lt3686a Linear Technology Corporation, lt3686a Datasheet - Page 13

• Manufacturer Part Number: lt3686a
• Description: 37v/1.2a Step-down Regulator In 3mm
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
The SYNC/MODE pin is used to synchronize the internal
oscillator with an external square wave. The synchronizing
clock signal to the LT3686A should be between 300kHz to
2.5MHz with pulse width of at least 200ns on and off time,
a low state below 0.4V and a high state above 0.8V. The
SYNC frequency must be higher than the RT programmed
frequency; see Table 1.
The inductor value should be chosen based on the RT
frequency rather than the highest synchronization fre-
quency.
The SYNC/MODE pin doubles as mode select for the
BD active load circuit. The active load is enabled when
SYNC/MODE is driven with clock pulses or tied greater
than 0.8V and disabled when SYNC/MODE is tied below
0.4V. See Fixed Frequency at Light Load section.
Inductor Selection and Maximum Output Current
A good fi rst choice for the inductor value is:
where V
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 effi ciency high, the
series resistance (DCR) should be less than 0.1Ω. Table 2
lists several vendors and types that are suitable. For small
size, the inductor can be chosen according to:
Table 2.
VENDOR
Sumida
Toko
Würth Elektronik
L =
L =
2(V
4(V
D
is the voltage drop of the catch diode (~0.4V), L
OUT
OUT
f
f
+ V
+ V
D
D
)
)
www.we-online.com
www.sumida.com
www.toko.com
URL
PART SERIES
WE-PD2(M)
WE-TPC(M)
CDRH4D28
CDRH8D28
CDRH5D28
WE-PD(S)
A916CY
D585LC
Using a smaller value inductor will increase inductor current
ripple and reduce the V
can keep the LT3686A at full switching frequency.
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. Finally, for duty cycles greater than
50% (V
required to avoid subharmonic oscillations. See Linear
Technology Application Note 19.
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 LT3686A 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 LT3686A 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
OUT
/V
INDUCTANCE RATE(μH)
IN
> 0.5), there is a minimum inductance
1.2 to 4.7
2.5 to 10
2.5 to 33
2.2 to 22
1.1 to 39
2 to 12
1 to 10
1 to 27
IN
voltage at which the active load
LT3686A
SIZE (mm)
4.5 x 4.5
5.5 x 5.5
8.3 x 8.3
6.3 x 6.2
4.8 x 4.8
5.2 x 5.8
7.3 x 7.3
8.1 x 8
13
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