Datasheet

LT8609/LT8609A/LT8609B
13
Rev. H
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APPLICATIONS INFORMATION
Achieving Ultralow Quiescent Current
To enhance efficiency at light loads, the LT8609/LT8609A
enters into 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
LT8609/LT8609A 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 LT8609/LT8609A consumes 1.7μA.
As the output load decreases, the frequency of single cur-
rent pulses decreases (see Figure1) and the percentage
of time the LT8609/LT8609A is in sleep mode increases,
resulting in much higher light load efficiency than for typi -
cal converters. By maximizing the time between pulses,
INPUT VOLTAGE (V)
8609 G23
0
LOAD CURRENT (mA)
100
40
60
20
80
90
30
50
10
70
0
30
5020 4010
V
OUT
= 5V
f
SW
= 700kHz
SYNC = FLOAT
Figure1b. Full Switching Frequency Minimum Load
vs V
IN
in Pulse Skipping Mode
Figure2. Burst Mode Operation
2.00µs/DIV
200mA/DIV
10mV/DIV
8609 F02
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.
While in Burst Mode operation the current limit of the
top switch is approximately 600mA 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 pro-
grammed by the resistor at the RT pin as shown in Table1.
The output load at which the LT8609/LT8609A reaches
the programmed frequency varies based on input voltage,
output voltage, and inductor choice.
For some applications, it is desirable for the LT8609/
LT8609A to operate in pulse-skipping mode, which is the
only mode available to the LT8609B. Pulse-skipping mode
offers two major differences from Burst Mode operation.
First is the clock stays awake at all times and all switch-
ing 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 as shown in Figure1b. To
enable pulse-skipping mode the SYNC pin is floated. To
achieve spread spectrum modulation with pulse-skipping
mode, the SYNC pin is tied high. While a clock is applied
to the SYNC pin the LT8609/LT8609A will also operate in
pulse-skipping mode.
LOAD CURRENT (mA)
0
FREQUENCY (kHz)
2500
1000
1500
500
2000
0
600200 400
L = 2.2µH
V
OUT
= 3.3V
V
IN
= 12V
SYNC = 0V
8609 G22
Figure 1a. SW Burst Mode Frequency vs Load
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