LTC3355_15 Datasheet, PDF (11/20 Pages) Linear Technology – 20V 1A Buck DC/DC with Integrated SCAP Charger and Backup Regulator

LTC3355
OPERATION
0.8V reference voltage and turns the charger off when
these voltages are the same. The V_CAP voltage represents
the fully charged supercapacitor voltage available to sup-
ply the boost regulator when V_IN has dropped out. When
CFB decays to 30mV below the CFB reference voltage the
charger will be turned on. The LTC3355 includes a soft-
start circuit to minimize the inrush current at the start of
charge. When the charger is enabled, the charge current
ramps from zero to full-scale over a period of approximately
1ms. This has the effect of minimizing the transient load
current on V_OUT.
The V_CAP output also has an overvoltage protection cir-
cuit which monitors the CFB voltage. If the CFB voltage
increases above the CFB reference voltage by 35mV a
hysteretic comparator switches in an 8k resistor from
V_CAP to ground. This will bleed any excess charge from
the supercapacitor. When the CFB voltage decays to the
CFB reference voltage the comparator will remove the
8k bleed resistor. Excess charge can come from leakage
currents associated with the boost rectifier diode.
V_IN Status Monitor
The PFI input always monitors the V_IN voltage and de-
termines when V_IN is in dropout. V_IN is divided down
by an external resistor divider and this voltage is then
compared to a reference voltage of 0.8V. If the PFI volt-
age is below the reference voltage the buck regulator and
the charger will be disabled and the boost regulator will
be enabled. The PFOB pin is a 5V open-drain output. This
pin is driven internally by the PFI comparator. When the
PFI comparator determines that V_IN has dropped out the
PFOB output switches low. PFOB is normally connected
to a low voltage supply, via an external pull-up resistor.
The pull-up resistor for this output can be connected to
V_OUT if another supply is not available.
V_OUT Status Monitor
The RSTB pin is a 5V open-drain output. An internal
comparator determines when V_OUT has reached 92.5% of
the programmed regulation voltage which then switches
the RSTB pin high. RSTB is normally connected to a low
voltage supply (V_OUT) via an external pull-up resistor.
V_CAP Status Monitor
The CPGOOD pin is a 5V open-drain output. An internal
comparator determines when V_CAP has reached 92.5% of
the programmed regulation voltage which then switches
the CPGOOD pin high. CPGOOD is normally connected to a
low voltage supply (V_OUT) via an external pull-up resistor.
Thermal Regulation
As the die temperature increases due to internal power
dissipation, a thermal regulator will limit the die tem-
perature to 110°C by reducing the charger current. The
thermal regulation protects the LTC3355 from excessive
temperature and allows the user to push the limits of the
power handling capability of a given circuit board without
the risk of damaging the LTC3355. Another feature is that
the charge current can be set according to typical, rather
than worst-case ambient temperatures for a given applica-
tion with the assurance that the charger will automatically
reduce the charge current in worst-case conditions.
Thermal Shutdown
The LTC3355 includes a thermal shutdown circuit in ad-
dition to the thermal regulator. If for any reason, the die
temperature exceeds 155°C, the entire part shuts down.
The part will resume normal operation once the temperature
drops about 15°C, to approximately 140°C.
V_OUT Overvoltage, Undervoltage Lockout
The LTC3355 includes an overvoltage protection circuit
to ensure that V_OUT does not exceed 5.65V (nominal).
An internal resistor divider from V_OUT is connected to an
amplifier that will regulate V_OUT as the overvoltage limit
is reached. The LTC3355 includes undervoltage lockout
which disables the boost when V_OUT is < 2V typical.
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