LTC3355
10
3355fb
For more information www.linear.com/LTC3355
OPERATION
The LTC3355 is a 1A buck regulator with a built-in
backup boost converter to allow temporary backup, or
ride-through, of V
OUT
during a sudden loss of V
IN
power.
The device contains all functions necessary to provide
seamless charging of a supercapacitor (or other storage
element), monitoring of V
IN
, V
OUT
and V
CAP
, and automatic
switch-over to backup power. When the buck is
disabled
an internal circuit blocks reverse current between V
OUT
and V
IN
.
Start-Up
When the part first starts up the only voltage available
is V
IN
since V
OUT
and V
CAP
are at zero volts. An internal
2.5V regulator powers INTV
CC
from V
IN
during start-up.
INTV
CC
powers all of the low voltage circuits. The buck
regulator is enabled and will drive V
OUT
positive through
an inductor until the feedback voltage at FB equals 0.8V.
When V
OUT
exceeds 2.5V INTV
CC
will exactly track V
OUT
and the current for the internal low voltage circuits will
now be supplied from V
OUT
instead of V
IN
. A 1µF external
ceramic capacitor is required for INTV
CC
to filter internal
switching noise.
Buck Switching Regulator
The LTC3355 uses a 1MHz constant frequency peak cur-
rent mode nonsynchronous monolithic buck regulator
with internal slope compensation to control the voltage at
V
OUT
when V
IN
is available. An error amplifier compares
the divided output voltage at FB with a reference voltage
of 0.8V and adjusts the peak inductor current accordingly.
Burst Mode operation can also be selected to optimize ef-
ficiency at low load currents via the MODE pin. The buck
is in
PWM mode when the MODE pin is low and in Burst
Mode operation when the MODE pin is high. The buck
is internally compensated and can operate over an input
voltage range of 3V to 20V. An internal soft-start ramp
limits inrush current during start-up. Frequency foldback
protection helps to prevent inductor current runaway
during start-up or short-circuit conditions.
Input Current Limit
The (optional
) input current limit is programmed via an
external sense resistor connected between V
INS
and V
IN
.
As the input current limit is reached the charge current
will be reduced. If the charge current has been reduced to
zero and the input current continues to increase the buck
regulator current drive capability will be reduced. The
maximum sense voltage is 50mV. The input current limit
includes the LTC
3355 quiescent current for high accuracy
over a wide current range.
Boost Switching Regulator
When V
IN
is not available, a monolithic 1MHz constant
frequency peak current mode boost regulator with internal
slope compensation is enabled and the buck regulator
is disabled via the PFI pin. The boost regulator uses the
voltage stored at V
CAP
as an input supply and regulates
the V
OUT
voltage. An error amplifier compares the divided
output voltage at FB with a reference voltage of 0.8V and
adjusts the peak inductor current accordingly. The I
BSTPK
pin sets the peak boost current over a range of 1A to 5A
allowing for lower current backup applications. The boost
switching regulator is compensated by adding a series RC
network from the V
CBST
pin to ground. The boost regulator
can operate over an
input voltage range (V
CAP
) of 0.5V
to 5V. The boost regulator uses the same feedback pin
and error amplifier as the buck and regulates to the same
V
OUT
voltage. The MODE pin is used to control the boost
switching regulator mode. The boost is in PWM mode
when the MODE pin is low and in Burst Mode operation
when the MODE pin is high. In PWM
mode as the load
current is decreased, the switch turns on for a shorter
period each cycle. If the load current is further decreased,
the boost converter will skip cycles to maintain output
voltage regulation.
Charger
The supercapacitor is charged by an internal 1A constant
current/constant voltage linear charger that supplies
current from V
OUT
to V
CAP
. The charger will be enabled
when V
IN
is above a programmable voltage via the PFI
pin, when the EN_CHG pin is high and when V
OUT
is in
regulation. The value of the resistor on the I
CHG
pin de-
termines the charger current. An internal amplifier servos
the I
CHG
voltage to 0.8V to create the reference current
for the charge. The V
CAP
voltage is divided down by an
external resistor divider that is connected
to the CFB pin.
A hysteretic comparator compares the CFB voltage to a
