LT1308BCF View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT1308BCF' PDF : 20 Pages View PDF
LT1308A/LT1308B
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 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 (VC) 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.
Without CPL, 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 CPL. Response is clean 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 CPL in tandem with the
series RC at the VC pin, loop response can be tailored to
optimize response using ceramic output capacitors.
VOUT
500mV/DIV
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 CPL 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. CPL 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.
L1
4.7μH
D1
VIN
5V
VOUT
12V
500mA
VIN
SW
SHDN
LT1308B
R3
10k
FB
VC
GND
+
C1
47μF
47k
R1
100k
R2
11.3k
CPL
330pF
C2
IL1
1A/DIV
LOAD 500mA
CURRENT 50mA
200μs/DIV
1308 F20
Figure 20. Load Step Response of LT1308B 5V to 12V
Boost Converter with 47μF Tantalum Output Capacitor
VOUT
500mV/DIV
IL1
1A/DIV
LOAD 500mA
CURRENT 50mA
200μs/DIV
1308 F21
Figure 21. Load Step Response with 47μF Tantalum
Output Capacitor and Phase Lead Capacitor CPL
VOUT
500mV/DIV
100pF
IL1
1A/DIV
C1: AVX TAJC476M010
C2: AVX TPSD476M016 (47μF) OR
TAIYO YUDEN TMK432BJ106MM (10μF)
D1: IR 10BQ015
L1: MURATA LQH6C4R7
1308 F19
Figure 19. 5V to 12V Boost Converter
14
LOAD 500mA
CURRENT 50mA
200μs/DIV
1308 F22
Figure 22. Load Step Response with 10μF X5R
Ceramic Output Capacitor
1308abfb
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