7
LT1304/LT1304-3.3/LT1304-5
Table 1 lists inductor suppliers along with appropriate part
numbers.
Table 1. Recommended Inductors
VENDOR SERIES PHONE NUMBER
Sumida CD54, CD43 (708) 956-0666
Coiltronics CTX20-1 (407) 241-7876
Dale LPT4545 (605) 665-9301
Coilcraft DO3316, DO1608, DO3308 (708) 639-6400
Capacitor Selection
Low ESR (Equivalent Series Resistance) capacitors should
be used at the output of the LT1304 to minimize output
ripple voltage. High quality input bypassing is also re-
quired. For surface mount applications AVX TPS series
tantalum capacitors are recommended. These have been
specifically designed for switch mode power supplies and
have low ESR along with high surge current ratings. A
100µF, 10V AVX TPS surface mount capacitor typically
limits output ripple voltage to 70mV when stepping up
from 2V to 5V at a 200mA load. For through hole applica-
tions Sanyo OS-CON capacitors offer extremely low ESR
in a small package size. Again, if peak switch current is
reduced using the I
LIM
pin, capacitor requirements can be
eased and smaller, higher ESR units can be used. Sug-
gested capacitor sources are listed in Table 2.
Table 2. Recommended Capacitors
VENDOR SERIES TYPE PHONE NUMBER
AVX TPS Surface Mount (803) 448-9411
Sanyo OS-CON Through Hole (619) 661-6835
Sprague 595D Surface Mount (603) 225-1961
Diode Selection
Best performance is obtained with a Schottky rectifier
such as the 1N5818. Motorola makes the MBRS130L
Schottky which is slightly better than the 1N5818 and
comes in a surface mount package. For lower switch
currents, the MBR0530 is recommended. It comes in a
very small SOD-123 package. Multiple 1N4148s in parallel
can be used in a pinch, although efficiency will suffer.
OPERATIO
U
I
LIM
Function
The LT1304’s current limit (I
LIM
) pin can be used for soft
start. Upon start-up, the LT1304 will draw maximum
current (about 1A) from the supply to charge the output
capacitor. Figure 3 shows V
OUT
and V
IN
waveforms as the
device is turned on. The high current flow can create IR
drops along supply and ground lines or cause the input
supply to drop out momentarily. By adding R1 and C1 as
shown in Figure 4, the switch current is initially limited to
well under 1A as detailed in Figure 5. Current flowing into
C1 from R1 and the I
LIM
pin will eventually charge C1 and
R1 effectively takes C1 out of the circuit. R1 also provides
a discharge path for C1 when SHUTDOWN is brought low
for turn-off.
V
OUT
2V/DIV
I
IN
500mA/DIV
V
SHDN
10V/DIV
1ms/DIV 1304 F03
Figure 3. Start-Up Response. Input Current Rises Quickly to
1A. V
OUT
Reaches 5V in Approximately 1ms. Output Drives
20mA Load
Figure 4. 2-Cell to 5V/200mA Boost Converter Takes Four
External Parts. Components with Dashed Lines Are for
Soft Start (Optional)
V
IN
SW
GND
I
LIM
SENSELBI
SHDNLB0
LT1304-5
100µF
C1
1µF
100µF
2 CELLS
22µH*
MBRS130L
5V
200mA
*SUMIDA CD54-220
1304 F04
SHUTDOWN
+
+
+
R1
1M
