Datasheet

Data Sheet ADP8866
Rev. A | Page 17 of 52
SHORT-CIRCUIT PROTECTION (SCP) MODE
The ADP8866 can protect against short circuits on the output
(V
OUT
). Short-circuit protection (SCP) is activated at the point
when V
OUT
< 55% of V
IN
. Note that this SCP sensing is disabled
during startup and restart attempts (fault recovery). SCP
sensing is reenabled 4 ms (typical) after activation. During a
short-circuit fault, the device enters a low current consumption
state and an interrupt flag is set. The device can be restarted at
any time after receiving a short-circuit fault by simply rewriting
nSTBY = 1 in Register 0x01. It then repeats another complete
soft start sequence. Note that the value of the output
capacitance (C
OUT
) should be small enough to allow V
OUT
to
reach approximately 55% (typical) of V
IN
within the 4 ms
(typical) time. If C
OUT
is too large, the device inadvertently
enters short-circuit protection.
OVERVOLTAGE PROTECTION (OVP)
Overvoltage protection is implemented on the VOUT pin.
There are two types of overvoltage events: normal (no fault) and
abnormal.
Normal (No Fault) Overvoltage
In this case, the VOUT pin voltage approaches V
OUT(REG)
(4.9 V
typical) during normal operation. This is not caused by a fault
or load change but is simply a consequence of the input voltage
times the gain reaching the clamped output voltage V
OUT(REG)
. To
prevent this, the ADP8866 detects when the output voltage rises
to V
OUT(REG)
. It then increases the effective R
OUT
of the gain stage
to reduce the voltage that is delivered. This effectively regulates
V
OUT
to V
OUT(REG)
; however, there is a limit to the effect that this
system can have on regulating V
OUT
. It is designed only for
normal operation and is not intended to protect against faults or
sudden load changes. During this mode, no interrupt is set, and
the operation is transparent to the LEDs and overall application.
The automatic gain selection equations take into account the
additional drop within R
OUT
to maintain optimum efficiency.
Abnormal (Fault/Sudden Load Change) Overvoltage
Because of the open loop behavior of the charge pump, as well
as how the gain transitions are computed, a sudden load change
or fault can abnormally force V
OUT
beyond 6 V. If the event
happens slowly enough, the system first tries to regulate the
output to 4.9 V as in a normal overvoltage scenario. However, if
this is not sufficient, or if the event happens too quickly, the
ADP8866 enters overvoltage protection mode when V
OUT
exceeds the OVP threshold (typically 5.7 V). In this mode, only
the charge pump is disabled to prevent V
OUT
from rising too
high. The current sources and all other device functionality
remain intact. When the output voltage falls by about 500 mV
(to 5.2 V typical), the charge pump resumes operation. If the
fault or load step recurs, the process may repeat. An interrupt
flag is set at each OVP instance.
THERMAL SHUTDOWN
(TSD)/OVERTEMPERATURE PROTECTION
If the die temperature of the ADP8866 rises above a safety limit
(150°C typical), the controllers enter TSD protection mode. In
this mode, most of the internal functions are shut down, the
part enters standby, and the TSD_INT interrupt is set (see
Register 0x02). When the die temperature decreases below
~130°C, the part is allowed to be restarted. To restart the part,
simply remove it from standby. No interrupt is generated when
the die temperature falls below 130°C. However, if the software
clears the pending TSD_INT interrupt and the temperature
remains above 130°C, another interrupt is generated.
The complete state machine for these faults (SCP, OVP, and
TSD) is shown in Figure 36.