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LT1308BCS8#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20
LT1308A/LT1308B
13
1308abfb
APPLICATIONS INFORMATION
COMPONENT SELECTION
Diodes
We have found ON Semiconductor MBRS130 and Inter-
national Rectifi er 10BQ015 to perform well. For applica-
tions where V
OUT
exceeds 30V, use 40V diodes such as
MBRS140 or 10BQ040.
Height limited applications may benefi t from the use of the
MBRM120. This component is only 1mm tall and offers
performance similar to the MBRS130.
Inductors
Suitable inductors for use with the LT1308A/LT1308B must
fulfi ll two requirements. First, the inductor must be able
to handle current of 2A steady-state, as well as support
transient and start-up current over 3A without inductance
decreasing by more than 50% to 60%. Second, the DCR
of the inductor should have low DCR, under 0.05 so
that copper loss is minimized. Acceptable inductance
values range between 2µH and 20µH, with 4.7µH best for
most applications. Lower value inductors are physically
smaller than higher value inductors for the same current
capability.
Table 1 lists some inductors we have found to perform
well in LT1308A/LT1308B application circuits. This is not
an exclusive list.
Table 1
VENDOR PART NO. VALUE PHONE NO.
Murata LQH6C4R7 4.7µH 770-436-1300
Sumida CDRH734R7 4.7µH 847-956-0666
Coiltronics CTX5-1 5µH 561-241-7876
Coilcraft LPO2506IB-472 4.7µH 847-639-6400
Capacitors
Equivalent Series Resistance (ESR) is the main issue
regarding selection of capacitors, especially the output
capacitors.
The output capacitors specifi ed for use with the LT1308A/
LT1308B circuits have low ESR and are specifi cally
designed for power supply applications. Output voltage
ripple of a boost converter is equal to ESR multiplied by
switch current. The performance of the AVX TPSD227M006
220µF tantalum can be evaluated by referring to Figure 3.
When the load is 800mA, the peak switch current is approxi-
mately 2A. Output voltage ripple is about 60mV
P-P
, so the
ESR of the output capacitor is 60mV/2A or 0.03. Ripple
can be further reduced by paralleling ceramic units.
Table 2 lists some capacitors we have found to perform
well in the LT1308A/LT1308B application circuits. This is
not an exclusive list.
Table 2
VENDOR SERIES PART NO. VALUE PHONE NO.
AVX TPS TPSD227M006 220µF, 6V 803-448-9411
AVX TPS TPSD107M010 100µF, 10V 803-448-9411
Taiyo Yuden X5R LMK432BJ226 22µF, 10V 408-573-4150
Taiyo Yuden X5R TMK432BJ106 10µF, 25V 408-573-4150
Figure 17. Start-Up Waveforms of Figure 16’s Circuit
without Soft-Start Components
Figure 18. Start-Up Waveforms of Figure 16’s Circuit
with Soft-Start Components Added
1308 F17
V
OUT
5V/DIV
I
L1
1A/DIV
12V
5V
V
SHDN
10V/DIV
50µs/DIV
1308 F18
V
OUT
I
L1
1A/DIV
12V
5V
V
SHDN
10V/DIV
5ms/DIV
LT1308A/LT1308B
14
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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
LT1308A/LT1308B
15
1308abfb
APPLICATIONS INFORMATION
GSM AND CDMA PHONES
The LT1308A/LT1308B are suitable for converting a single
Li-Ion cell to 5V for powering RF power stages in GSM or
CDMA phones. Improvements in the LT1308A/LT1308B
error amplifi ers allow external compensation values to be
reduced, resulting in faster transient response compared
to the LT1308. The circuit of Figure 24 (same as Figure 1,
printed again for convenience) provides a 5V, 1A output
from a Li-Ion cell. Figure 25 details transient response at
the LT1308A operating at a V
IN
of 4.2V, 3.6V and 3V. Ripple
voltage in Burst Mode operation can be seen at 10mA
load. Figure 26 shows transient response of the LT1308B
under the same conditions. Note the lack of Burst Mode
ripple at 10mA load.
Figure 23. Load Step Response with 10μF X5R
Ceramic Output Capacitor and C
PL
Figure 25. LT1308A Li-Ion to 5V Boost Converter
Transient Response to 1A Load Step
Figure 26. LT1308B Li-Ion to 5V Boost
Converter Transient Response to 1A Load Step
1308 F23
LOAD
CURRENT
I
L1
1A/DIV
500mA
50mA
V
OUT
500mV/DIV
200µs/DIV
1308 F25
I
LOAD
1A
1mA
V
OUT
V
IN
= 4.2V
V
OUT
V
IN
= 3.6V
V
OUT
V
IN
= 3V
200µs/DIV
V
OUT
TRACES =
200mV/DIV
1308 F26
I
LOAD
1A
10mA
V
OUT
V
IN
= 4.2V
V
OUT
V
IN
= 3.6V
V
OUT
V
IN
= 3V
100µs/DIV
V
OUT
TRACES =
200mV/DIV
Figure 24. Li-Ion to 5V Boost Converter Delivers 1A
V
IN
SW
FB
LT1308B
L1
4.7µH
D1
47k
R2
100k
R1
309k
5V
1A
100pF
1308A/B F24
C1
47µF
C2
220µF
Li-Ion
CELL
V
C
GND
SHDNSHUTDOWN
C1: AVX TAJC476M010
C2: AVX TPSD227M006
+
+
D1: IR 10BQ015
L1: MURATA LQH6N4R7
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20

LT1308BCS8#PBF

Mfr. #:
Buy LT1308BCS8#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 1x Cell Hi C uP 600kHz Boost DC/DC Conv
Lifecycle:
New from this manufacturer.
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