LT1308A_1 Datasheet, PDF (14/20 Pages) Linear Technology – High Current, Micropower Single Cell, 600kHz DC/DC Converters

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 (V_C) so component val-
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_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.
L1
4.7μH
D1
V_IN
5V
V_OUT
12V
500mA
V_IN
SW
SHDN
LT1308B
R3
10k
FB
V_C
GND
+
C1
47μF
47k
R1
100k
R2
11.3k
C_PL
330pF
C2
100pF
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
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 mar-
gin 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 benefit 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.
V_OUT
500mV/DIV
I_L1
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
V_OUT
500mV/DIV
I_L1
1A/DIV
LOAD ^500mA
CURRENT _50mA
200μs/DIV
1308 F21
Figure 21. Load Step Response with 47μF Tantalum
Output Capacitor and Phase Lead Capacitor C_PL
V_OUT
1V/DIV
I_L1
1A/DIV
LOAD ^500mA
CURRENT _50mA
200μs/DIV
1308 F22
Figure 22. Load Step Response with 10μF X5R
Ceramic Output Capacitor
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