Datasheet

LT8620
11
8620fa
For more information www.linear.com/LT8620
APPLICATIONS INFORMATION
Achieving Ultralow Quiescent Current
To enhance efficiency at light loads, the LT8620 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 LT8620
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
LT8620 consumes 1.7μA.
As the output load decreases, the frequency of single cur-
rent pulses decreases (see Figure 1a) and the percentage
of time the LT8620 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 LT8620 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.
While in Burst Mode operation the current limit of the top
switch is approximately 400mA resulting in output voltage
ripple shown in Figure 2. 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 1a
. The out-
put load at which the LT8620 reaches the programmed
frequency varies based on input voltage, output voltage,
and inductor choice.
Figure 1. SW Frequency vs Load Information in Burst Mode Operation (1a) and Pulse-Skipping Mode (1b)
Minimum Load to Full Frequency (SYNC DC High)
Burst Frequency
(1a) (1b)
LOAD CURRENT (mA)
0
SWITCHING FREQUENCY (kHz)
400
500
600
200
8620 F01a
300
200
0
50
100
150
100
800
V
IN
= 12V
V
OUT
= 5V
700
INPUT VOLTAGE (V)
LOAD CURRENT (mA)
60
80
100
15 25 55 65
8620 F01b
40
20
0
5 35 45
V
OUT
= 5V
f
SW
= 700kHz
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