Datasheet

LT1308A/LT1308B
14
1308abfb
APPLICATIONS INFORMATION
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 benefi cial to loop stability.
Ceramics do not have appreciable ESR, so the zero is lost
when they are used. However, the LT1308A/LT1308B have
external compensation pin (V
C
) so component values 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-
tor C
PL
parallels feedback resistor R1. Figure 20 shows
load step response of a 50mA to 500mA load step using a
47µF tantalum capacitor at the output. Without the phase
lead capacitor, there is some ringing, suggesting the
phase margin is low. C
PL
is then added, and response to
the same load step is pictured in Figure 21. Some phase
margin is restored, improving the response. Next, C2 is
replaced by a 10µF, X5R dielectric, ceramic capacitor.
Without C
PL
, load step response is pictured in Figure 22.
Although the output settles faster than the tantalum case,
there is appreciable ringing, again suggesting phase margin
is low. Figure 23 depicts load step response using the 10µF
ceramic output capacitor and C
PL
. Response is clean and
no ringing is evident. Ceramic capacitors have the added
benefi t of lowering ripple at the switching frequency due
to their very low ESR. By applying C
PL
in tandem with the
series RC at the V
C
pin, loop response can be tailored to
optimize response using ceramic output capacitors.
Figure 19. 5V to 12V Boost Converter
Figure 20. Load Step Response of LT1308B 5V to 12V
Boost Converter with 47μF Tantalum Output Capacitor
Figure 21. Load Step Response with 47μF Tantalum
Output Capacitor and Phase Lead Capacitor C
PL
Figure 22. Load Step Response with 10μF X5R
Ceramic Output Capacitor
1308 F20
LOAD
CURRENT
I
L1
1A/DIV
500mA
50mA
V
OUT
500mV/DIV
200µs/DIV
1308 F21
LOAD
CURRENT
I
L1
1A/DIV
500mA
50mA
V
OUT
500mV/DIV
200µs/DIV
1308 F22
LOAD
CURRENT
I
L1
1A/DIV
500mA
50mA
V
OUT
500mV/DIV
200µs/DIV
V
IN
SW
LT1308B
GND
V
C
FB
SHDN
+
C
PL
330pF
R1
100k
C1
47µF
V
IN
5V
D1
L1
4.7µH
V
OUT
12V
500mA
C2
100pF
R2
11.3k
47k
R3
10k
C1: AVX TAJC476M010
C2: AVX TPSD476M016 (47µF) OR
TAIYO YUDEN TMK432BJ106MM (10µF)
D1: IR 10BQ015
L1: MURATA LQH6C4R7
1308 F19