lt3990 Linear Technology Corporation, lt3990 Datasheet - Page 11

• Manufacturer Part Number: lt3990
• Description: 60v, 350ma Step-down Regulator With 2.5?a Quiescent Current And Integrated Diodes
• Manufacturer: Linear Technology Corporation

applicaTions inFormaTion
(due to longer on-times). If the input power source has
high impedance, or there is significant 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 sup-
ply in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage ripple
at the LT3990 and to force this very high frequency switch-
ing 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 LT3990 (see the PCB Layout section). A second
precaution regarding the ceramic input capacitor concerns
the maximum input voltage rating of the LT3990. A ceramic
input capacitor combined with trace or cable inductance
forms a high quality (under damped) tank circuit. If the
LT3990 circuit is plugged into a live supply, the input volt-
age can ring to twice its nominal value, possibly exceeding
the LT3990’s voltage rating. This situation is easily avoided
(see the Hot Plugging Safely section).
Output Capacitor and Output Ripple
The output capacitor has two essential functions. It stores
energy in order to satisfy transient loads and stabilize the
LT3990’s control loop. Ceramic capacitors have very low
equivalent series resistance (ESR) and provide the best
ripple performance. A good starting value is:
where f
capacitance in μF . Use X5R or X7R types. This choice will
provide low output ripple and good transient response.
Transient performance can be improved with a higher value
capacitor if combined with a phase lead capacitor (typically
22pF) between the output and the feedback pin. A lower
value of output capacitor can be used to save space and
cost but transient performance will suffer.
The second function is that the output capacitor, along
with the inductor, filters the square wave generated by the
LT3990 to produce the DC output. In this role it determines
C
OUT
SW
=
is in MHz and C
V
OUT
50
•
f
SW
OUT
is the recommended output
the output ripple, so low impedance (at the switching
frequency) is important. The output ripple decreases with
increasing output capacitance, down to approximately
1mV. See Figure 1. Note that a larger phase lead capacitor
should be used with a large output capacitor.
When choosing a capacitor, look carefully through the
data sheet to find out what the actual capacitance is under
operating conditions (applied voltage and temperature).
A physically larger capacitor or one with a higher voltage
rating may be required. Table 3 lists several capacitor
vendors.
Table 3. Recommended Ceramic Capacitor Vendors
Ceramic Capacitors
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT3990 due to their piezoelectric nature.
When in Burst Mode operation, the LT3990’s switching
frequency depends on the load current, and at very light
loads the LT3990 can excite the ceramic capacitor at audio
frequencies, generating audible noise. Since the LT3990
Figure 1. Worst-Case Output Ripple Across Full Load Range
MANUFACTURER
Vishay Siliconix
Taiyo Yuden
18
14
12
10
16
8
6
4
0
2
Murata
AVX
TDK
0
V
20
IN
= 12V
FRONT PAGE APPLICATION
C
LEAD
40
C
OUT
= 47pF FOR C
V
(µF)
IN
60
= 24V
OUT
www.avxcorp.com
www.t-yuden.com
www.murata.com
www.vishay.com
80
www.tdk.com
≥ 47µF
WEBSITE
3990 F01
100
LT3990

3990p