LT8708-1
17
Rev 0
In addition, all LT8708-1s’ V
IN
and V
OUT
should be con-
nected to the LT8708’s V
IN
and V
OUT
, respectively.
See the Quick-Start Multiphase Setup Guidelines section
in the Applications Information for a step-by-step design
procedure to add LT8708-1s to your system.
START-UP
Figure 3 illustrates the start-up sequence for the LT8708-1.
Start-Up: SWEN Pin
The LT8708-1 and LT8708’s SWEN pins share the same
functionality. Refer to Start-Up: SWEN Pin of the LT8708
OPERATION
SOFT-START 1
• SS CHARGES UP
SWITCHER OFF 2
• SWITCHER DISABLED
• INTV
CC
AND LDO33 OUTPUTS ENABLED
• SS PULLED LOW
INITIALIZE
SS < 50mV
ICP OR ICN > 510mV
• SS PULLED LOW
• V
C
FORCED TO COMMAND NEAR ZERO
CURRENT LIMIT
(INTV
CC
AND GATEV
CC
> 4.81V) AND
LDO33 > 3.075V AND SWEN > 1.208V
T
JUNCTION
< 160°C AND
SHDN
> 1.221V AND V
INCHIP
> 2.5V AND
((INTV
CC
AND GATEV
CC
< 4.65V)
OR LDO33 < 3.04V)
SWITCHER OFF 1
• SWITCHER DISABLED
• INTV
CC
AND LDO33 OUTPUTS ENABLED
• SWEN AND SS PULLED LOW
CHIP OFF
SHDN
< 1.181V OR
V
INCHIP
< 2.5V OR
T
JUNCTION
> 165°C
• SWITCHER OFF
• LDOs OFF
• SWEN PULLED LOW
(INTV
CC
AND GATEV
CC
> 4.81V) AND
LDO33 > 3.075V AND SWEN < 0.8V
SOFT-START 3
• SS CHARGES UP
• SWITCHER ENABLED
• M1, M4 ON-TIME SOFT-START
• V
C
IS FREE TO SLEW
SOFT-START 2
• SS CHARGES UP
ICP AND ICN < 510mV
NORMAL MODE
• NORMAL OPERATION
SS > 1.8V
87081 F02
SS > 0.8V
data sheet for more details. SWEN is internally pulled
down by the LT8708 and/or LT8708-1(s) when the respec-
tive switching regulator is unable or not ready to operate
(see CHIP OFF and SWITCHER OFF 1 states in Figure 3).
In a multiphase system, the SWEN pins are connected
between all phases. Due to the internal SWEN pull-down
on the LT8708 and LT8708-1, the external pull-up for
the common SWEN node should always have a current
limiting resistor. Typically, the common SWEN node is
pulled up, through a resistor, to the LT8708’s LDO33 pin.
In other cases, the common SWEN node can be digitally
driven through a current limiting resistor.
Figure 3. Start-Up Sequence (All Values are Typical)
