LT8708
16
Rev 0
For more information
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
IN
to V
OUT
or
from V
OUT
to V
IN
as needed. Some terms, listed below, are
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
IN
and I
IN
: The V
IN
side of circuits drawn in this data
sheet will always be on the left. V
IN
is con-
nected to the SW1 side of the buck-boost
inductor through M1. I
IN
is the V
IN
current.
V
OUT
and
I
OUT
:
The V
OUT
side of circuits drawn in this data
sheet will always be on the right. V
OUT
is
connected to the SW2 side of the buck-boost
inductor through M4. I
OUT
is the V
OUT
current.
Supply
(Input):
Power Source. The power source is most
commonly applied to V
IN
. However, V
OUT
can be a Supply (or Input) when power is
being delivered from V
OUT
to V
IN
.
Load
(Output):
Devices that are consuming the power. The
Load is most commonly connected to V
OUT
.
However, V
IN
can connect to the Load (or
Output) when power is being delivered from
V
OUT
to V
IN
.
Forward
Conduction:
Current or power flowing from the V
IN
or
SW1 node (or side) to the V
OUT
or SW2 node
(or side) of the circuit. This is generally left
to right on schematics.
Reverse
Conduction:
Current or power flowing from the V
OUT
or
SW2 node (or side) to the V
IN
or SW1 node
(or side) of the circuit. This is general right
to left on schematics.
Positive
Current:
Current that flows from the SW1 side of the
buck-boost inductor to the SW2 side. Also
refers to current that flows from V
IN
and/
or into V
OUT
.
Reverse
Current:
Current that flows from the SW2 side of the
buck-boost inductor to the SW1 side. Also
refers to current that flows from V
OUT
and/
or into V
IN
.
Refer to the Block Diagram (Figure 1) when reading the
following sections about the operation of the LT8708.
START-UP
Figure 2 illustrates the start-up sequence for the LT8708.
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
CC
and
LDO33 regulators are enabled (SWITCHER OFF 1 state).
External devices powered by LDO33 can become active at
this time if enough voltage is available on V
INCHIP
or EXTV
CC
to raise INTV
CC
, and thus LDO33, to an adequate voltage.
Start-Up: SWEN Pin
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
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.
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
0.8V (typical), the internal SWEN pull-down is disabled
and start-up can proceed past the SWITCHER OFF1 state.
LDO33 or INTV
CC
are convenient nodes to pull SWEN up
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
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 unpowered. The SWEN output low
is 0.2V when
SHDN
is 3V and V
INCHIP
is powered.
