LT8708
17
Rev 0
Figure 2. Start-Up Sequence (All Values are Typical)
OPERATION
SOFT-START
• SS CHARGES UP
• WHEN SS > 0.2V ...
• SWITCHER ENABLED
• V
C
SOFT-START
• M1, M4 ON-TIME SOFT-START
SWITCHER OFF 2
• SWITCHER DISABLED
• INTV
CC
AND LDO33 OUTPUTS ENABLED
• SS PULLED LOW
NORMAL MODE
• NORMAL OPERATION
INITIALIZE
SS < 50mV
SS > 1.8V
• 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
8708 F02
Start-Up: Soft-Start of Switching Regulator
In the INITIALIZE state, the SS (soft-start) pin is pulled
low to prepare for soft-starting the switching regulator.
After SS has been discharged to less than 50mV, the
SOFT-START state begins. In this state, as SS gradually
rises, the soft-start circuitry provides a gradual ramp of
V
C
and the inductor current in the appropriate direction
(refer to the V
C
vs SS Voltage graph in the Typical Per-
formance Characteristics section). This prevents abrupt
surges of inductor current and helps the output voltage
ramp smoothly into regulation. See the Switch Control:
Soft-Start section for information about the power switch
control during soft-start.
During soft-start, an integrated 180k (typical) resistor pulls
SS up to 3.3V. The rising ramp rate of the SS pin voltage
is set by this 180k resistor and the external capacitor
connected to this pin. When SS reaches 1.8V (typical),
the LT8708 exits soft-start and enters normal operation.
Typical values for the external soft-start capacitor range
from 220nF to 2μF. A minimum of 220nF is recommended.
CONTROL OVERVIEW
The LT8708 is a current mode controller that provides
an output voltage above, below or equal to the input volt-
age. It also provides bidirectional current monitoring and
regulation capabilities at both the input and the output.
The ADI proprietary control architecture employs an inductor
current-sensing resistor (R
SENSE
) in buck, boost or buck-
boost regions of operation. The inductor current is controlled
by the voltage on the V
C
pin, which is the combined output
of six internal error amplifiers EA1 – EA6. These amplifiers
