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LTC3106

3106f

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operaTion

present. If an extended duration output short is expected,

protection for the primary battery should be considered.

Start-Up

The LTC3106 will start up from either input voltage source

but gives priority to VIN.TheAUXoutputisinitiallycharged

with the synchronous rectifiers disabled. Once VAUX has

reached its terminal voltage the output voltage is then also

chargedasynchronouslyuntilVOUTreachesapproximately

1.2V.Theconverterthenleavestheasynchronousmodein

favor of a more efficient synchronous start-up mode until

VOUT is in regulation and the part enters normal operation.

ItisnormalfortheoutputvoltagetoriseasVAUXischarging.

The AUXSW switch and the SWDI switch are in parallel so

even when switched off there is still some asynchronous

body diode conduction to the output. The rate at which

this occurs is related to the VAUX/VOUT output capacitor

ratio and operating conditions at start-up (i.e., any static

load on VOUT). A minimum 10:1 ratio of VOUT to VAUX cap

is recommended to allow for proper start-up.

Starting from Very Low Current Input Sources

Many solar cells that are optimized for indoor use have

very low available power at low light levels and therefore

very low output current, often less than 100µA at 200Lux.

If the LTC3106 is to start up using only a weak source on

VINandwithnobackupbatteryonVSTOREtheinputcapaci-

tance must be sized larger than that for normal operation.

Although dependent on the specific operating conditions

for the application, in general, starting from low current

sources on VIN at low light levels alone will require larger

input capacitances than those calculated using the CVIN

equation in the VIN and VOUT Capacitor Selection section.

For example if the LTC3106 application in Figure 14 needs

to start from the AM-1454 solar cell without the benefit

of a battery on VSTORE, the required input capacitance

increases from 470µF to 2.2mF minimum.

If a battery is connected to VSTORE but is disabled by

bringing ENVSTR low and is therefore not used to start the

LTC3106, the input source on VIN needs to have an output

current equal to or greater than 100µA (typ) regardless

of the input capacitor size for the internal VCC decision

circuit to run properly during start up. If the input source

has less than a 100µA capability, startup could stall until

more input current is available from the source or until

the VSTORE battery is enabled. The 100µA limitation also

applies where the LTC3106’s output is used to charge a

battery or a large super capacitor. For typical applications

where the input capacitance is greater than the output

capacitance the 100µA limitation does not apply.

Operating from a Low Power VIN

Controlling the minimum input voltage is essential when

using high impedance or intermittent input sources. The

LTC3106hasseveraloptionsforVINvoltagecontrolduring

start-up and during normal operation.

If a valid VSTORE voltage exists or if VAUX is in regulation,

there are several LTC3106 configurations allowing accu-

rate control at lower input voltages on VIN. The accurate

RUN comparator can be used to control the VIN turn-on

threshold at any arbitrary voltage equal to or above 600mV

as discussed in the Accurate RUN Pin section of this

data sheet. The 300mV UVLO on VIN could also be used

to maintain VIN but is fixed at the 300mV threshold. If a

higher sleep current can be tolerated, the MPP pin can

be used to control VIN at any arbitrary threshold above

300mV. These latter two methods of controlling VIN are

discussed in later sections of the data sheet.

Even if no other input source is present (VSTORE/VCAP

disabled, not used or too low), a crude VIN comparator

will control VIN during start-up. If the RUN pin is tied to

VIN or held above the RUN enable threshold (>0.4V typ)

the LTC3106 has a typical start-up voltage of 0.85V with

input currents as low as 15µA or ~12µW of input power.

If the source impedance is high enough to cause VIN to

drop below the VIN comparator threshold, start-up is

terminated until the input capacitance is again charged to

approximately 0.85V. Operation continues in this manner

until start-up is complete. Input source impedance due

to the source itself or due to the input source’s expected

environmental conditions determine the required size

of the input capacitance on VIN to facilitate a successful

start-up. Recommendations are presented in the Input

Capacitor Selection and Typical Applications sections of

this document.

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