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LT8708
OPERATION
TYPOGRAPHICAL CONVENTIONS
The LT8708 is a high performance 4-switch buck-boost
controller that includes features to facilitate bidirectional
current and power flow. Using the LT8708, an application
can command power to be delivered from V
from V
to V
as needed. Some terms, listed below, are
OUT
IN
used throughout this data sheet in reference to the direction
of current and power flow. In order to clarify these direction-
based concepts, these terms are defined as follows:
V
and I
: The V
side of circuits drawn in this data
IN
IN
IN
sheet will always be on the left. V
nected to the SW1 side of the buck-boost
inductor through M1. I
V
and
The V
side of circuits drawn in this data
OUT
OUT
I
:
sheet will always be on the right. V
OUT
connected to the SW2 side of the buck-boost
inductor through M4. I
Supply
Power Source. The power source is most
(Input):
commonly applied to V
can be a Supply (or Input) when power is
being delivered from V
Load
Devices that are consuming the power. The
(Output):
Load is most commonly connected to V
However, V
Output) when power is being delivered from
V
to V
OUT
IN
Forward
Current or power flowing from the V
Conduction:
SW1 node (or side) to the V
(or side) of the circuit. This is generally left
to right on schematics.
Reverse
Current or power flowing from the V
Conduction:
SW2 node (or side) to the V
(or side) of the circuit. This is general right
to left on schematics.
Positive
Current that flows from the SW1 side of the
Current:
buck-boost inductor to the SW2 side. Also
refers to current that flows from V
or into V
OUT
Reverse
Current that flows from the SW2 side of the
Current:
buck-boost inductor to the SW1 side. Also
refers to current that flows from V
or into V
.
IN
16
to V
IN
is con-
IN
is the V
current.
IN
IN
is the V
current.
OUT
OUT
. However, V
IN
to V
.
OUT
IN
can connect to the Load (or
IN
.
or SW2 node
OUT
or SW1 node
IN
IN
.
OUT
For more information
Refer to the Block Diagram (Figure 1) when reading the
following sections about the operation of the LT8708.
START-UP
or
Figure 2 illustrates the start-up sequence for the LT8708.
OUT
Start-Up: SHDN Pin
The master shutdown pin for the chip is SHDN. When driven
below 0.35V (LT8708E, LT8708I) or 0.3V (LT8708H), the
chip is disabled (CHIP OFF state) and quiescent current is
minimal. Increasing the SHDN voltage can increase qui-
escent current but will not enable the chip until SHDN is
driven above 1.221V (typical) after which the INTV
LDO33 regulators are enabled (SWITCHER OFF 1 state).
External devices powered by LDO33 can become active at
is
OUT
this time if enough voltage is available on V
to raise INTV
CC
Start-Up: SWEN Pin
OUT
The SWEN pin is used to enable the switching regulator
after the chip has also been enabled by driving SHDN high.
SWEN must be pulled high through a resistor to enable
the switching regulator. The typical activation threshold is
.
OUT
1.208V as shown in the Electrical Characteristics section.
When the SWEN pin voltage is below the activation thresh-
old, the CSP-CSN, CSPIN-CSNIN and CSPOUT-CSNOUT
current sense circuits on the chip are disabled.
or
IN
SWEN has an internal pull-down that is activated when
the switching regulator is unable to operate (see CHIP OFF
and SWITCHER OFF 1 states in Figure 2). After the chip is
able to operate and SWEN is internally pulled down below
or
OUT
0.8V (typical), the internal SWEN pull-down is disabled
and start-up can proceed past the SWITCHER OFF1 state.
LDO33 or INTV
to. Choose a pull-up resistor value that limits the current
to less than 200μA when SWEN is pulled low. The SWEN
pin can also be digitally driven through a current limiting
and/
resistor. Note in the Electrical Characteristics section, the
SWEN output low voltage is 0.9V (typical) when SHDN is
low and/or V
INCHIP
is 0.2V when SHDN is 3V and V
and/
www.analog.com
, and thus LDO33, to an adequate voltage.
are convenient nodes to pull SWEN up
CC
is unpowered. The SWEN output low
is powered.
INCHIP
and
CC
or EXTV
INCHIP
CC
Rev 0
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