LT3690 LINER [Linear Technology], LT3690 Datasheet - Page 13

• Manufacturer Part Number: LT3690
• Description: 42V, 2.5A Synchronous Step-Down Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
The LT8610 is capable of a maximum duty cycle of greater
than 99%, and the V
R
switch cycles, resulting in a lower switching frequency
than programmed by RT.
For applications that cannot allow deviation from the pro-
grammed switching frequency at low V
the following formula to set switching frequency:
where V
skipped cycles, V
V
respectively at maximum load), f
quency (set by RT), and t
off-time. Note that higher switching frequency will increase
the minimum input voltage below which cycles will be
dropped to achieve higher duty cycle.
Inductor Selection and Maximum Output Current
The LT8610 is designed to minimize solution size by
allowing the inductor to be chosen based on the output
load requirements of the application. During overload or
short-circuit conditions the LT8610 safely tolerates opera-
tion with a saturated inductor through the use of a high
speed peak-current mode architecture.
A good first choice for the inductor value is:
where f
the output voltage, V
(~0.15V) and L is the inductor value in μH.
To avoid overheating and poor efficiency, an inductor must
be chosen with an RMS current rating that is greater than
the maximum expected output load of the application. In
addition, the saturation current (typically labeled I
rating of the inductor must be higher than the load current
plus 1/2 of in inductor ripple current:
V
SW(BOT)
DS(ON)
IN(MIN)
L =
I
L(PEAK)
V
SW
OUT
of the top switch. In this mode the LT8610 skips
IN(MIN)
=
are the internal switch drops (~0.3V, ~0.15V,
1– f
=I
V
is the switching frequency in MHz, V
+ V
OUT
f
LOAD(MAX)
SW
SW
SW(BOT)
is the minimum input voltage without
+ V
OUT
• t
SW(BOT)
OFF(MIN)
IN
is the output voltage, V
SW(BOT)
-to-V
+
OFF(MIN)
2
1
OUT
∆I
– V
is the bottom switch drop
L
dropout is limited by the
SW(BOT)
SW
is the minimum switch
is the switching fre-
IN
+ V
/V
OUT
SW(TOP)
SW(TOP)
ratios use
OUT
SAT
and
(5)
(6)
(7)
is
)
where ∆I
Equation 9 and I
for a given application.
As a quick example, an application requiring 1A output
should use an inductor with an RMS rating of greater than
1A and an I
overload or short-circuit conditons, the inductor RMS
routing requirement is greater to avoid overheating of the
inductor. To keep the efficiency high, the series resistance
(DCR) should be less than 0.04Ω, and the core material
should be intended for high frequency applications.
The LT8610 limits the peak switch current in order to
protect the switches and the system from overload faults.
The top switch current limit (I
duty cycles and decreases linearly to 2.8A at DC = 0.8. The
inductor value must then be sufficient to supply the desired
maximum output current (I
of the switch current limit (I
The peak-to-peak ripple current in the inductor can be
calculated as follows:
where f
L is the value of the inductor. Therefore, the maximum
output current that the LT8610 will deliver depends on
the switch current limit, the inductor value, and the input
and output voltages. The inductor value may have to be
increased if the inductor ripple current does not allow
sufficient maximum output current (I
switching frequency, and maximum input voltage used in
the desired application.
The optimum inductor for a given application may differ
from the one indicated by this design guide. A larger value
inductor provides a higher maximum load current and
reduces the output voltage ripple. For applications requir-
ing smaller load currents, the value of the inductor may
be lower and the LT8610 may operate with higher ripple
I
∆I
OUT(MAX)
L
=
SW
L
L • f
V
is the switching frequency of the LT8610, and
is the inductor ripple current as calculated in
SAT
OUT
SW
=I
of greater than 1.3A. During long duration
LIM
• 1–
LOAD(MAX)
⎛
⎜
⎝
–
∆I
V
2
IN(MAX)
L
V
OUT
OUT(MAX)
is the maximum output load
LIM
LIM
) and the ripple current.
⎞
⎟
⎠
) is at least 3.5A at low
), which is a function
OUT(MAX)
LT8610
) given the
13
8610p
(8)
(9)