Datasheet

LT8640/LT8640-1
15
Rev.C
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APPLICATIONS INFORMATION
The exposed pad on the bottom of the package should be
soldered to SW to reduce thermal resistance to ambient. 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 (Burst Mode
Operation)
To enhance efficiency at light loads, the LT8640/LT8640-1
operates in low ripple Burst Mode operation, which keeps
the output capacitor charged to the desired output voltage
while minimizing the input quiescent current and minimiz
-
ing output voltage ripple. In Burst Mode operation the
LT8640/LT8640-1 delivers single small pulses of current
to the output capacitor followed by sleep periods where
the output power is supplied by the output capacitor. While
in sleep mode the LT8640/LT8640-1 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 LT8640/LT8640-1 is in sleep mode increases,
resulting in much higher light load efficiency than for typi
-
cal converters. By maximizing the time between pulses,
the converter quiescent current approaches 2.5µA for a
typical 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
LT8640/LT8640-1 can stay in sleep mode longer be
-
tween 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 switching frequency
application, if high light load efficiency is desired, a
higher inductor value should be chosen. See curve in
Typical Performance Characteristics.
Figure2. SW Frequency vs Load Information in Burst Mode Operation (2a) and Pulse-Skipping Mode (2b)
Minimum Load to Full Frequency (Pulse-Skipping Mode)Burst Frequency
(2a)
(2b)
LOAD CURRENT (mA)
0
0
SWITCHING FREQUENCY (kHz)
200
400
600
800
1000
1200
100 200 300 400
8640 F02a
FRONT PAGE APPLICATION
V
IN
= 12V
V
OUT
= 5V
INPUT VOLTAGE (V)
5
LOAD CURRENT (mA)
60
80
100
20 30 45
8640 F02b
40
20
0
10 15
25
35 40
FRONT PAGE APPLICATION
V
OUT
= 5V
f
SW
= 1MHz
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