ACT5880
Rev 2, 03-Sep-13
writing to the byte. It is highly recommended to
reset the UNLOCK bit after each writing, to avoid
possible accidental change to the voltage setting for
VBBAT.
The Always-on LDO and the Backup
Battery Charger
Two regulators are specialized for always-on
powers, as a backup battery charger and a low
voltage, low quiescent current always-on regulator.
The backup battery charger is a low power voltage
regulator with preset current limit of up to 5mA
nominal as a constant-voltage and constant-current
charger. If applications like the RAM data keeping
requires larger current option, contact factory for
ordering.
For configuration and manipulation, refer to THE
REGISTERS and BITS DESCRIPTION for detail.
Irregular Conditions
The LDO circuit monitors the output and asserts
interrupt if out of regulation happens, if the function
is allowed. When short circuit or over current
happens, the output current limit folds back to about
the 0.45 times of the maximum output current. Each
LDO is safe in output short circuit.
The always-on LDO and the backup battery charger
have programmable output voltages, which are set
individually as other LDOs but the backup battery
charger uses different code to voltage cross as
what in the VBBAT OUTPUT VOLTAGE SETTING
table. As the VBBAT is internally connected to
power the always-on LDO, and the always-on LDO
powers the RTC, if the RTC function is used, the
outputs of both regulator should have enough
headroom for the RTC operation.
The individual LDO does not have its own over
temperature protection circuit. Subject to the whole
die's thermal condition, the LDOs are turned off
when die over temperature happens.
Component Selection
Input Capacitor Selection
The backup battery charger LDO outputs at slightly
lower voltage than the code voltage with a current
related drop before it falls into current regulation,
which would have output the exact code voltage
when the output current were zero. This drop has
close to logarithmic trend to the load current (i.e.,
doubled every decade change of load current).
Each LDO requires a small ceramic input capacitor
to supply current to support fast transients at the
input of the LDO.
Output Capacitor Selection
Each LDO requires
a small ceramic output
capacitor for stability. Each output capacitor should
be connected directly between the output and GA
pins, as close to the output as possible, and with a
short, direct connection.
Typically the backup battery charger input, the
BBCIN connects to the main battery; Its output, the
VBBAT connects to a backup battery coin cell or a
super cap, charges and floats at the programmed
voltage. Its precise voltage setting and current
setting accommodate to various battery chemicals
of 1.5V to 3.6V output and super caps. When the
input power removes, the backup battery charger
turns itself shutdown with almost 0 leakage
backward from the cell.
The input and output capacitor values are listed in
Table 6. High quality ceramic capacitors such as
X7R and X5R dielectric types are strongly
recommended.
Table 6
Component Recommendation for LDOs
The input of the always-on regulator is internally
routed to available powers, which is the internal
bias in the ACT5880, the VSYS, or the VBBAT. The
always-on regulator output is internally connected
to power the RTC block. When the RTC function is
enabled, the minimum voltage to start oscillation or
to maintain the RTC functional is greater than the
minimum available programmable output voltage.
OUTPUT CAP
INPUT CAP
LDO NAME
(μF)
(μF)
REG4, REG6,
REG7, REG8
3.3
1
REG5, REG9, REG12
REG10, REG13
REG11
2.2
1.5
1
1
1
-
The data in register bits for the always-on LDO
output voltage setting are kept until both the system
power and VALIVE fail.
1
LDO tied in REG2
1~1.5
Two accesses are designed for change the VBBAT
regulation voltage. The first access is to set
UNLOCK bit, to allow the change to the voltage
configuration byte, then the VBBAT is changed by
Backup Battery
Charger
1~1.5
1~1.5
-
-
Always-on LDO
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