ANALOG BRIGHTNESS CONTROL

30045318

The current though the LED could be varied dynamically by

changing the I

The current though the LED can be calculated as follows.

30045317

CAPACITOR SELECTION

The LM2758 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 LM2758 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 LM2758. 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

LM2758. 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

LM2758. The voltage rating of the output capacitor should be

6.3V or more. For example, a 6.3V 0603 2.2

μF output ca-

pacitor (TDK C1608X5R0J225) is acceptable for use with the

LM2758, as long as the capacitance on the output does not

fall below a minimum of 1

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.

Suggested Capacitors and Suppliers

MFG Part No.

Type

MFG

Voltage Rating

Case Size

Inch (mm)

2.2 µF for C

C1608X5R0J225

Ceramic X5R

TDK

6.3V

0603 (1608)

JMK107BJ225

Ceramic X5R

Taiyo-Yuden

6.3V

0603 (1608)

1 µF for C1 and C2

C1608X5R0J105

Ceramic X5R

TDK

6.3V

0603 (1608)

JMK107BJ105M

Ceramic X5R

Taiyo-Yuden

6.3V

0603 (1608)

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