R1 is then:

= 7.959 k

:

(T

R

)

x

:

=

k

100

e

-

+

298

1

273

85

1

E

»

¼

º

0.8V x 7.959 k

:

=

1.8V ± 0.8V

6.367 k

:

(

)

TRIP

- V

BIAS

V

=

1

R

)

TRIP

(

T

R

TRIP

V

e

x

( )

C

25

R

=

T

R

°

298

1

273

+

C

°

T

1

E

-

¹

·

©

§

LM3550

SNVS569B – MAY 2009 – REVISED MAY 2013

www.ti.com

NOTE

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

Boost Capacitors

Surface-mount multi-layer ceramic capacitors are recommended. These capacitors are small, inexpensive and

have very low equivalent series resistance (ESR <20 m

Ω typ.). 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 10V to account for DC bias capacitance losses.

Current Source FET

Choosing the proper current source MOSFET is required to ensure accurate flash current delivery. N-Channel

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 NFET's pulsed performance in the electrical characteristics section of

their datasheets. A MOSFET's performance 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

ALD/TEMP Components

NTC SELECTION

NTC thermistors have a temperature-to-resistance relationship of:

(11)

where

it is equal to:

(12)

nominal value at +25°C is 100 k

Ω and a β = 4500K, the trip point is chosen to be +85°C. The value of R(T) at

85°C is:

(13)

28

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