LT8471
10
8471fd
For more information www.linear.com/8471
Input Supply Requirements
V
IN1
is the main power supply of the LT8471. It powers
channel 1, the Skyhook channel and most of the internal
control and bias circuits for both channels. It must be
powered up to enable any channel of the LT8471. V
IN2
is the power supply for channel 2, and only needs to be
powered up when channel 2 is used. When V
IN2
is not
powered up, channel 2 will shut down.
Switch Configurations and the Skyhook Regulator
The primary channel NPN power switches can be connected
in low side or high side configurations. A low side connec
-
tion is
when the power switch is on the lower voltage side
of
the inductor while the switch is on. The boost, SEPIC,
flyback and dual-inductor inverting configurations use low
side power switches. Conversely, a high side connection is
when the power switch is on the higher voltage side of the
inductor when it is on. The buck, ZETA and single-inductor
inverting configurations use high side power switches.
Channels 1 and 2 can be configured for high side or low
side switching and do not need to be configured in the
same way as the other.
applicaTions inForMaTion
operaTion
In the low side configurations, the E pin is typically tied
to ground and the respective C pin toggles. V
IN
for the
respective channel must operate in a range of 2.6V to 40V.
High side configurations require that the C pin be tied to a
positive DC voltage supply and 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 drive to the
base of the NPN power switch. When configured with a
high side switch, the channel’s V
IN
can operate up to 50V
above ground, 60V above the respective E pin voltage, and
40V above the respective C pin voltage.
The Skyhook boost regulator is available to provide ad
-
ditional V
IN
voltage when it is needed to support high side
switch topologies. When enabled, the Skyhook output
voltage (SHOUT) is regulated to 4.25V (typical) above
the C2 pin voltage. Figure 1 shows an example where the
application input voltage is 6V to 32V. The Skyhook boost
converter regulates SHOUT, V
IN1
and V
IN2
to 10.25V to
36.25V insuring that the V
IN1
and V
IN2
pins are typically
4.25V above the C1 and C2 pins. More information about
the Skyhook regulator is available in later sections.
and
a gradual ramp-up of duty cycle for the Skyhook
channel. When the part is brought out of shutdown, the
external SS capacitors are first discharged (providing
protection against OV/UV pin glitches and slow ramp
-
ing). Next
, internal 250k resistors pull the SS pins up
to
~2.15V. By connecting an external capacitor to each
of the SS pins, the voltage ramp rates on the pins can
be set. Typical values for the soft-start capacitor range
from 100nF to 1μF.
• Finally, the primary channels’ switching frequency is
folded back by 2, 4, or 8 times when the correspond
-
ing FB pin voltage is below certain thresholds (see
the
Typical Performance Characteristics section). This
feature reduces the minimum duty cycle that the part
can achieve, thus allowing better control of the switch
current during start-up. The slope compensation func
-
tion is disabled during foldback to increase the available
current that can be delivered to the output.
Thermal Shutdown Operation
Not shown in the Block Diagram is the thermal shutdown
circuit. If the temperature of the part exceeds approximately
164°C, the SR21 and
SR22 latches
are set. A full soft-start
cycle will then be initiated after the temperature drops
below approximately 162.5°C. The thermal shutdown
circuit protects the power switches as well as the external
components connected to the LT8471.
