LT1308A/ LT1308B
U
W U U
APPLICATIONS INFORMATION
Table 1
VENDOR
Murata
LAYOUT HINTS
PART NO.
LQH6C4R7
CDRH734R7
CTX5-1
VALUE
4.7µH
4.7µH
5µH
PHONE NO.
770-436-1300
847-956-0666
561-241-7876
TheLT1308A/LT1308Bswitchcurrentathighspeed, man-
dating careful attention to layout for proper performance.
You will not get advertised performance with careless
layouts. Figure 4 shows recommended component place-
ment for a boost (step-up) converter. Follow this closely in
your PC layout. Note the direct path of the switching loops.
Input capacitor C1 must be placed close (<5mm) to the IC
Sumida
Coiltronics
Capacitors
package. As little as 10mm of wire or PC trace from C to
IN
Equivalent Series Resistance (ESR) is the main issue
regarding selection of capacitors, especially the output
capacitors.
V will cause problems such as inability to regulate or
IN
oscillation.
The negative terminal of output capacitor C2 should tie
close to Pin 4 of the LT1308A/LT1308B. Doing this
reduces dI/dt in the ground copper which keeps high
frequency spikes to a minimum. The DC/DC converter
ground should tie to the PC board ground plane at one
place only, to avoid introducing dI/dt in the ground plane.
The output capacitors specified for use with the LT1308A/
LT1308B circuits have low ESR and are specifically
designed for power supply applications. Output voltage
ripple of a boost converter is equal to ESR multiplied by
switchcurrent.TheperformanceoftheAVXTPSD227M006
220µF tantalum can be evaluated by referring to Figure 4.
When the load is 800mA, the peak switch current is
A SEPIC (Single-Ended Primary Inductance Converter)
schematic is shown in Figure 5. This converter topology
produces a regulated output over an input voltage range
that spans (i.e., can be higher or lower than) the output.
Recommended component placement for a SEPIC is
shown in Figure 6.
approximately 2A. Output voltage ripple is about 60mV
P-
P,sotheESRoftheoutputcapacitoris 60mV/2Aor0.03Ω.
Ripplecanbefurtherreducedbyparallelingceramicunits.
Table 2 lists some capacitors we have found to perform
well in the LT1308A/LT1308B application circuits. This is
not an exclusive list.
COMPONENT SELECTION
Inductors
Table 2
VENDOR
AVX
SERIES
TPS
PART NO.
VALUE
PHONE NO.
TPSD227M006 220µF, 6V 803-448-9411
TPSD107M010 100µF, 10V 803-448-9411
LMK432BJ226 22µF, 10V 408-573-4150
TMK432BJ106 10µF, 25V 408-573-4150
Suitable inductors for use with the LT1308A/LT1308B
must fulfill 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
inductancedecreasingbymorethan50%to60%.Second,
theDCRoftheinductorshouldhavelowDCR,under0.05Ω
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.
AVX
TPS
Taiyo Yuden
Taiyo Yuden
X5R
X5R
Diodes
We have found Motorola MBRS130 and International
Rectifier10BQ015toperformwell. Forapplications where
VOUT exceeds 30V, use 40V diodes such as MBRS140 or
10BQ040.
Table1lists someinductors wehavefoundtoperformwell
in LT1308A/LT1308B application circuits. This is not an
exclusive list.
7
Linear [ Linear ]
