.
Buck Mode Control Loop
If the PFI voltage falls below 1.15V, power will be delivered
from CAP to V
IN
. If the system load at V
IN
is high enough,
the regulator will switch continuously. In a
typical cycle,
the top switch is turned on for a fixed duration (on-time).
Once that duration expires, the top switch turns off and
the bottom switch is turned on; inductor current is allowed
to discharge until a valley current level is reached, at a
threshold set by an internally compensated I
TH
voltage.
Once that current level is reached, the bottom switch
turns off and the top
switch turns back on again and the
cycle repeats.
The on-time is once again calculated such that in steady
state, the regulator will operate at a frequency around
1MHz. Thus, the on-time is equal to:
t
ON−BUCK
= 1µs•
IN
V
The buck mode control loop is compensated by an internal
RC network and the external ITH pin is grounded through
an internal switch.
Buck Mode Low Current Operation
When V
IN
reaches the programmed regulation voltage and
a minimal load is present, the regulator will automatically
transition into Burst Mode operation. In this mode, the ITH
voltage will transition above and below the sleep threshold
depending on the
V
IN
voltage. If the V
IN
voltage decreases
slightly below its regulation point, the regulator will wake
up from sleep, and the top switch will turn on for a fixed
duration, t
ON-BUCK
. Then, the top switch will turn off and the
bottom switch will turn on until the inductor current reaches
0A. This switching cycle will repeat until the V
IN
voltage
has replenished enough
to force the I
TH
voltage below the
sleep threshold. Once that happens, both switches are
turned off and the quiescent current is decreased to 400µA.
