Protection IC Datasheet

© 2024 Littelfuse, Inc.

Specifications are subject to change without notice.

Revised: JC. 08/12/24

LS0502SCD33

Super Capacitor Protection IC for Backup Power Applications

8

Operation

The system should have three states: STANDBY State, CHARGE State, DISCHARGE State and UVLO State.

STANDBY State:

1.VCAP > VSYS, VIN is still available and PFB higher than threshold voltage. Or,

2.VCAP reaches setting point by CFB and VIN is still available and PFB higher than threshold. Or,

3.Any of the two super capacitor voltage reaches internal protection point and VIN is still available.

In STANDBY State, M3 FET should be kept OFF. M3 body diode need to be switches according to VSYS VCAP voltage.

CHARGE State:

VSYS > VCAP, VIN is available and healthy; VCAP is below setting point by CFB. Both super capacitors are below overvoltage protection

point. In this state, M3 will turn off with current limit set by ICHG.

Two possible features need to be evaluated:

1. Input DPPM. If input voltage drop and PFB voltage drops close to set point, we can fold back charge current to limit current drain from

weak input source

2. Soft charging ending. When VCAP is charged and CFB voltage getting close to set point, we can fold back charge current so that when

charge stops, voltage drop on any impedance in series with capacitor doesn’t cause oscillation between charge mode/standby mode.

DISCHARGE State:

When PFB is below threshold, we consider input power source is gone. In this case, OVP FET is turned off. Ideal diode should be turned

off too. Power path control FET M3 is turned on to connect VCAP to VSYS. Transition from Charge Mode or Standby Mode to Discharge

Mode need to be smooth without huge inrush current, at the same time, it cannot let VSYS drop too much which might cause downstream

system to shut down.

In Discharge mode, all the bias current is supplied by VCAP. To extend operating time, quiescent current need to be below 1 μA in this

case. Circuits need to be alive are: 1. M3 overcurrent detection circuit; 2. UVLO detection circuit for VCAP.

UVLO State:

In case the system runs on Super Capacitor for long time without recharge, eventually VCAP will drop too low to sustain the function of

the whole system When VCAP drops too low, we will enter UVLO state. In this state, all the circuit is shut down. This state can only be

cleared if VIN is supplied.

Charge Mode

Path: 1 & 2

OVP FET M1:ON

Ideal Diode M2:ON

Power Path M3: ON

Standby Mode

Path: 1

OVP FET M1:ON

Ideal Diode M2:ON

Power Path M3: OFF

UVLO Mode

Turn Off

OVP FET M1:OFF

Discharge Mode

Path: 3

OVP FET M1:OFF

Ideal Diode M2:OFF

Power Path M3: ON

Ideal Diode M2:OFF

Power Path M3: OFF

Charging Done

PFB> Vth

VSYS<VCAP or

CFB ≥ Vth

Vin Good

PFB> Vth

Re-Charging

PFB> Vth

VSYS> VCAP or

CFB< Vth

Vin Good

PFB> Vth

Vin Fault

PFB< Vth

Vin Fault

PFB< Vth

VCAP>UVLO

Vcap Low

PFB< Vth

VCAP<UVLO

PCB Layout Guideline

■ The high current paths (GND, VIN, VSYS and VCAP) should be placed very close to the IC with short, direct and wide traces.

■ Put the input capacitors and VSYS capacitors as close to the VIN/VSYS and GND pins as possible.

■ Keep the VIN and GND pads connected with large copper and use at least two layers for IN and GND trace to achieve better

thermal performance. Also, add several vias with 10 mil_drill/18 mil_copper_width close to the VIN and GND pads to help on

thermal dissipation.

■ Four-layer layout is strongly recommended to achieve better thermal performance.