Datasheet

LTC3677-3
39
36773f
operaTion
Figure 18. Power-Up Sequencing
V
OUT
UVLO
PBSTAT
BUCK1
BUCK2
LDO1
PGOOD
PWR_ON
STATE PON
36773 F17
ON (PB)
50ms
1.8ms
>14 SEC
14 SEC
Figure 17. Hard Reset Timing
36773 F18
1
0
0V
0V
0V
2ms/DIV
PBSTAT
BUCK1
1V/DIV
BUCK2
2V/DIV
LDO2
1V/DIV
Hard Reset Timing
Hard reset provides a way to reset the μC/μP in case of a
software lockup. To initiate a hard reset, the pushbutton is
pressed (ON LOW) and held for greater than 14 seconds.
Once the hard reset time is exceeded the PGOOD input
will go LOW for 1.8ms which resets the μC/μP. Operation
of the enabled supplies is not effected by the hard reset
event. All enabled supplies should remain in regulation
and operating correctly assuming specified operating
conditions are met (i.e., no shorted supplies, etc).
There are only two methods to power down the LTC3677-3
supplies: 1) PWR_ON goes LOW; 2) V
OUT
drops below the
V
OUT
UVLO threshold. If the μC/μP controls shutdown by
bringing PWR_ON LOW, it is possible that the application
can hang with all supplies enabled if the μC/μP fails to
reset correctly on hard reset. In this case the battery will
continue to be drained until V
OUT
drops below the V
OUT
UVLO threshold, or the user intervenes to shut down the
application manually. The application can be shut down
manually by removing the battery and any external supplies,
or by providing a suicide button that will bring PWR_ON
LOW when pressed.
Power-Up Sequencing
Figure 18 shows the actual power-up sequencing of the
LTC3677-3. Buck1, Buck2 and LDO2 are all initially disabled
(0V). Once the pushbutton has been applied (ON LOW)
for 50ms PBSTAT goes LOW and LDO2 is enabled. Once
enabled, LDO2 slews up and enters regulation. The actual
slew rate is controlled by the soft-start function of LDO2
which ramps the LDO reference up over a 200µs period
typically. After a 14ms delay from LDO2 being enabled,
Buck1 is enabled and slews up into regulation. When Buck1
is within about 8% of final regulation, Buck2 is enabled
and slews up into regulation. The bucks also have a soft-
start function to limit inrush current at start-up. 230ms
after Buck2 is within 8% of final regulation, the PGOOD
output will go high impedance (not shown in Figure 18).
The regulators in Figure 18 are slewing up with nominal
output capacitors and no load. Adding a load or increasing
output capacitance on any of the outputs will reduce the
slew rate and lengthen the time it takes the regulator to
get into regulation.