Datasheet

LT8611
12
8611fa
For more information www.linear.com/LT8611
APPLICATIONS INFORMATION
Achieving Ultralow Quiescent Current
To enhance efficiency at light loads, the LT8611 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 LT8611
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
LT8611 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 LT8611
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
q
u
iescent 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.
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 LT8611 reaches the programmed
frequency varies based on input voltage, output voltage,
and inductor choice.
For some applications it is desirable for the LT8611
to
operate in pulse-skipping mode, offering two major differ
-
ences from Burst Mode operation. First is the clock stays
awake
at
all times and all switching cycles are aligned to
the clock. In this mode much of the internal circuitry is
awake at all times, increasing quiescent current to several
hundred µA. Second is that full switching frequency is
reached at lower output load than in Burst Mode operation
(see Figure 1b). To enable pulse-skipping mode, the SYNC
pin is tied high either to a logic output or to the INTV
CC
pin. When a clock is applied to the SYNC pin the LT8611
will also operate in pulse-skipping mode.
Figure 1. SW Frequency vs Load Information in
Burst Mode Operation (1a) and Pulse-Skipping Mode (1b)
Figure 2. Burst Mode Operation
Minimum Load to Full Frequency (SYNC DC High)
Burst Frequency
(1a)
(1b)
LOAD CURRENT (mA)
0
SWITCHING FREQUENCY (kHz)
400
500
600
200
8611 F01a
300
200
0
50
100
150
100
800
V
IN
= 12V
V
OUT
= 3.3V
700
INPUT VOLTAGE (V)
LOAD CURRENT (mA)
60
80
100
15 25 40 45
8611 F01b
40
20
0
5 10
20
30 35
5V
OUT
700kHz
I
L
200mA/DIV
V
OUT
10mV/DIV
5µs/DIVV
SYNC
= 0V
8611 F02
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