LTC3355_15 Datasheet, PDF (10/20 Pages) Linear Technology – 20V 1A Buck DC/DC with Integrated SCAP Charger and Backup Regulator

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 LTC3355 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
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