14

LT1308A/LT1308B

1308abfa

SW

LT1308B

GND

FB

SHDN

+

330pF

R1

100k

C1

47

μF

5V

D1

L1

4.7

μH

12V

500mA

C2

100pF

R2

11.3k

47k

R3

10k

C1: AVX TAJC476M010

C2: AVX TPSD476M016 (47

μF) OR

TAIYO YUDEN TMK432BJ106MM (10

μF)

D1: IR 10BQ015

L1: MURATA LQH6C4R7

1308 F19

Figure 19. 5V to 12V Boost Converter

Ceramic Capacitors

Multilayer ceramic capacitors have become popular, due

to their small size, low cost, and near-zero ESR. Ceramic

capacitors can be used successfully in LT1308A/LT1308B

designs provided loop stability is considered. A tantalum

capacitor has some ESR and this causes an "ESR zero" in

the regulator loop. This zero is beneficial to loop stability.

Ceramics do not have appreciable ESR, so the zero is lost

when they are used. However, the LT1308A/LT1308B

ues can be adjusted to achieve stability. A phase lead

capacitor can also be used to tune up load step response

to optimum levels, as detailed in the following paragraphs.

Figure 19 details a 5V to 12V boost converter using either

a tantalum or ceramic capacitor for C2. The input capaci-

tor has little effect on loop stability, as long as minimum

capacitance requirements are met. The phase lead capaci-

load step response of a 50mA to 500mA load step using a

47

μF tantalum capacitor at the output. Without the phase

lead capacitor, there is some ringing, suggesting the

the same load step is pictured in Figure 21. Some phase

margin is restored, improving the response. Next, C2 is

replaced by a 10

μF, X5R dielectric, ceramic capacitor.

Although the output settles faster than the tantalum case,

there is appreciable ringing, again suggesting phase mar-

gin is low. Figure 23 depicts load step response using the

10

and no ringing is evident. Ceramic capacitors have the

added benefit of lowering ripple at the switching frequency

to optimize response using ceramic output capacitors.

Figure 21. Load Step Response with 47

μF Tantalum

500mV/DIV

200

μs/DIV

1308 F20

Figure 20. Load Step Response of LT1308B 5V to 12V

Boost Converter with 47

μF Tantalum Output Capacitor

500mV/DIV

200

μs/DIV

1308 F21

1V/DIV

1A/DIV

200

μs/DIV

1308 F22

Figure 22. Load Step Response with 10

μF X5R

Ceramic Output Capacitor

LOAD

CURRENT

50mA

500mA

1A/DIV

LOAD

CURRENT

50mA

500mA

1A/DIV

LOAD

CURRENT

50mA

500mA