6

LT1930/LT1930A

CAPACITOR SELECTION

Low ESR (equivalent series resistance) capacitors should

be used at the output to minimize the output ripple voltage.

Multi-layer ceramic capacitors are an excellent choice, as

they have 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 10µF 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 OSCON 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 LT1930/LT1930A. A 1

µF to 4.7µF input

capacitor is sufficient for most applications. Table 3 shows

a list of several ceramic capacitor manufacturers. Consult

the manufacturers for detailed information on their entire

selection of ceramic parts.

Table 3. 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

The decision to use either low ESR (ceramic) capacitors or

the higher ESR (tantalum or OSCON) capacitors can affect

the stability of the overall system. The ESR of any capaci-

tor, along with the capacitance itself, contributes a zero to

the system. For the tantalum and OSCON capacitors, this

zero is located at a lower frequency due to the higher value

of the ESR, while the zero of a ceramic capacitor is at a

much higher frequency and can generally be ignored.

A phase lead zero can be intentionally introduced by

placing a capacitor (C3) in parallel with the resistor (R1)

of the zero is determined by the following equation.

ƒ=

Z

RC

1

21

3

π ••

By choosing the appropriate values for the resistor and

capacitor, the zero frequency can be designed to improve

the phase margin of the overall converter. The typical

target value for the zero frequency is between 35kHz to

55kHz. Figure 3 shows the transient response of the step-

up converter from Figure 1 without the phase lead capaci-

tor C3. The phase margin is reduced as evidenced by more

ringing in both the output voltage and inductor current. A

10pF capacitor for C3 results in better phase margin,

which is revealed in Figure 4 as a more damped response

and less overshoot. Figure 5 shows the transient response

when a 33

µF tantalum capacitor with no phase lead

capacitor is used on the output. The higher output voltage

ripple is revealed in the upper waveform as a set of double

lines. The transient response is not greatly improved

which implies that the ESR zero frequency is too high to

increase the phase margin.

0.2V/DIV

AC COUPLED

0.5A/DIV

AC COUPLED

250mA

150mA

LOAD

CURRENT

50

µs/DIV

1930 F03

Figure 3. Transient Response of Figure 1's Step-Up

Converter without Phase Lead Capacitor

Figure 4. Transient Response of Figure 1's Step-Up

Converter with 10pF Phase Lead Capacitor

0.2V/DIV

AC COUPLED

0.5A/DIV

AC COUPLED

250mA

150mA

LOAD

CURRENT

50

µs/DIV

1930 F04