LM3550SPXSLASHNOPB datasheet(29/43 Pages) TI2

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LM3550

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SNVS569C – MAY 2009 – REVISED OCTOBER 2016

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8.2.2 Detailed Design Procedure

8.2.2.1 Component Selection

8.2.2.1.1

Super-Capacitor

Super-capacitors, or electrochemical double-layer capacitors (EDLC's), have a very high energy density

compared to other capacitor types. Most super-capacitors aimed at applications requiring voltages higher than

3V are three-terminal devices (two super-capacitor cells stacked in series). Special care must be taken to ensure

that the voltage on each cell of the super-capacitor does not exceed the maximum rating (typically 2.75 V to

2.85 V, depending on the manufacturer). The LM3550 is capable of safely charging super-capacitors of many

different capacitances up to a VOUT(maximum) = 5.3 V typical.

The capacitor balance pin (BAL) on the LM3550 ensures that the voltage on each cell is equal to half of the

output voltage to prevent an overvoltage condition on either cell. If either cell fails as a short, the BAL pin does

not prevent the second cell from being damaged.

NOTE

The LM3550 is not designed to work with low-voltage, single-cell super-capacitors.

8.2.2.1.2

Boost Capacitors

The LM3550 requires 4 external capacitors for proper operation (C1 = C2 = 1 µF; CIN = 4.7 µF; COUT = 2.2 µF); TI

recommends surface-mount multi-layer ceramic capacitors are recommended. These capacitors are small,

inexpensive and have very low equivalent series resistance (ESR < 20 m

Ω typical). Tantalum capacitors, OS-

CON capacitors, and aluminum electrolytic capacitors are not recommended for use with the LM3550 due to their

high ESR, as compared to ceramic capacitors.

For most applications, ceramic capacitors with X7R or X5R temperature characteristic are preferred for use with

the LM3550. These capacitors have tight capacitance tolerance (as good as ±10%) and hold their value over

temperature (X7R: ±15% over

−55°C to +125°C; X5R: ±15% over −55°C to +85°C).

Capacitors with Y5V or Z5U temperature characteristic are generally not recommended for use with the LM3550.

Capacitors with these temperature characteristics typically have wide capacitance tolerance (+80%,

−20%) and

vary significantly over temperature (Y5V: 22%,

−82% over −30°C to +85°C range; Z5U: 22%, −56% over 10°C to

85°C range). Under some conditions, a nominal 1µF Y5V or Z5U capacitor could have a capacitance of only 0.1

µF. Such detrimental deviation is likely to cause Y5V and Z5U capacitors to fail to meet the minimum

capacitance requirements of the LM3550.

The recommended voltage rating for the capacitors is 10 V to account for DC bias capacitance losses.

8.2.2.1.3

Current Source FET

Choose the proper current source MOSFET to ensure accurate flash current delivery. N-channel MOSFETs

(NFET) with allowed drain-to-source voltages (VDS) greater than 5.5 V are required. In order to prevent damage

to the current source NFET, special attention must be given to the pulsed-current rating of the MOSFET. The

NFET must be sized appropriately to handle the desired flash current and flash duration. Most MOSFET

manufacturers provide curves showing the pulsed performance of the NFET in the electrical characteristics

section of their data sheets. The performance of the MOSFET rating at temperature, primarily temperatures

greater than 40°C, must also be investigated to ensure NFET does not become thermally damaged during a

flash pulse. An NFET possessing low RDSON values helps improve the efficiency of the flash pulse.

LM3550SPX/NOPB Datasheet(PDF) 29 Page - Texas Instruments

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