LT8471
21
8471fd
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Skyhook Configuration Requirements
The Skyhook provides the boosted V
IN
voltage required
for channels operating in the high side configuration. High
side channels have their respective C pin tied to a posi
-
tive DC voltage supply (usually V
CC
) while the respective
E pin toggles. The channel’s V
IN
pin should be at least
2.2V (typical) higher than the respective C pin to provide
adequate base drive for the NPN power switch. Internal
circuits monitor the voltage difference between V
IN1
and
E1 (and V
IN2
and E2). If the voltage difference is less
than 2.2V (typical), the power switch will be turned off
immediately for that clock cycle. Increasing voltage dif
-
ference between V
IN
and the respective C pin increases
power loss and reduces efficiency. V
IN
must not be more
than 40V higher than the respective C pin for high side
configurations. If use of Skyhook channel is not desired,
then the boosted V
IN
voltage can instead be provided by
an external power supply or by the output of the opposite
channel if the voltage is high enough.
The Skyhook output (SHOUT) is regulated to ~4.25V above
the C2 pin voltage and can be connected to the
appropriate
V
IN
pin(s) as shown in the Typical Applications section.
When in use, the Skyhook can only be configured as a
boost converter (i.e., as in Figure 1a). Also, since SHOUT
is regulated to ~4.25V above C2, the C2 pin must be
connected to a DC voltage (usually V
CC
) and must not be
toggling. Because of this requirement, if channel 2 is used
while the Skyhook is operating, channel 2 must be in the
high side configuration such as buck or single-inductor
inverting. If not being used, the Skyhook channel can be
disabled by connecting the C3 pin to ground. When the
Skyhook channel is disabled V
IN1
current is reduced.
Capacitor and Diode Selection (Skyhook)
A low ESR capacitor should be used at the Skyhook output
to minimize voltage ripple. Ceramic capacitors make a
good choice for this (see the discussion in the Capacitor
Selection (Primary Channels) section). The capacitor value
can affect stability. Read the upcoming Compensation
(Skyhook) section for more information.
For the best noise performance, the Skyhook output
capacitor should be connected from SHOUT to GND, and
the capacitors should be placed close to the pins (V
IN1
or V
IN2
) that SHOUT is shorted to. The Skyhook output
capacitor can also be connected from SHOUT to the C2
pin (usually V
CC
), as shown in Figure 9a. By doing this, the
output voltage of the Skyhook, or the boosted base drive
voltage for the primary channels will have better tracking
with the supply voltage of the channel. In addition, the
voltage across the capacitor is lower, thus reducing the
size and required voltage rating of the capacitor.
The Skyhook has a Schottky diode built on-chip. Nev
-
ertheless, an
external Schottky diode can be connected
from C3 to SHOUT to improve performance when load
currents are high. The diode choice can be made based
on the discussion in the Diode Selection (Primary Chan
-
nels) section.
The output current (I
OUT
) for the Skyhook
channel can be estimated as:
I
OUT
≅
CC
+
OUT1
1
+
OUT2
2
β • V
CC
• η
where:
V
CC
= Input voltage of the Skyhook.
I
OUT1
= Average output current of channel 1 if V
IN1
is
connected to SHOUT (0 otherwise).
I
OUT2
= Average output current of channel 1 if V
IN2
is
connected to SHOUT (0 otherwise).
DC1 = Duty cycle of channel 1 in steady state.
DC2 = Duty cycle of channel 2 in steady state.
η = Power conversion efficiency of the Skyhook (typically
87%).
β = Channel 1/channel 2 power switch beta (typically 35)
applicaTions inForMaTion
