Datasheet

LT3686
13
3686fc
The MODE pin serves as mode select for the BD active
load circuit. The active load is enabled when MODE is
tied high and disabled when MODE is tied low. See Fixed
Frequency at Light Load section.
Inductor Selection and Maximum Output Current
A good first choice for the inductor value is:
L =
4(V
OUT
+ V
D
)
f
where V
D
is the voltage drop of the catch diode (~0.4V), L
is in μH, frequency is in MHz. With this value there will be
no subharmonic oscillation. The inductor’s RMS current
rating must be greater than the maximum load current
and its saturation current should be about 30% higher. For
robust operation during fault conditions, the saturation
current should be above 2A. To keep efficiency high, the
series resistance (DCR) should be less than 0.1Ω. Table
2 lists several vendors and types that are suitable.
There are several graphs in the Typical Performance
Characteristics section of this data sheet that show the
maximum load current as a function of input voltage and
inductor value for several popular output voltages. Low
inductance may result in discontinuous mode opera-
tion, which is okay, but further reduces maximum load
current. For details of the maximum output current and
discontinuous mode operation, see Linear Technology
Application Note 44.
ApplicAtions inForMAtion
Catch Diode
A low capacitance 1-2A 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 MBRM140 is a good choice; it is rated for
1A continuous forward current and a maximum reverse
voltage of 40V.
Input Capacitor
Bypass the input of the LT3686 circuit with a 2.2μF or
higher value ceramic capacitor of X7R or X5R type. Y5V
types have poor performance over temperature and ap-
plied voltage and should not be used. A 2.2μF ceramic is
adequate to bypass the LT3686 and will easily handle the
ripple current. However, 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 supply in pulses with very fast rise and
fall times. The input capacitor is required to reduce the
resulting voltage ripple at the LT3686 and to force this very
high frequency switching current into a tight local loop,
minimizing EMI. A 2.2μF capacitor is capable of this task,
but only if it is placed close to the LT3686 and the catch
diode (see the PCB Layout section). A second precaution
regarding the ceramic input capacitor concerns the maxi-
mum input voltage rating of the LT3686. A ceramic input
capacitor combined with trace or cable inductance forms
Table 2
VENDOR URL PART SERIES INDUCTANCE RATE (µH) SIZE (mm)
Sumida www.sumida.com CDRH4D28
CDRH5D28
CDRH8D28
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