LT8705
14
8705ff
For more information www.linear.com/LT8705
operaTion
Refer to the Block Diagram (Figure 1) when reading the
following sections about the operation of the LT8705.
Main Control Loop
The LT8705 is a current mode controller that provides an
output voltage above, equal to or below the input voltage.
The LTC proprietary topology and control architecture
employs a current-sensing resistor (R
SENSE
) in buck or
boost modes. The inductor current is controlled by the
voltage on the V
C
pin, which is the diode-AND of error
amplifiers EA1-EA4. In the simplest form, where the output
is regulated to a constant voltage, the FBOUT pin receives
the output voltage feedback signal, which is compared to
the internal reference voltage by EA4. Low output voltages
would create a higher V
C
voltage, and thus more current
would flow into the output. Conversely, higher output volt-
ages would
cause V
C
to drop, thus reducing the current
fed into the output.
The LT8705 contains four error amplifiers (EA1-EA4)
allowing it to regulate or limit the output current (EA1),
input current (EA2), input voltage (EA3) and/or output
voltage (EA4). In a typical application, the output voltage
might be regulated using EA4, while the remaining error
amplifiers are monitoring
for excessive input or output
current
or an input undervoltage condition. In other ap-
plications, such
as a battery charger, the output current
regulator (EA1) can facilitate constant current charging
until a predetermined voltage is reached where the output
voltage (EA4) control would take over.
INTV
CC
/EXTV
CC
/GATEV
CC
/LDO33 Power
Power for the top and bottom MOSFET drivers, the LDO33
pin and most internal circuitry is derived from the INTV
CC
pin. INTV
CC
is regulated to 6.35V (typical) from either the
V
IN
or EXTV
CC
pin. When the EXTV
CC
pin is left open or
tied to a voltage less than 6.22V (typical), an internal low
dropout regulator regulates INTV
CC
from V
IN
. If EXTV
CC
is taken above 6.4V (typical), another low dropout regula-
tor will
instead regulate INTV
CC
from EXTV
CC
. Regulating
INTV
CC
from EXTV
CC
allows the power to be derived from
the lowest supply voltage (highest efficiency) such as the
LT8705 switching regulator output (see INTV
CC
Regulators
and EXTV
CC
Connection in the Applications Information
section for more details).
The GATEV
CC
pin directly powers the bottom MOSFET
drivers for switches M2 and M3. GATEV
CC
should always
be connected to INTV
CC
and should not be powered or
connected to any other source. Undervoltage lock outs
(UVLOs) monitoring INTV
CC
and GATEV
CC
disable the
switching regulator when the pins are below 4.65V (typical).
The LDO33 pin is available to provide power to external
components such as a microcontroller and/or to provide an
accurate bias voltage. Load current is limited to 17.25mA
(typical). As long as SHDN is high the LDO33 output is
linearly regulated from the INTV
CC
pin and is not affected
by the INTV
CC
or GATEV
CC
UVLOs or the SWEN pin voltage.
LDO33 will remain regulated as long as SHDN is high and
sufficient voltage is available on INTV
CC
(typically > 4.0V).
An undervoltage lockout, monitoring LDO33, will disable the
switching regulator when LDO33 is below 3.04V (typical).
Start-Up
Figure 2 illustrates the start-up sequence for the LT8705.
The master shutdown pin for the chip is SHDN. When
driven below 0.35V (LT8705E, LT8705I) or 0.3V (LT8705H,
LT8705MP) the chip is disabled (chip off state) and qui
-
escent current
is minimal. Increasing the SHDN voltage
can increase quiescent current but will not enable the chip
until SHDN is driven above 1.234V (typical) after which
the INTV
CC
and LDO33 regulators are enabled (switcher
off state). External devices powered by the LDO33 pin can
become active at this time if enough voltage is available
on V
IN
or EXTV
CC
to raise INTV
CC
, and thus LDO33, to an
adequate voltage.
Starting up the switching regulator happens after SWEN
(switcher enable) is also driven above 1.206V (typical),
INTV
CC
and GATEV
CC
have risen above 4.81V (typical) and
the LDO33 pin has risen above 3.08V (typical) (initialize
state). The SWEN pin is not available in the TSSOP pack
-
age. In this package the SWEN pin is internally connected
to INTV
CC
.
Start-Up: Soft-Start of Switch Current
In the initialize state, the SS (soft-start) pin is pulled low
to prepare for soft starting the regulator. If forced continu
-
ous mode is selected (MODE pin low), the part is put into
discontinuous
mode during soft-start to prevent current
