LT8708-1
18
Rev 0
For more information
SWEN is used to synchronize the start-up between all
phases of the system. If one or more of the phases is
unable to operate, SWEN is pulled low by those chips,
thus preventing the entire system from starting. After all
phases are ready to operate and SWEN has been pulled
down below 0.8V (typical) SWEN rises, due to the pull-
up resistor, and start-up proceeds, for all phases, to the
SWITCHER OFF 2 state.
When the common SWEN node rises above 1.208V (typi-
cal), all the phases proceed to the INITIALIZE state at the
same time.
Start-Up: Soft-Start of Switching Regulator
The soft-start sequence, described in this section, happens
independently and in parallel for each phase since each
phase has its own SS pin, external capacitor and related
circuitry. The remaining discussion concerns the LT8708-1
soft-start behavior. The LT8708 soft-start differs slightly.
In the INITIALIZE state, the SS pin is pulled low to prepare
for soft-starting the switching regulator. Also, V
C
is forced
to command near zero current, and IMON_OP is forced to
~1.209V (typical) to improve the transient behavior when
the LT8708-1 subsequently starts switching.
After SS has been discharged to less than 50mV, the
SOFT-START 1 state begins. In this state, an integrated
180k (typical) resistor from 3.3V pulls SS up. The rising
ramp rate of the SS pin voltage is set by this 180k resistor
and the external capacitor connected to this pin.
After SS reaches 0.2V (typical), the LT8708-1’s integrated
pull-up resistor is reduced from 180k to 90k to increase
the rising ramp rate of the SS pin voltage. This ensures
that the slave chip enters the normal mode in Figure 3
before the master chip, preventing saturation of the regu-
lation loop during start-up.
Switching remains disabled until either (1) ICP or ICN
voltage becomes higher than 510mV (typical) (SOFT-
START 3) or (2) SS reaches 0.8V (typical) (SOFT-START 2).
As soon as switching is enabled, V
C
is free to slew under
the control of the internal error amplifiers (EA1–EA6). This
allows the average I
OUT(SLAVE)
to quickly follow the aver-
age I
OUT(MASTER)
without saturating the slave’s regulation
loop. During soft-start the LT8708-1 employs the same
switch control mechanism as the LT8708. See the Switch
Control: Soft-Start section of the LT8708 data sheet for
more information.
When SS reaches 1.8V (typical), the LT8708-1 exits
soft-start and enters normal mode. Typical values for the
external soft-start capacitor range from 220nF to 2µF.
It is recommended to use the same brand and value SS
capacitor for all the synchronized LT8708/ LT8708-1(s).
Using a slave SS capacitor value significantly higher than
the master SS capacitor value can result in undesirable
start-up behavior.
CONTROL OVERVIEW
The LT8708-1 is a slave current mode controller that
regulates the average I
OUT(SLAVE)
based on the mas-
ter’s ICP and ICN voltages, or equivalently, the average
I
OUT(MASTER)
. The main regulation loop involves EA6 (see
Figure 1). In a simple example of I
OUT(SLAVE)
regulation,
the CSPOUT–CSNOUT pins receive the I
OUT(SLAVE)
feed-
back signal which is summed with the ICP and ICN signals
from the LT8708 to generate the IMON_OP voltage using
A1 (see Figure 1). The IMON_OP voltage is compared to
the internal reference voltage using EA6. Low IMON_OP
voltages raise V
C
, which causes I
OUT(SLAVE)
to become
more positive (or less negative) and increases the current
out of the IMON_OP pin. Conversely, higher IMON_OP
voltages reduce V
C
, which causes I
OUT(SLAVE)
to become
less positive (or more negative) and reduces the current
flowing out of the IMON_OP pin.
The V
C
voltage typically has a Min to Max range of about
1.2V. The maximum V
C
voltage commands the most
positive inductor current, and thus, commands the most
power flow from V
IN
to V
OUT
. The minimum V
C
voltage
commands the most negative inductor current, and thus,
commands the most power flow from V
OUT
to V
IN
.
V
C
is the combined output of five internal error amplifiers
EA1–EA6 as shown in Table 3. In a common application,
I
OUT(SLAVE)
would be regulated using the main regulation
error amplifier EA6, while error amplifiers EA1 and EA5
are monitoring for excessive input current and EA3 and
EA4 are disabled.
OPERATION
