10
Rev. C
For more information
APPLICATIONS INFORMATION
Achieving Ultralow Quiescent Current
To enhance efficiency at light loads, the LT8607 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 mini-
mizing output voltage ripple. In Burst Mode operation the
LT8607 delivers single small pulses of current to the out-
put capacitor followed by sleep periods where the output
power is supplied by the output capacitor. While in sleep
mode the LT8607 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 LT8607 is in sleep mode increases, result-
ing in much higher light load efficiency than for typical
converters. By maximizing the time between pulses, the
converter quiescent current approaches 3.0µA for a typi-
cal 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 250mA resulting in output
voltage ripple shown in Figure 3. 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
Table 1. The output load at which the LT8607 reaches the
programmed frequency varies based on input voltage,
output voltage, and inductor choice.
For some applications it is desirable for the LT8607 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
as shown in Figure 2. To enable pulse-skipping mode the
SYNC pin is floated. To achieve spread spectrum modula-
tion with pulse-skipping mode, the SYNC pin is tied high.
While a clock is applied to the SYNC pin the LT8607 will
also operate in pulse-skipping mode. The LT8607 DFN is
Figure 1. Burst Frequency vs Output Current
R
T
= 18.2kΩ
V
IN
= 12V
L = 2.2µH
V
OUT
= 3.3V
INPUT VOLTAGE (V)
0
5
10 15 20 25 30 35 40 45
0
25
50
75
100
125
150
OUTPUT CURRENT (mA)
8607 F02
2µs/DIV
V
OUT
20mV/DIV
I
LOAD
200mA/DIV
V
SW
10V/DIV
8607 F03
programmed for Burst Mode operation and cannot enter
pulse-skipping mode. The LT8607B DFN is programmed
for pulse-skipping mode and cannot enter Burst Mode
operation.
OUTPUT CURRENT (mA)
0
25
50
75
100
125
0
250
500
750
1000
1250
1500
1750
2000
2250
2500
SWITHCING FREQUENCY (kHz)
8607 F01
V
IN
= 12V
L = 2.2µH
V
OUT
= 3.3V
SYNC = 0V OR LT8607 DFN
Figure 2. Minimum Load to Full Frequency
(SYNC Float to 1.9V) (MSOP or LT8607B DFN)
Figure 3. Burst Mode Operation
