Datasheet

LT8641
14
Rev B
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APPLICATIONS INFORMATION
Achieving Ultralow Quiescent Current
To enhance efficiency at light loads, the LT8641 operates
in low ripple Burst Mode operation, which keeps the out-
put capacitor charged to the desired output voltage while
minimizing the input quiescent current and minimizing
output voltage ripple. In Burst Mode operation the LT8641
delivers single small pulses of current to the output capaci-
tor followed by sleep periods where the output power is
supplied by the output capacitor. While in sleep mode the
LT8641 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 LT8641 is in sleep mode increases, resulting in
much higher light load efficiency than for typical convert-
ers. 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 LT8641 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
curve in Typical Performance Characteristics.
While in Burst Mode operation the current limit of the top
switch is approximately 950mA (as shown in Figure 3),
resulting in low output voltage ripple. Increasing the output
capacitance will decrease output ripple proportionally. As
load ramps upward from zero the switching frequency
will increase but only up to the switching frequency pro-
grammed by the resistor at the RT pin as shown in Figure
2a. The output load at which the LT8641 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)
(2a)
(2b)
Figure 3. Burst Mode Operation
FRONT PAGE APPLICATION
V
IN
= 12V
V
OUT
= 5V
LOAD CURRENT (mA)
0
200
400
600
800
0
200
400
600
800
1000
1200
SWITCHING FREQUENCY (kHz)
Burst Frequency
8641 F02a
FRONT PAGE APPLICATION
V
OUT
= 5V
f
SW
= 1MHz
INPUT VOLTAGE (V)
5
15
25
35
45
55
65
0
20
40
60
80
100
120
140
LOAD CURRENT (mA)
(Pulse-Skipping Mode)
Minimum Load to Full Frequency
8641 F02b
Mode Operation
Switching Waveforms, Burst
10µs/DIV
V
SW
5V/DIV
I
L
500mA/DIV
8641 F03
FRONT PAGE APPLICATION
12V
IN
TO 5V
OUT
AT 10mA
V
SYNC
= 0V
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