Datasheet

LT8608/LT8608B
10
Rev. D
For more information www.analog.com
OPERATION
The LT8608 is a monolithic constant frequency current
mode step-down DC/DC converter. An oscillator with
frequency set using a resistor on the RT pin turns on
the internal top power switch at the beginning of each
clock cycle. Current in the inductor then increases until
the top switch current comparator trips and turns off the
top power switch. The peak inductor current at which the
top switch turns off is controlled by the voltage on the
internal VC node. The error amplifier servos the VC node
by comparing the voltage on the V
FB
pin with an internal
0.778V reference. When the load current increases it
causes a reduction in the feedback voltage relative to the
reference leading the error amplifier to raise the VC volt-
age until the average inductor current matches the new
load current. When the top power switch turns off the
synchronous power switch turns on until the next clock
cycle begins or inductor current falls to zero. If overload
conditions result in excess current flowing through the
bottom switch, the next clock cycle will be delayed until
switch current returns to a safe level.
If the EN/UV pin is low, the LT8608 is shut down and draws
1µA from the input. When the EN/UV pin is above 1.05V,
the switching regulator becomes active.
To optimize efficiency at light loads, the LT8608 enters
Burst Mode operation during light load situations. Between
bursts, all circuitry associated with controlling the output
switch is shut down, reducing the input supply current to
1.7μA. In a typical application, 2.5μA will be consumed
from the input supply when regulating with no load. The
SYNC pin is tied low to use Burst Mode operation and
can be floated to use pulse-skipping mode. If a clock is
applied to the SYNC pin the part will synchronize to an
external clock frequency and operate in pulse-skipping
mode. While in pulse-skipping mode the oscillator oper-
ates continuously and positive SW transitions are aligned
to the clock. During light loads, switch pulses are skipped
to regulate the output and the quiescent current will be
several mA. The SYNC pin may be tied high for spread
spectrum modulation mode, and the LT8608 will oper-
ate similar to pulse-skipping mode but vary the clock
frequency to reduce EMI. The LT8608 DFN has no SYNC
pin and will always operate in Burst Mode operation. The
LT8608B DFN has no SYNC pin and will always operate
in pulse-skipping mode.
Comparators monitoring the FB pin voltage will pull the
PG pin low if the output voltage varies more than ±8.5%
(typical) from the set point, or if a fault condition is present.
The oscillator reduces the LT8608’s operating frequency
when the voltage at the FB pin is low. This frequency fold-
back helps to control the inductor current when the output
voltage is lower than the programmed value which occurs
during start-up. When a clock is applied to the SYNC pin
the frequency foldback is disabled.
APPLICATIONS INFORMATION
Achieving Ultralow Quiescent Current
To enhance efficiency at light loads, the LT8608 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 minimizing
output voltage ripple. In Burst Mode operation the LT8608
delivers single small pulses of current to the output capaci-
tor followed by sleep periods where the output power is
supplied by the output capacitor. While in sleep mode the
LT8608 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 LT8608 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 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.
Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.