LTC4155 datasheet(17/26 Pages) AD

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LTC4041

Rev A

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Programming the Supercapacitor Charge Voltage

The charge voltage for a supercapacitor or a stack of

supercapacitors is set by an external resistor divider as

shown in Figure 2. The charge voltage is given by the

following equation:

VCHG = 0.8V • 1+

RFB1

RFB2

⎛

⎝

⎜

⎜⎜

⎞

⎠

⎟

⎟⎟

where 0.8V is the typical CAPFB pin servo voltage

(VCAPFB). Typical values for RFB1 and RFB2 are in the range

of 40k to 2MΩ. Small resistor values result in higher leak-

age current that will discharge the supercapacitor. If the

resistor values are too large, the parasitic capacitance on

the CAPFB pin could create an additional pole and cause

loop instability.

APPLICATIONS INFORMATION

Programming the Input Voltage Threshold for the

Power-Fail Comparator

The input voltage threshold below which the power-fail

status pin PFO indicates a power-fail condition and the

LTC4041 activates the backup boost operation can be

programmed by using a resistor divider from the supply

to GND via the PFI pin such that:

VIN(PF) = 1.19V • 1+

RPF1

RPF2

⎛

⎝

⎜

⎜⎜

⎞

⎠

⎟

⎟⎟

where 1.19V is the typical power fail threshold voltage

(VPFI). See Block Diagram. The power fail threshold volt-

age should be set to a level between 200mV to 300mV

Figure 2. Programming the Charge Voltage

below the nominal input supply voltage so that supply

transients do not trip the comparator. On the other hand,

it should be set high enough so that the VSYS voltage

does not drop enough to trip the SYSGD comparator dur-

ing the transition to backup mode. For applications using

the overvoltage protection (OVP) module, select a value

greater than 35k for RPF1.

Programming the Supercapacitor Charge Current

Supercapacitor charge current is programmed using a

single resistor from the PROG pin to ground. To set a

charge current of ICHG, the PROG pin resistor value can

be determined using the following equation:

RPROG = 2500 •

0.8V

ICHG

2000V

ICHG

where 0.8V is the typical PROG pin servo voltage (VPROG).

For example, to set the charge current to 1A, the value

of the PROG pin resistor should be 2k. The minimum

recommended charge current is 500mA, below which the

accuracy of the charge current suffers. This corresponds

to a maximum RPROG resistor of 4k. The maximum charge

current is 2.5A.

Programming the Input Current Limit and IMON

Monitor

The input current limit is programmed by connecting a

series resistor between the VIN and CLN pins. To limit the

total system current to ISYSLIM, the value of the required

resistor can be calculated using the following equation:

RS =

25mV

ISYSLIM

For example, to set the current limit to 2A, the series

resistor should be 12.5mΩ. As discussed in the Operation

section, the LTC4041 does not limit the system current

but reduces the charge current to zero in the event the

system load exceeds this limit.

LTC4041

RFB1

RFB2

4041 F02

SCAP

CAPFB

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