PEAK

I

LOAD

I

=

K

x

L

I

+ '

where

I

'

V

(

)

IN

V

OUT

SW

V

x

L

x

f

x

2

IN

OUT

V

V

28

LM3555

SNVS594G – DECEMBER 2008 – REVISED APRIL 2016

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Copyright © 2008–2016, Texas Instruments Incorporated

8.2.2 Detailed Design Procedure

8.2.2.1 Inductor Current Limit

To prevent damage to the inductor of the LM3555 and to limit the power drawn by the LM3555 during a flash

event, an inductor current limit circuit is present. The LM3555 monitors the current through the inductor during

the charge phase of the boost cycle. In the event that the inductor current reaches the current limit, the NFET of

the converter terminates the charge phase for that cycle. The process repeats itself until the flash event has

ended or until the input voltage increases to the point where the peak current is no longer reached. Hitting the

peak inductor current limit does not disable the part. It does, however, limit the output power delivery to the

LEDs.

limit can be set to 1.25 A, 1.5 A, 1.75 A, and 2 A depending on the values of the IL1 and IL0 bits in the Control

Register (address 0x04). The peak inductor current limit value can be used to help size the inductor to the

appropriate saturation current level. For more information on inductor sizing, please refer to the Inductor

Selection.

8.2.2.2 Inductor Selection

of the biggest sources of efficiency loss in the circuit. Therefore, choosing an inductor with the lowest possible

series resistance is important. Additionally, the saturation rating of the inductor must be greater than the

maximum operating peak current of the LM3555. This prevents excess efficiency loss that can occur with

inductors that operate in saturation and prevents over heating of the inductor and possible damage. For proper

inductor operation and circuit performance ensure that the inductor saturation and the peak current limit setting of

(1)

Table 11. Recommended Inductors

MANUFACTURER

PART NUMBER

Toko

FDSE312-2R2M

2.2 µH / 2.3 A

Coilcraft

LPS4012-222ML

2.2 µH / 2.3 A

TDK

VLF4014ST-2R2M1R9

2.2 µH / 2 A

8.2.2.3 Capacitor Selection

pin. 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 LM3555 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 LM3555. 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 LM3555.

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 1-µF 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 LM3555.

The recommended voltage rating for the input capacitor is 10 V (minimum = 6.3 V). The recommended output

capacitor voltage rating is 16 V (minimum = 10 V). The recommended value takes into account the DC bias

capacitance losses, while the minimum rating takes into account the OVP trip levels.