LTC3350 datasheet(22/46 Pages) LINER

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LTC3350

3350fc

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applicaTions inForMaTion

The peak inductor current limit, IPEAK, is 80% higher than

the maximum charge current and is equal to:

IPEAK =

58mV

RSNSC

Note that the input current limit does not include the part’s

quiescent and gate drive currents. The total current drawn

by the part will be IIN(MAX) + IQ + IG, where IQ is the non-

switchingquiescentcurrentandIGisthegatedrivecurrent.

Low Current Charging and High Current Backup

The LTC3350 can accommodate applications requiring

low charge currents and high backup currents. In these

applications, program the desired charge current using

RSNSI. The higher current needed during backup can be

set using RSNSC. The input current limit will override the

chargecurrentlimitwhenthesupercapacitorsarecharging

while the charge current limit provides sufficient current

capability for backup operation.

The charge current will be limited to ICHG(MAX) at low

VCAP (i.e., low duty cycles). As VCAP rises, the switching

controller’s input current will increase until it reaches

IIN(MAX). The input current will be maintained at IIN(MAX)

and the charge current will decrease as VCAP rises further.

Some applications may want to use only a portion of the

inputcurrentlimittochargethesupercapacitors.Twoinput

current sense resistors placed in series can be used to

accomplish this as shown in Figure 3. VOUTSP is kelvin

connected to the positive terminal of RSNSI1 and VOUTSN

is kelvin connected to the negative terminal of RSNSI2.

The load current is pulled across RSNSI1 while the input

current to the charger is pulled across RSNSI1 and RSNSI2.

The input current limit is:

32mV = RSNSI1 • ILOAD + (RSNSI1 + RSNSI2) • IINCHG

For example, suppose that only 2A of input current is de-

sired to charge the supercapacitors but the system load

and charger combined can pull a total of up to 4A from the

supply. Setting RSNSI1 = RSNSI2 = 8mΩ will set a 4A cur-

rent limit for the load + charger while setting a 2A limit for

the charger. With no system load, the charger can pull up

to 2A of input current. As the load pulls 0A to 4A of current

thecharger’sinputcurrentwillreducefrom2Adownto0A.

The following equation can be used to determine charging

input current as a function of system load current:

IINCHG =

32mV

RSNSI1+RSNSI2

–

RSNSI1

RSNSI1+RSNSI2

•ILOAD

ThecontactresistanceofthenegativeterminalofRSNSI1and

the positive terminal of RSNSI2 as well as the resistance of

the trace connecting them will cause variability in the input

currentlimit.Tominimizetheerror,placebothinputcurrent

sense resistors close together with a large PCB pad area

betweenthemasthesystemloadcurrentispulledfromthe

trace connecting the two sense resistors.

Note that the backup current will flow through RSNSI2. The

RSNSI2 package should be sized accordingly to handle the

power dissipation.

Figure 3

VIN

INFET VOUTSP

RSNSI1 RSNSI2

LTC3350

VOUTSN

IINCHG

ILOAD

VOUT (TO SYSTEM)

TGATE

BGATE

3350 F03

Setting VCAP Voltage

The LTC3350 VCAP voltage is set by an external feedback

resistordivider,asshowninFigure4.Theregulatedoutput

voltage is determined by:

VCAP = 1+

RFBC1

RFBC2

⎛

⎝⎜

⎞

⎠⎟

CAPFBREF

where CAPFBREF is the output of the VCAP DAC, pro-

grammed in the vcapfb_dac register. Great care should

be taken to route the CAPFB line away from noise sources,

such as the SW line.

Power-Fail Comparator Input Voltage Threshold

The input voltage threshold below which the power-fail

status pin, PFO, indicates a power-fail condition and the

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