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LTC3350

3350fc

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Digital Configuration

Although the LTC3350 has extensive digital features, only

a few are required for basic use. The shunt voltage should

be programmed via the vshunt register if a value other

than the default 2.7V is required. The capacitor voltage

feedback reference defaults to 1.2V; it may be changed

in the vcapfb_dac register.

All other digital features are optional and used for moni-

toring. The ADC automatically runs and stores conver-

sionstoregisters(e.g.,meas_vcap).CapacitanceandESR

measurements only run if requested, however, they may

bescheduledtorepeatifdesired(ctl_strt_capesrandcap_

esr_per). Each measured parameter has programmable

limits (e.g., vcap_uv_lvl and vcap_ov_lvl) which may

trigger an alarm and SMBALERT when enabled. These

alarms are disabled by default.

Capacitor Configuration

The LTC3350 may be used with one to four supercapaci-

tors. If less than four capacitors are used, the capacitors

must be populated from CAPRTN to CAP4, and the unused

CAP pins must be tied to the highest used CAP pin. For

example, if three capacitors are used, CAP4 should be tied

to CAP3. If only two capacitors are used, both CAP4 and

CAP3 should be tied to CAP2. The number of capacitors

used must be programmed on the CAP_SLCT0 and

CAP_SLCT1 pins by tying the pins to VCC2P5 for a one

and ground for a zero as shown in Table 1. The value

programmed on these pins may be read back from the

num_caps register via I2C/SMBus.

Table 1

CAP_SLCT1

CAP_SLCT0

num_caps

NUMBER OF

CAPACITORS

Capacitor Shunt Regulator Programming

VSHUNT is programmed via the I2C/SMBus interface and

defaults to 2.7V at initial power-up. VSHUNT serves to limit

the voltage on any individual capacitor by turning on a

shunt around that capacitor as the voltage approaches

VSHUNT. CAPRTN, CAP1, CAP2, CAP3 and CAP4 must be

connected to the supercapacitors through resistors which

serve as ballasts for the internal shunts. The shunt cur-

rent is approximately VSHUNT divided by twice the shunt

resistance value. For a VSHUNT of 2.7V, 2.7Ω resistors

should be used for 500mA of shunt current. The shunts

have a duty cycle of up to 75%. The power dissipated in

a single shunt resistor is approximately:

PSHUNT ≈

3VSHUNT

16RSHUNT

andtheresistorsshouldbesizedaccordingly.Iftheshunts

are disabled, make RSHUNT 100Ω.

Since the shunt current is less than what the switcher can

supply, the on-chip logic will automatically reduce the

chargingcurrenttoallowtheshunttoprotectthecapacitor.

This greatly reduces the charge rate once any one shunt is

activated. For this reason, VSHUNT should be programmed

as high as possible to reduce the likelihood of it activating

during a charge cycle. Ideally, VSHUNT would be set high

enough so that any likely capacitor mismatches would not

cause the shunts to turn on. This keeps the charger operat-

ing at the highest possible charge current and reduces the

charge time. If the shunts never turn on, the charge cycle

completes quickly and the balancers eventually equalize

the voltage on the capacitors. The shunt setting may also

be used to discharge the capacitors for testing, storage

or other purposes.

Setting Input and Charge Currents

The maximum input current is determined by the resis-

tance across the VOUTSP and VOUTSN pins, RSNSI. The

maximum charge current is determined by the value of

the sense resistor, RSNSC, used in series with the induc-

tor. The input and charge current loops servo the voltage

across their respective sense resistor to 32mV. Therefore,

the maximum input and charge currents are:

IIN(MAX) =

32mV

RSNSI

ICHG(MAX) =

32mV

RSNSC

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