Datasheet

LT8612
11
8612fa
For more information www.linear.com/LT8612
APPLICATIONS INFORMATION
Achieving Ultralow Quiescent Current
To enhance efficiency at light loads, the LT8612 oper-
ates in 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
LT8612 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 LT8612 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 LT8612 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
quiescent current approaches 3µA 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 700mA 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 output load at which the LT8612 reaches
the programmed frequency varies based on input voltage,
output voltage, and inductor choice.
For some applications it is desirable for the LT8612 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 LT8612
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
SWITCH FREQUENCY (kHz)
0
500
600
800
8612 F01a
400
300
200
100
700
100
200 500300 400
V
IN
= 12V
V
OUT
= 5V
L = 3.9µH
I
L
0.5A/DIV
V
SW
5V/DIV
5µs/DIV
12V
IN
TO 5V
OUT
AT 20mA; FRONT PAGE APP
V
SYNC
= 0V
8612 F02
INPUT VOLTAGE (V)
0
MINIMUM LOAD (mA)
0
40
50
60
8612 F01b
30
20
10
10
20 5030 40
FRONT PAGE
APPLICATION
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