LTC3355 datasheet(11/18 Pages) AD

11 / 18 page

LTC3355-1

Rev. 0

For more information www.analog.com

OPERATION

CFB decays to 30mV below the CFB reference voltage

the charger will be turned on. The LTC3355-1 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 VOUT.

The VCAP 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

VCAP 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.

VIN Status Monitor

The PFI input always monitors the VIN voltage and deter-

mines when VIN is in dropout. VIN is divided down by an

external resistor divider and this voltage is then compared

to a reference voltage of 0.8V. If the PFI voltage is below

the reference voltage the buck regulator and the charger

will be disabled and the boost regulator will be enabled.

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 LTC3355-1 from exces-

sive 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 LTC3355-1. Another

feature is that the charge current can be set according

to typical, rather than worst-case ambient tempera-

tures for a given application with the assurance that the

charger will automatically reduce the charge current in

worst-case conditions.

Thermal Shutdown

The LTC3355-1 includes a thermal shutdown circuit in

addition 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 tempera-

ture drops about 15°C, to approximately 140°C.

VOUT Overvoltage, Undervoltage Lockout

The LTC3355-1 includes an overvoltage protection circuit

to ensure that VOUT does not exceed 5.65V (nominal).

An internal resistor divider from VOUT is connected to an

amplifier that will regulate VOUT as the overvoltage limit

is reached. The LTC3355-1 includes undervoltage lockout

which disables the boost when VOUT is < 2V typical.

LTC3355 Datasheet(PDF) 11 Page - Analog Devices

LTC3355 Datasheet(HTML) 11 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Datasheet Download

Link URL