Datasheet

LT3504
14
3504fa
For more information www.linear.com/LT3504
applicaTions inForMaTion
Catch Diode
Use a 1A Schottky diode. The diode must have a reverse
voltage rating equal to or greater than the maximum input
voltage. The ON Semiconductor MBRM140 is a good
choice; it is rated for 1A continuous forward current and
a maximum reverse voltage of 40V.
Input Capacitor
The input of the LT3504 circuit must be bypassed with a
X7R or X5R type ceramic capacitor. Y5V types have poor
performance over temperature and amplified voltage
and should not be used. There are four V
IN
pins. Each
V
IN
pin should be bypassed to the nearest ground pin.
However it is not necessary to use a dedicated capaci-
tor for each V
IN
pin. Pins 9 and 11 may be tied together
on the board layout so that both pins can share a single
bypass capacitor. Since the channels running on Pins 9
and 11 are 180 degrees out-of-phase, it is not necessary
to double the capacitor value either. Similarly, Pins 26
and 28 may be tied together on the board layout to save
a bypass capacitor. For switching frequencies greater than
750kHz, a 1µF capacitor or higher value ceramic capacitor
should be used to bypass each group of two V
IN
pins. For
switching frequencies less than 750kHz, a 2.2µF or higher
value ceramic capacitor should be used to bypass each
group of two V
IN
pins. The ceramic bypass capacitors
should be located as close to the V
IN
pins as possible.
See the sample layout shown in the PCB Layout section.
All four V
IN
pins should be tied together on the board and
bypassing with a low performance electrolytic capacitor
is recommended especially if the input power source has
high impedance, or there is significant inductance due to
long wires or cables.
Step-down regulators draw current from the input sup
-
ply in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage
ripple at the LT3504 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
To accomplish this task, the input bypass capacitor must
be placed close to the LT3504 and the catch diode; see
the PCB Layout section. A second precaution regarding
the ceramic input capacitor concerns the maximum input
voltage rating of the LT3504. A ceramic input capacitor
combined with trace or cable inductance forms a high
quality (underdamped) tank circuit. If the LT3504 circuit
is plugged into a live supply, the input voltage can ring to
Table 2. Inductor Vendors
VENDOR URL PART SERIES INDUCTANCE (µH) SIZE (mm)
Sumida www.sumida.com CDRH4D28
CDRH5D28
CDRH5D28
1.2 TO 4.7
2.5 TO 10
2.5 TO 33
4.5 × 4.5
5.5 × 5.5
8.3 × 8.3
Toko www
.toko.com A916CY
D585LC
2 TO 12
1.1 TO 39
6.3 × 6.2
8.1 × 8
Würth Elektronik www.we-online.com
WE-TPC(M)
WE-PD2(M)
WE-PD(S)
1 TO 10
2.2 TO 22
1 TO 27
4.8 × 4.8
5.2 × 5.8
7.3 × 7.3
Table 3. Capacitor Vendors
VENDOR PHONE URL PART SERIES COMMENTS
Panasonic [phone] www.panasonic.com Ceramic, Polymer, Tantalum EEF Series
Kemet [phone] www.kemet.com Ceramic, Tantalum T494, T495
Sanyo [phone] www.sanyovideo.com Ceramic, Polymer, Tantalum POSCAP
Murata [phone] www.murata.com Ceramic
AVX www.avxcorp.com Ceramic, Tantalum TPS Series
Taiyo Yuden [phone] www.taiyo-yuden.com Ceramic