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LT1932

APPLICATIO S I FOR ATIO

Capacitor Selection

Low ESR (equivalent series resistance) capacitors should

be used at the output to minimize the output ripple

voltage. Because they have an extremely low ESR and are

available in very small packages, multilayer ceramic ca-

pacitors are an excellent choice. X5R and X7R type

capacitors are preferred because they retain their capaci-

tance over wider voltage and temperature ranges than

other types such as Y5V or Z5U. A 1

µF or 2.2µF output

capacitor is sufficient for most applications. Always use a

capacitor with a sufficient voltage rating. Ceramic capaci-

tors do not need to be derated (do not buy a capacitor with

a rating twice what your application needs). A 16V ce-

ramic capacitor is good to more than 16V, unlike a 16V

tantalum, which may be good to only 8V when used in

certain applications. Low profile ceramic capacitors with

a 1mm maximum thickness are available for designs

having strict height requirements.

Ceramic capacitors also make a good choice for the input

decoupling capacitor, which should be placed as close as

possible to the LT1932. A 2.2

µF or 4.7µF input capacitor

is sufficient for most applications. Table 2 shows a list of

several ceramic capacitor manufacturers. Consult the

manufacturers for detailed information on their entire

selection of ceramic parts.

Table 2. Recommended Ceramic Capacitor Manufacturers

VENDOR

PHONE

URL

Taiyo Yuden

(408) 573-4150

www.t-yuden.com

Murata

(814) 237-1431

www.murata.com

Kemet

(408) 986-0424

www.kemet.com

Diode Selection

Schottky diodes, with their low forward voltage drop and

fast switching speed, are the ideal choice for LT1932

applications. Table 3 shows several different Schottky

diodes that work well with the LT1932. Make sure that the

diode has a voltage rating greater than the output voltage.

The diode conducts current only when the power switch is

turned off (typically less than one-third the time), so a 0.4A

or 0.5A diode will be sufficient for most designs.

Table 3. Recommended Schottky Diodes

PART

VENDOR

MBR0520

ON Semiconductor

MBR0530

(800) 282-9855

MBR0540

www.onsemi.com

ZHCS400

Zetex

ZHCS500

(631) 543-7100

www.zetex.com

Programming LED Current

The LED current is programmed with a single resistor

connected to the RSET pin (see Figure 1). The RSET pin is

internally regulated to 100mV, which sets the current

flowing out of this pin, ISET, equal to 100mV/RSET. The

LT1932 regulates the current into the LED pin, ILED, to 225

times the value of ISET. For the best accuracy, a 1% (or

better) resistor value should be used. Table 4 shows

several typical 1% RSET values. For other LED current

values, use the following equation to choose RSET.

SET

LED









225

•

Table 4. RSET Resistor Values

ILED (mA)

RSET VALUE

562

Ω

750

Ω

1.13k

1.50k

2.26k

4.53k

Most white LEDs are driven at maximum currents of 15mA

to 20mA. Some higher power designs will use two parallel

strings of LEDs for greater light output, resulting in 30mA

to 40mA (two strings of 15mA to 20mA) flowing into the

LED pin.

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