LT3502 LINER [Linear Technology], LT3502 Datasheet - Page 12

• Manufacturer Part Number: LT3502
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LT3502/LT3502A
Catch Diode
A low capacitance 500mA Schottky diode is recommended
for the catch diode, D1. The diode must have a reverse
voltage rating equal to or greater than the maximum input
voltage. The Phillips PMEG4005AEA is a good choice; it
is related for 500mA continuous forward current and a
maximum reverse voltage of 40V.
Input Capacitor
Bypass the input of the LT3502/LT3502A circuit with a 1μF
or higher value ceramic capacitor of X7R or X5R type. Y5V
types have poor performance over temperature and applied
voltage and should not be used. A 1μF ceramic is adequate
to bypass the LT3502/LT3502A and will easily handle the
ripple current. However, if the input power source has
high impedance, or there is signifi cant inductance due to
long wires or cables, additional bulk capacitance may be
necessary. This can be provided with a low performance
electrolytic capacitor.
Step-down regulators draw current from the input supply
in pulses with very fast rise and fall times. The input ca-
pacitor is required to reduce the resulting voltage ripple at
the LT3502/LT3502A and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
A 1μF capacitor is capable of this task, but only if it is placed
close to the LT3502/LT3502A and the catch diode (see the
PCB Layout section). A second precaution regarding the
ceramic input capacitor concerns the maximum input volt-
age rating of the LT3502/LT3502A. A ceramic input capaci-
tor combined with trace or cable inductance forms a high
Table 1
VENDOR
Sumida
Toko
Würth Elektronik
12
URL
www.sumida.com
www.toko.com
www.we-online.com
PART SERIES
WE-PD2(M)
WE-TPC(M)
CDRH4D28
CDRH5D28
CDRH8D28
WE-PD(S)
A916CY
D585LC
quality (underdamped) tank circuit. If the LT3502/LT3502A
circuit is plugged into a live supply, the input voltage can
ring to twice its nominal value, possibly exceeding the
LT3502/LT3502A’s voltage rating. This situation is easily
avoided; see the Hot Plugging Safely section.
Output Capacitor
The output capacitor has two essential functions. Along
with the inductor, it fi lters the square wave generated by
the LT3502/LT3502A to produce the DC output. In this role
it determines the output ripple so low impedance at the
switching frequency is important. The second function is to
store energy in order to satisfy transient loads and stabilize
the LT3502/LT3502A’s control loop. Ceramic capacitors
have very low equivalent series resistance (ESR) and
provide the best ripple performance. A good value is:
where C
in mind that a ceramic capacitor biased with V
have less than its nominal capacitance. This choice will
provide low output ripple and good transient response.
Transient performance can be improved with a high value
capacitor, but a phase lead capacitor across the feedback
resistor, R1, may be required to get the full benefi t (see
the Compensation section).
C
C
OUT
OUT
OUT
=
=
V
V
INDUCTANCE RATE (μH)
33
66
OUT
OUT
is in μF . Use an X5R or X7R type and keep
for the LT3502A
for the LT3502
1.2 to 4.7
2.5 to 10
2.5 to 33
1.1 to 39
2.2 to 22
2 to 12
1 to 10
1 to 27
SIZE (mm)
4.5 × 4.5
5.5 × 5.5
8.3 × 8.3
6.3 × 6.2
4.8 × 4.8
5.2 × 5.8
7.3 × 7.3
8.1 × 8
OUT
3502fc
will