LT8471
9
8471fd
For more information www.linear.com/8471
operaTion
The LT8471 consists of two primary channels, each with
a 2A power switch. One Skyhook channel is also avail-
able with
a 500mA power switch to support the primary
channels
when performing step-down conversions. The
maximum voltage between V
IN1
and E1 (or V
IN2
and E2) is
40V when E1 (or E2) is grounded. This is the case for boost,
SEPIC, flyback and dual-inductor inverting topologies. The
maximum allowed voltage between V
IN1
and E1 (or V
IN2
and E2) is 60V when E1 (or E2) is allowed to toggle, such
as in buck, ZETA and single-inductor inverting topologies.
Primary Channels
The two primary channels, 1 and 2, can be independently
configured as boost, buck, SEPIC, ZETA, flyback or invert
-
ing DC/DC converters to adapt into various applications.
Both channels use a constant-frequency, current mode
control scheme to provide line and load regulation (refer
to the Block Diagram). The channel 1 clock is in phase
with the internal oscillator or the SYNC pin if it is toggling.
In order to reduce transient switching spikes, the clock
for channel 2 is approximately 180° out-of-phase with
the channel 1 clock.
At the start of each clock phase, an
SR latch (
SR11/SR12
in the Block Diagram) is set, turning on the internal power
switch (Q1/Q2 in the Block Diagram) for the respective
channel. An amplifier (A41/A42 in the Block Diagram) and
a comparator (A31/A32 in the Block Diagram) monitor the
current flowing through the internal power switch, turning
the switch off when the current reaches a level determined
by the voltage at VC1/VC2. An error amplifier measures
the output voltage through an external resistor divider tied
to the FB1/FB2 pin and servos the VC1/VC2 voltage. If
the error amplifier’s output (VC1/VC2) increases, more
current is delivered to the output; if it decreases, less
current is delivered. An internal clamp on the VC1/VC2
voltage provides current limit.
Both of the primary channels contain a power good com
-
parator which trips when the corresponding FB pin voltage
is at 92.5% of its regulated value. The PG1 and PG2 outputs
are driven by open-drain N-channel MOSFET devices that
are off when the respective output is in regulation, allow
-
ing external resistors to pull the PG1/PG2 pins high. The
PG1
and PG2 pin states are only valid when the respective
channel is enabled and V
IN1
is above 2.6V.
Skyhook Channel
When either channel is configured as a buck, ZETA or
single-inductor inverting converter, the respective V
IN
pin(s) must be boosted above the input voltage, V
CC
. The
boosted supply provides base current to the appropriate
Q1 and/or Q2 NPN power switch. The Skyhook channel
provides this boosted voltage to the SHOUT pin which
must also be connected to V
IN2
and/or V
IN1
as needed.
The Skyhook is a constant-frequency, voltage mode
boost converter including a Schottky diode integrated on
chip. The Skyhook output, SHOUT, is regulated to a fixed
voltage (4.25V typical) above the C2 pin which must be
connected to a DC voltage (typically V
CC
). If the Skyhook is
not needed it can be disabled by tying the C3 pin to GND.
This also reduces the current draw from V
IN1
. Refer to
the Applications Information section for more information
about the proper use of the Skyhook channel.
The Skyhook operates as follows: An error amplifier mea
-
sures the output voltage (SHOUT) through the SHOUT-C2
voltage
comparator and servos an internal control volt-
age. The
control voltage determines the on-time of the
Q3
power switch for each cycle, and thus, the amount of
current being delivered to SHOUT. Loop compensation
is integrated in the chip. Comparator A43 monitors the
current in the power switch Q3 in order to detect over
current conditions. If current in excess of 500mA (typ) is
detected, switch Q3 is immediately turned off.
Start-Up Operation
Several functions are provided to enable a clean start-up
for the LT8471.
• First, the OV/UV pin voltage is monitored by an inter
-
nal voltage
reference to give a precise turn-on voltage
range.
An external resistor (or resistor divider) can be
connected from the input power supply to the OV/UV
pin to provide a user-programmable undervoltage and
overvoltage lockout function.
• Second, the soft-start circuitry provides for a gradual
ramp-up of the switch current for the primary channels
