Datasheet

ADP8863
Rev. A | Page 13 of 52
Soft Start Feature
At startup (either from UVLO activation or fault/standby
recovery), the output is first charged by I
SS
(3.75 mA typical)
until it reaches about 92% of V
IN
. This soft start feature reduces
the inrush current that is otherwise present when the output
capacitance is initially charged to V
IN
. When this point is
reached, the controller enters G = 1× mode. If the output
voltage is not sufficient, then the automatic gain selection
determines the optimal point as defined in the Automatic Gain
Selection section.
OPERATING MODES
There are four different operating modes: active, standby,
shutdown, and reset.
Active Mode
In active mode, all circuits are powered up and in a fully
operational state. This mode is entered when Bit nSTBY (in
Register MDCR) is set to 1.
Standby Mode
Standby mode disables all circuitry except for the I
2
C receivers.
Current consumption is reduced to less than 1 A. This mode is
entered when the nSTBY bit is set to 0 or when the nRST pin is
held low for more than 100 s (maximum). When standby is
exited, a soft start sequence is performed.
Shutdown Mode
Shutdown mode disables all circuitry, including the I
2
C receivers.
Shutdown occurs when V
IN
is below the undervoltage thresholds.
When V
IN
rises above V
IN(START)
(2.05 V typical), all registers are
reset and the part is placed into standby mode.
Reset Mode
In reset mode, all registers are set to their default values and
the part is placed into standby. There are two ways to reset the
part: by power-on reset (POR) or using the nRST pin. POR is
activated any time that the part exits shutdown mode. After a
POR sequence is complete, the part automatically enters
standby mode.
After startup, the part can be reset by pulling the nRST pin low.
As long as the nRST pin is low, the part is held in a standby
state, but no I
2
C commands are acknowledged (all registers are
kept at their default values). After releasing the nRST pin, all
registers remain at their default values, and the part remains in
standby; however, the part does accept I
2
C commands.
The nRST pin has a 50 s (typical) noise filter to prevent inad-
vertent activation of the reset function. The nRST pin must be
held low for this entire time to activate reset.
The operating modes function according to the timing diagram
in Figure 28.
STANDBY
nRST
SHUTDOWN
25µs TO 100µs NOISE FILTER
1×
1.5×
2×
SOFT STARTSOFT START
10µs 100µs
08392-026
V
IN
V
OUT
V
IN
V
IN
CROSSES ~2.05V AND TRIGGERS POWER-ON RESET
BIT nSTBY IN REGISTER
MDCR GOES LOW
~100µs DELAY BETWEEN POWER-UP AND
WHEN I
2
C COMMANDS CAN BE RECEIVED
nRST MUST BE HIGH FOR 20µs (MAX)
BEFORE SENDING I
2
CCOMMANDS
nRST IS LOW, WHICH FORCES STANDBY
LOW AND RESETS ALL I
2
CREGISTERS
GAIN CHANGES OCCUR ONLY WHEN NECESSARY,
BUT HAVE A MIN TIME BEFORE CHANGING
~3.75mA CHARGES
V
OUT
TO V
IN
LEVEL
Figure 28. Typical Timing Diagram