LTC4040 datasheet(20/26 Pages) AD

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LTC4041

Rev A

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APPLICATIONS INFORMATION

2.5A and the system load is also very high, e.g., 2.5A.

Under this scenario, as soon as the LTC4041 initiates

backup mode, the inductor current has to reverse from

2.5A (from SW to SCAP) to as high as the boost current

limit of approximately 6.5A (from SCAP to SW). That is a

9A current change in the inductor with a slope of VSCAP/L.

At a low supercapacitor voltage of 3.2V, this would take

almost 3μs even with a 1μH inductor. During this transi-

tion, CSYS, the capacitor on the VSYS pin, has to deliver

the shortfall until the inductor current catches up with

the system load demand, and the capacitor will deplete

according to the following equation:

CSYS = ILOAD •

Δt

ΔV

The size of the capacitor should be big enough to hold

the system voltage, VSYS, up above the SYSGD threshold

during this transition. For a system load ILOAD = 2.5A and

transition time Δt = 3μs, if the maximum droop ΔV allowed

in the system output is 100mV, the required capacitance

at the VSYS pin should be at least 75μF. The other consid-

eration for choosing the VSYS capacitor size is the maxi-

mum acceptable output voltage ripple during steady-state

backup boost operation. For a given duty cycle D and

load ILOAD, the output ripple VRIP of a boost converter is

calculated using the following equation:

VRIP =

ILOAD

CSYS

• D •

fOSC

If the maximum allowable ripple is 20mV under 2.5A

steady-state load while boosting from 3.2V to 5V

(D = 36%), the required capacitance at VSYS is calculated to

be at least 40μF using the above equation. Refer to Table 3

for recommended ceramic capacitor manufacturers.

Table 3. Recommended Ceramic Capacitor Manufacturers

AVX

www.avx.com

Murata

www.murata.com

Taiyo Yuden

www.t-yuden.com

Vishay Siliconix

www.vishay.com

TDK

www.tdk.com

Choosing a Supercapacitor

The backup energy requirement is the main consider-

ation when selecting a supercapacitor. The capacitance

per cell and the number of cells (maximum of two) needed

depends on the system load (ISYS), system voltage (VSYS),

backup boost efficiency (η), supercapacitor charge volt-

age (VCHG) and the duration of the backup (tBACKUP). The

following equation can be used to estimate the amount

of capacitance required for a given backup application:

CSCAP =

VSYS •ISYS • tBACKUP

η • (VCHG)

Another factor to be considered is the current rating of

the supercapacitor. With the LTC4041, the supercapacitor

could be charged with a current as high as 2.5A. During a

backup event, the supercapacitor could be discharged at a

current level as high as 7.5A. It is also important to select

a supercapacitor with low ESR to minimize power losses

in the supercapacitor during charging or backup. Other

factors to be considered are the lifetime of the superca-

pacitor at the charge voltage, and the capacitance degra-

dation over time.

The internal balancer of the LTC4041 is designed to bal-

ance supercapacitors with capacitances greater than

100mF per cell. For lower capacitances, the balancer

servo loop could be unstable.

A list of supercapacitor suppliers is provided in Table 4.

Table 4. Supercapacitor Suppliers

AVX

www.avx.com

Bussman

www.cooperbussman.com

CAP-XX

www.cap-xx.com

Illinois Capacitor

www.illcap.com

Maxwell

www.maxwell.com

Murata

www.murata.com

NESS CAP

www.nesscap.com

Tecate Group

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