SETTING LED CURRENT

The current through the LED is set by programming the ap-

propriate register with the desired current level code for Flash

and Torch. The time that Flash mode is active is dependent

the Strobe pin), or the duration of the Flash Timeout. Use the

tables below to select the desired current level.

Using the part in conditions where the junction temperature

might rise above the rated maximum requires that the oper-

ating ranges and/or conditions be de-rated. The printed circuit

board also must be carefully laid out to account for high ther-

mal dissipation in the part.

Flash Current Table (Reg xB0)

CODE (Hex)

FLASH CURRENT (mA)

00

80

01

150

02

220

03

280

04

350

05

410

06

470

07

530

08

590

09

650

0A

710

0B

770

0C

830

0D

890

0E

950

0F

1010

Torch Current Table (Reg xA0)

CODE (Hex)

TORCH CURRENT (mA)

00

15

01

30

02

40

03

50

04

65

05

80

06

90

07

100

08

110

09

120

0A

130

0B

140

0C

150

0D

160

0E

170

0F

180

FLASH TIME-OUT FEATURE

Time-out Protection Circuitry disables the current sink when

either the Strobe pin is held at logic high or the Flash mode is

grammed timeout duration. This prevents the device from

self-heating due to the high power dissipation during Flash

conditions. During the time-out condition, voltage will still be

present on V

in no current through the Flash LED. When the device goes

into a time-out condition, disabling and then re-enabling the

device will reset the time-out. Use the table below to set the

desired Flash timeout duration.

Flash Timeout Duration (Reg xC0)

CODE (Hex)

TIME (ms)

00

60

01

125

02

250

03

375

04

500

05

625

06

750

07

1100

CAPACITOR SELECTION

The LM2759 requires 4 external capacitors for proper opera-

tion. Surface-mount multi-layer ceramic capacitors are rec-

ommended. These capacitors are small, inexpensive and

have very low equivalent series resistance (ESR <20 m

Ω

typ.). Tantalum capacitors, OS-CON capacitors, and alu-

minum electrolytic capacitors are not recommended for use

with the LM2759 due to their high ESR, as compared to ce-

ramic capacitors. For most applications, ceramic capacitors

with X7R or X5R temperature characteristic are preferred for

use with the LM2759. These capacitors have tight capaci-

tance 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

LM2759. 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 detri-

mental deviation is likely to cause Y5V and Z5U capacitors to

fail to meet the minimum capacitance requirements of the

LM2759. The voltage rating of the output capacitor should be

6.3V or more. For example, a 6.3V 0603 4.7

μF output ca-

pacitor (TDK C1608X5R0J475) is acceptable for use with the

LM2759, as long as the capacitance on the output does not

fall below a minimum of 3

μF in the intended application. All

other capacitors should have a voltage rating at or above the

maximum input voltage of the application and should have a

minimum capacitance of 1

μF.

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