Datasheet

9
LT1317/LT1317B
Table 1. Inductors Suitable for Use with the LT1317
MAX HEIGHT
PART VALUE DCR MFR (mm) COMMENT
LQH3C100 10µH 0.57 Murata-Erie 2.0 Smallest Size,
Limited Current
Handling
DO1608-103 10µH 0.16 Coilcraft 3.0
CD43-100 10µH 0.18 Sumida 3.2
CD54-100 10µH 0.10 Sumida 4.5 Best Efficiency
CTX32CT-100 10µH 0.50 Coiltronics 2.2 1210 Footprint
Capacitor Selection
Low ESR (Equivalent Series Resistance) capacitors should
be used at the output of the LT1317. For most applications
a solid tantalum in a C or D case size works well. Accept-
able capacitance values range from 10µF to 330µF with
ESR falling between 0.1Ω and 0.5Ω. If component size is
an issue, tantalum capacitors in smaller case sizes can be
used but they have high ESR and output voltage ripple may
reach unacceptable levels.
Ceramic capacitors are an alternative because of their
combination of small size and low ESR. A 10µF ceramic
capacitor will work for some applications but the ex-
tremely low ESR of these capacitors may cause loop
stability problems. Compensation components will need
APPLICATIONS INFORMATION
WUU
U
to be adjusted to ensure a stable system for the entire input
voltage range. Figure 4 shows a 2V to 3.3V converter with
new values for R
C
and C
C
. Figure 5 details transient
response for this circuit. Also, ceramic caps are prone to
temperature effects and the designer must check loop
stability over the operating temperature range (see section
on Frequency Compensation).
Input bypass capacitor ESR is less critical and smaller
units may be used. If the input voltage source is physically
near the V
IN
pin (<5mm), a 10µF ceramic or a 10µF A case
tantalum is adequate.
Diodes
Most of the application circuits on this data sheet specify
the Motorola MBR0520L surface mount Schottky diode.
In lower current applications, a 1N4148 can be used,
although efficiency will suffer due to the higher forward
drop. This effect is particularly noticeable at low output
voltages. For higher voltage output applications, such as
LCD bias generators, the extra drop is a small percentage
of the output voltage so the efficiency penalty is small. The
low cost of the 1N4148 makes it attractive wherever it can
be used. In through hole applications the 1N5818 is the all
around best choice.
V
IN
SW
FB
LT1317B
L1
10µH
D1
SHDN
R
C

20k
R2
604k
1%
V
OUT

3.3V
R1
1M
1%
C
C

1500pF
1317 F04
C1
10µF
V
IN

2V
C2
10µF
CERAMIC
D1: MBR0520
L1: SUMIDA CD43-100
V
C
GND
Figure 4. 2V to 3.3V Converter with a 10µF Ceramic Output
Capacitor. R
C
and C
C
Have Been Adjusted to Give Optimum
Transient Response.
200µs/DIV 1317 F05
V
OUT
200mV/DIV
AC COUPLED
I
LOAD
5mA TO
200mA
Figure 5. Transient Response for the Circuit of Figure 4.