Applications Information - Linear Technology Analog Devices LT8607B Datasheet

LT8607/LT8607B

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
10
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.
2500
2250
2000
1750
1500
1250
1000
Figure 1. Burst Frequency vs Output Current
V
20mV/DIV
I
200mA/DIV
10V/DIV
For more information www.analog.com
L = 2.2µH
V = 12V
IN
V = 3.3V
OUT
SYNC = 0V OR LT8607 DFN
750
500
250
0
0 25 50 75
OUTPUT CURRENT (mA)
150
L = 2.2µH
V = 12V
IN
125
V = 3.3V
OUT
R = 18.2kΩ
T
100
75
50
25
0
0 5 10 15 20 25 30
INPUT VOLTAGE (V)
Figure 2. Minimum Load to Full Frequency
(SYNC Float to 1.9V) (MSOP or LT8607B DFN)
OUT
LOAD
V
SW
2µs/DIV
Figure 3. Burst Mode Operation
100 125
8607 F01
35 40 45
8607 F02
8607 F03
Rev. C