1308AI Linear Technology Corporation, 1308AI Datasheet - Page 14

• Manufacturer Part Number: 1308AI
• Description: Lt1308b - Single Cell High Currentmicropower 600khz Boost Dc/dc Converter
• Manufacturer: Linear Technology Corporation

LT1308A/LT1308B
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
have external compensation pin (V
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-
tor C
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
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.
14
APPLICATIONS
V
5V
IN
+
PL
C1: AVX TAJC476M010
C2: AVX TPSD476M016 (47μF) OR
D1: IR 10BQ015
L1: MURATA LQH6C4R7
parallels feedback resistor R1. Figure 20 shows
TAIYO YUDEN TMK432BJ106MM (10μF)
C1
47μF
Figure 19. 5V to 12V Boost Converter
SHDN
V
V
IN
C
47k
LT1308B
100pF
4.7μH
L1
U
GND
PL
SW
INFORMATION
FB
is then added, and response to
U
D1
10k
R3
C
W
R1
100k
R2
11.3k
) so component val-
C
330pF
PL
1308 F19
U
C2
V
12V
500mA
OUT
CURRENT
CURRENT
Without C
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
10μF ceramic output capacitor and C
and no ringing is evident. Ceramic capacitors have the
added benefit of lowering ripple at the switching frequency
due to their very low ESR. By applying C
the series RC at the V
to optimize response using ceramic output capacitors.
CURRENT
LOAD
LOAD
LOAD
500mV/DIV
500mV/DIV
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
1A/DIV
1V/DIV
1A/DIV
1A/DIV
500mA
500mA
500mA
50mA
50mA
50mA
V
V
V
OUT
Figure 22. Load Step Response with 10μF X5R
Ceramic Output Capacitor
OUT
OUT
I
I
I
L1
L1
L1
PL
, load step response is pictured in Figure 22.
C
pin, loop response can be tailored
200μs/DIV
200μs/DIV
200μs/DIV
PL
. Response is clean
PL
in tandem with
1308 F21
1308 F22
1308 F20
PL
1308abfa