LT1946

6

sn1946 1946fs

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.

Multilayer ceramic capacitors are an excellent choice, as

they have an extremely low ESR and are available in very

small packages. X5R dielectrics are preferred, followed by

X7R, as these materials retain the capacitance over wide

voltage and temperature ranges. A 4.7

µF to 20µF output

capacitor is sufficient for most applications, but systems

with very low output currents may need only a 1

µFor2.2µF

output capacitor. Solid tantalum or OS-CON capacitors

can be used, but they will occupy more board area than a

ceramic and will have a higher ESR. Always use a capacitor

with a sufficient voltage rating.

Ceramic capacitors also make a good choice for the input

decoupling capacitor, which should be placed as close as

possible to the LT1946. A 2.2

µF to 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. Ceramic Capacitor Manufacturers

Taiyo Yuden

(408) 573-4150

www.t-yuden.com

AVX

(803) 448-9411

www.avxcorp.com

Murata

(714) 852-2001

www.murata.com

Compensation—Adjustment

To compensate the feedback loop of the LT1946, a series

resistor-capacitor network should be connected from the

COMP pin to GND. For most applications, a capacitor in

the range of 220pF to 680pF will suffice. A good starting

to 100k. A good technique to compensate a new applica-

tion is to use a 100k

eter while observing the transient response, the optimum

process for the circuit of Figure 1 with a load current

stepped from 250mA to 300mA. Figure 3a shows the tran-

20mV/DIV

AC COUPLED

0.5A/DIV

AC COUPLED

200

µs/DIV

1946 F03a

Figure 3a. Transient Response Shows Excessive Ringing

20mV/DIV

AC COUPLED

0.5A/DIV

AC COUPLED

200

µs/DIV

1946 F03b

Figure 3b. Transient Response is Better

20mV/DIV

AC COUPLED

0.5A/DIV

AC COUPLED

200

µs/DIV

1946 F03b

Figure 3c. Transient Response is Well Damped

poor as evidenced by the excessive ringing in the output

is increased to 18k, which results in a more damped re-

further to 49.9k. The transient response is nicely damped

and the compensation procedure is complete. The COMP

pin provides access to an internal resistor (120k) and

capacitor (90pF). For some applications, these values will