Datasheet

LT8614
13
Rev. E
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APPLICATIONS INFORMATION
traces will shield them from the SW and BOOST nodes.
The exposed pad on the bottom of the package should
be soldered to SW to reduce thermal resistance to ambi-
ent. To keep thermal resistance low, extend the ground
plane from GND1 and GND2 as much as possible, and
add thermal vias to additional ground planes within the
circuit board and on the bottom side.
Achieving Ultralow Quiescent Current
To enhance efficiency at light loads, the LT8614 oper-
ates in low ripple Burst Mode operation, which keeps the
output capacitor charged to the desired output voltage
while minimizing the input quiescent current and mini-
mizing output voltage ripple. In Burst Mode operation the
LT8614 delivers single small pulses of current to the out-
put capacitor followed by sleep periods where the output
power is supplied by the output capacitor
. While in sleep
mode the LT8614 consumes 1.7μA.
As the output load decreases, the frequency of single cur-
rent pulses decreases (see Figure2a) and the percentage
of time the LT8614 is in sleep mode increases, result-
ing in much higher light load efficiency than for typical
converters. By maximizing the time between pulses, the
converter quiescent current approaches 2.5µA for a typi
-
cal application when there is no output load. Therefore,
to optimize the quiescent current performance at light
loads, the current in the feedback resistor divider must
be minimized as it appears to the output as load current.
In order to achieve higher light load efficiency, more energy
must be delivered to the output during the single small
pulses in Burst Mode operation such that the LT8614 can
stay in sleep mode longer between each pulse. This can be
achieved by using a larger value inductor (i.e., 4.7µH), and
should be considered independent of switching frequency
when choosing an inductor. For example, while a lower
inductor value would typically be used for a high switch-
ing frequency application, if high light load efficiency is
desired, a higher inductor value should be chosen. See
cur
ve in Typical Performance Characteristics.
While in Burst Mode operation the current limit of the
top switch is approximately 600mA resulting in output
voltage ripple shown in Figure3. Increasing the output
capacitance will decrease the output ripple proportionally.
As load ramps upward from zero the switching frequency
will increase but only up to the switching frequency
programmed by the resistor at the RT pin as shown in
Figure2a. The output load at which the LT8614 reaches
the programmed frequency varies based on input voltage,
output voltage, and inductor choice.
Figure2. SW Frequency vs Load Information in Burst
Mode Operation (2a) and Pulse-Skipping Mode (2b)
Minimum Load to Full Frequency (SYNC DC High)
Burst Frequency
(2a)
(2b)
LOAD CURRENT (mA)
0
0
SWITCHING FREQUENCY (kHz)
200
400
600
800
1000
1200
50 100 150 200
8614 F02a
FRONT PAGE APPLICATION
V
IN
= 12V
V
OUT
= 5V
INPUT VOLTAGE (V)
5
LOAD CURRENT (mA)
60
80
100
20 30 45
8614 F02b
40
20
0
10 15
25
35 40
FRONT PAGE APPLICATION
V
OUT
= 5V
f
SW
= 1MHz
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