LT8610A/LT8610AB Series
19
8610abfa
For more information www.linear.com/LT8610A
APPLICATIONS INFORMATION
Figure 3. Reverse V
IN
Protection
V
IN
IN
LT8610A/
LT8610AB
EN/UV
GND
to 2.2MHz range. The R
T
resistor should be chosen to set
the LT8610A/LT8610AB switching frequency equal to or
below the lowest synchronization input. For example, if the
synchronization signal will be 500kHz and higher, the R
T
should be selected for 500kHz. The slope compensation is
set by the R
T
value, while the minimum slope compensation
required to avoid subharmonic oscillations is established
by the inductor size, input voltage, and output voltage.
Since the synchronization frequency will not change the
slopes of the inductor current waveform, if the inductor
is large enough to avoid subharmonic oscillations at the
frequency set by R
T
, then the slope compensation will be
sufficient for all synchronization frequencies.
For some applications it is desirable for the LT8610A/
LT8610AB to operate in pulse-skipping mode, offering two
major differences from Burst Mode operation. First is the
clock stays awake at all times and all switching cycles are
aligned to the clock. Second is that full switching frequency
is reached at lower output load than in Burst Mode operation.
These two differences come at the expense of increased
quiescent current. To enable pulse-skipping mode, the SYNC
pin is tied high
either to
a logic output or to the INTV
CC
pin.
The LT8610A/LT8610AB does not operate in forced con-
tinuous mode
regardless of SYNC signal. Never leave the
SYNC pin floating.
Shorted and Reversed Input Protection
The LT8610A/LT8610AB will tolerate a shorted output.
Several features are used for protection during output
short-circuit and brownout conditions. The first is the
switching frequency will be folded back while the output
is lower than the set point to maintain inductor current
control. Second, the bottom switch current is monitored
such that if inductor current is beyond safe levels switch
-
ing of the top switch will be delayed until such time as the
inductor current falls to safe levels.
Frequency
foldback behavior depends on the state of the
SYNC pin: If the SYNC pin is low the switching frequency
will slow while the output voltage is lower than the pro
-
grammed level
.
If the SYNC pin is connected to a clock
source or tied high, the LT8610A/LT8610AB will stay at
the programmed frequency without foldback and only
slow switching if the inductor current exceeds safe levels.
There is another situation to consider in systems where
the
output
will be held high when the input to the LT8610A/
LT8610AB is absent. This may occur in battery charging
applications or in battery-backup systems where a battery
or some other supply is diode ORed with the LT8610A/
LT8610AB’s output. If the V
IN
pin is allowed to float
and the EN pin is held high (either by a logic signal or
because it is tied to V
IN
), then the LT8610A/LT8610AB’s
internal circuitry will pull its quiescent current through
its SW pin. This is acceptable if the system can tolerate
several μA in this state. If the EN pin is grounded the SW
pin current will drop to near 1µA. However, if the V
IN
pin
is grounded while the output is held high, regardless of
EN, parasitic body diodes inside the LT8610A/LT8610AB
can pull current from the output through the SW pin and
the V
IN
pin. Figure 3 shows a connection of the V
IN
and
EN/UV pins that will allow the LT8610A/LT8610AB to run
only
when the input voltage is present and that protects
against a shorted or reversed input.
PCB
Layout
For proper
operation and minimum EMI, care must be taken
during printed circuit board layout. Figure 4 shows the
recommended component placement with trace, ground
plane and via locations. Note that large, switched currents
flow in the LT8610A/LT8610AB’s V
IN
pins, GND pins, and
the input capacitor (C1). The loop formed by the input
capacitor should be as small as possible by placing the
capacitor adjacent to the V
IN
and GND pins. When using
a physically large input capacitor the resulting loop may
become too large in which case using a small case/value
capacitor placed close to the V
IN
and GND pins plus a larger
capacitor further away is preferred. These components,
along with the inductor and output capacitor, should be
placed on the same side of the circuit board, and their
connections should be made on that layer. Place a local,
unbroken ground plane under the application circuit on
