Datasheet

LT8608/LT8608B
11
Rev. D
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APPLICATIONS INFORMATION
Figure 2. Full Switching Frequency Minimum Load vs V
IN
in
Pulse Skipping Mode (MSOP Only)
Figure 1. SW Burst Mode Frequency vs Load
LOAD CURRENT (mA)
0
100
200
300
400
500
0
250
500
750
1000
1250
1500
1750
2000
2250
2500
SWITHCING FREQUENCY (kHz)
Burst Frequency vs Load Current
8608 F01
V
IN
= 12V
L = 2.2µH
V
OUT
= 3.3V
SYNC = 0V
INPUT VOLTAGE (V)
0
5
10
15
20
25
30
35
40
0
25
50
75
100
125
LOAD CURRENT (mA)
8608 F02
L = 2.2µH
V
OUT
= 5V
R
T
= 18.2k
For some applications it is desirable for the LT8608 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 LT8608 will
also operate in pulse-skipping mode. The LT8608 DFN is
programmed for Burst Mode operation and can not enter
pulse-skipping mode. The LT8608B DFN is programmed
for pulse-skipping and cannot enter Burst Mode operation.
While in Burst Mode operation the current limit of the
top switch is approximately 550mA resulting in output
voltage ripple shown in Figure 3 and Figure 4. Increasing
the output capacitance will decrease the output ripple
proportionally. As load ramps upward from zero the switch-
ing frequency will increase but only up to the switching
frequency programmed by the resistor at the RT pin as
shown in Figure 1. The output load at which the LT8608
reaches the programmed frequency varies based on input
voltage, output voltage, and inductor choice.
Figure 4. Burst Mode Operation (Zoomed In)
Figure 3. Burst Mode Operation
V
OUT
20mV/DIV
INDUCTOR
CURRENT
500mA/DIV
8608 F03
SW
5V/DIV
20μs/DIV
V
OUT
20mV/DIV
INDUCTOR
CURRENT
500mA/DIV
8608 F04
SW
5V/DIV
500ns/DIV
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