Datasheet

LT8705
15
8705fb
For more information www.linear.com/LT8705
operaTion
T
JUNCTION
< 160°C
AND
SHDN > 1.234V AND V
IN
> 2.5V
AND
(SWEN* < 1.184V OR (INTV
CC
AND GATEV
CC
< 4.65V)
OR LDO33 < 3.04V)
SOFT-START
• SS CHARGES UP
• SWITCHER ENABLED
• SS SLOWLY DISCHARGES
SWITCHER OFF
• SWITCHER DISABLED
• INTV
CC
AND LDO33 OUTPUTS
ENABLED
NORMAL MODE
POST FAULT DELAY
• SS CHARGES UP
• SWITCHER DISABLED
• CLKOUT DISABLED
FAULT DETECTED
• NORMAL OPERATION
• WHEN SS > 1.6V ...
• CLKOUT ENABLED
• ENABLE FORCED
CONTINUOUS MODE
IF SELECTED
INITIALIZE
SS < 50mV
FAULT
FAULT FAULT
SS < 50mV
*SWEN IS CONNECTED TO INTV
CC
IN THE TSSOP PACKAGE
8705 F02
FAULT
• SS PULLED LOW
• FORCE DISCONTINOUS
MODE UNLESS Burst Mode
OPERATION SELECTED
CHIP OFF
TYPICAL VALUES
SHDN < 1.184V OR
V
IN
< 2.5V OR
T
JUNCTION
> 165°C
• SWITCHER OFF
• LDOs OFF
TYPICAL VALUES
SHDN > 1.234V AND V
IN
> 2.5V
AND SWEN* > 1.206V AND
(INTV
CC
AND GATEV
CC
> 4.81V) AND
LDO33 > 3.075V
SS > 1.6V AND
NO FAULT CONDITIONS
STILL DETECTED
TYPICAL VALUES
FAULT = OVERVOLTAGE (IMON_IN OR IMON_OUT > 1.61V TYP)
Figure 2. Start-Up and Fault Sequence
from being drawn out of the output and forced into the
input. After SS has been discharged to less than 50mV,
a soft-start of the switching regulator begins (soft-start
state). The soft-start circuitry provides for a gradual
ramp-up of the inductor current by gradually allowing the
V
C
voltage to rise (refer to V
C
vs SS Voltage in the Typical
Performance Characteristics). This prevents abrupt surges
of current from being drawn out of the input power sup
-
ply. An integrated 100k resistor pulls the SS pin to ≅2.5V.
The ramp rate of the SS pin voltage is set by this 100k
resistor and the external capacitor connected to this pin.
Once SS gets to 1.6V, the CLKOUT pin is enabled, the part
is allowed to enter forced continuous mode (if MODE is
low) and an internal regulator pulls SS up quickly to ≅2.5V.
Typical values for the external soft-start capacitor range
from 100nF to 1μF. A minimum of 100nF is recommended.
Fault Conditions
The LT8705 activates a fault sequence under certain op
-
erating conditions. If any of these conditions occur (see
Fi
gure2) the CLKOUT pin and internal switching activity
are disabled. At the same time, a timeout sequence com
-
mences where the SS pin is charged up to a minimum
of 1.6V (fault detected state). The SS pin will continue