LT3669EUFD データシートの表示（PDF） - Linear Technology
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LT3669EUFD Datasheet PDF : 40 Pages
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LT3669/LT3669-2
APPLICATIONS INFORMATION
Input Voltage Range
The minimum input voltage is determined by either the
LT3669’s minimum operating voltage of 7.5V or by its
maximum duty cycle (see equation in the Operating Fre-
quency Trade-Offs section). The minimum input voltage
due to duty cycle is:
VL +(M IN)
=
1−
VOUT + VD
fSW • tOFF(MIN)
− VD
+
VSW
where VL+(MIN) is the minimum input voltage, and tOFF(MIN)
is the minimum switch-off time. Note that higher switch-
ing frequency will increase the minimum input voltage.
If a lower dropout voltage is desired, a lower switching
frequency should be used.
The maximum input voltage for LT3669 applications
depends on switching frequency, the absolute maximum
ratings of the L+ and BST pins, and the operating mode.
The LT3669 can operate continuously from input voltages
up to 40V. Input voltage transients of up to 60V are also
safely withstood. However, note that if VL+ exceeds VOVLO
(43V typical), the LT3669 will stop switching, allowing the
output to fall out of regulation.
For a given application in which the switching frequency
and the output voltage are already fixed, the maximum
input voltage that guarantees optimum output voltage
ripple for that application can be found by applying the
following expression:
VL +(MAX )
=
VOUT + VD
fSW • tON(MIN)
−
VD
+ VSW
where VL+(MAX) is the maximum operating input voltage,
VOUT is the output voltage, VD is the catch diode drop
(~0.72V in LT3669) and VSW is the internal drop from L+
to SW pins (~1.0V in LT3669 and ~1.4V in LT3669-2 at
maximum load), fSW is the switching frequency (set by
RT), and tON(MIN) is the minimum switch-on time. Note
that a higher switching frequency will reduce the maximum
operating input voltage. Conversely, a lower switching
frequency is necessary to achieve optimum operation at
high input voltages.
Special attention must be paid when the output is in
start-up, short-circuit, or other overload conditions.
In these cases, the LT3669 tries to bring the output in
regulation by driving lots of current into the output load.
During these events, the inductor peak current might easily
reach and even exceed the maximum current limit of the
LT3669, especially in those cases where the switch already
operates at minimum on-time. The circuitry monitoring
the current through the catch diode prevents the switch
from turning on again if the inductor valley current is above
0.2A and 0.45A nominal values for LT3669 and LT3669-2,
respectively. In these cases, the inductor peak current is
therefore the maximum current limit of the LT3669 plus
the additional current overshoot during the turn-off delay
due to minimum on-time:
I L(PEAK )
= ISW(LIM) +
VL+(MAX) − VOUTOL
L
• tON(MIN)
where IL(PEAK) is the peak inductor current, ISW(LIM) is
the switch current limit (0.325A in LT3669 and 0.65A
in LT3669-2), VL+(MAX) is the maximum expected input
voltage, L is the inductor value, tON(MIN) is the minimum
on-time and VOUTOL is the output voltage under the overload
condition. The part is robust enough to survive prolonged
operation under these conditions as long as the peak in-
ductor current does not exceed 0.6A in LT3669 and 1.3A
in LT3669-2. Inductor current saturation and excessive
junction temperature may further limit performance.
Inductor Selection and Maximum Output Current
A good first choice for the inductor value is:
L
=
(VOUT
+
VD )
•
k
fSW
(k = 9 in LT3669, k = 3.6 in LT3669-2)
where fSW is the switching frequency in MHz, VOUT is
the output voltage, VD is the catch diode drop (~0.72V in
LT3669) and L is the inductor value in μH.
The inductor’s RMS current rating must be greater than
the maximum load current and its saturation current
should be about 30% higher. To keep the efficiency high,
the series resistance (DCR) should be less than 0.1Ω, and
the core material should be intended for high frequency
applications. Table 3 lists several vendors of inductors.
3669fa
28
For more information www.linear.com/LT3669
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