ACT4910QW
Rev 1.0, 15-Sept-2017
As an example, with BSTVSET[4:0] = 10111 (sets
VBSTVSET = 28V) and RBSET = 34.2kΩ, VSTR = 12.8V. Note
that RBSET should be set between 20kΩ and 75kΩ.
Setting RBSET below 20kΩ gives the same result as
using a 20kΩ resistor. Setting RBSET above 75kΩ gives
the same result as using a 75kΩ resistor. Using RBSET
outside the range is acceptable and does not damage
the IC.
HMON_THR[3]
HMON_THR[2:0]
0
1
000
001
010
011
100
101
110
111
95.0%
95.2%
95.4%
95.6%
95.8%
96.0%
96.2%
96.4%
96.6%
96.8%
97.0%
97.2%
97.4%
97.6%
97.8%
98.0%
Input Capacitor Selection
There are no special considerations for the boost
converter input capacitor. The IC topology uses the
buck converter output capacitor for the boost input
capacitor. Proper selection of the buck converter output
capacitor automatically results in an acceptable boost
converter input capacitor.
Storage Capacitor Health Monitor
Output Inductor Selection
General Description
There are no special considerations for the boost
converter inductor. The IC topology uses the buck
converter inductor for the boost converter inductor.
Proper selection of the buck converter output capacitor
automatically results in an acceptable boost converter
input capacitor.
The ACT4910 has an internal health monitor for the
storage capacitors. It applies a constant current sink to
the capacitors and monitors the voltage drop. If the
voltage drops below a predetermined threshold, the IC
asserts nIRQ to indicate that the storage capacitance
has dropped below the allowable threshold. Health
monitoring is completely autonomous, but can also be
manually initiated by the system uP. Health monitoring
can also be configured so that it is only a manual
operation so it can be triggered on demand to avoid any
critical system operations. The health monitor
parameters are adjustable via I2C registers to allow
flexibility for different capacitor values.
Output Capacitor Selection
The IC topology uses the buck converter input capacitor
for the boost converter output capacitor. Note that the
storage capacitors are also included in the boost
converter output capacitors. Proper selection of the
buck converter input capacitor typically results in an
acceptable boost converter output capacitor. The boost
output capacitor has two criteria. The first is that it must
consist of at least 10µF of high quality X5R or X7R
ceramic capacitance placed directly between the STR
pin and PGND. The second criteria is that the sum of
the ceramic capacitor and storage capacitors must be
greater than 100µF. There is no requirement for the
additional capacitance dielectric material. This provides
flexibility for the storage capacitors, which allows the
use of super capacitors, electrolytic, polymer, Tantalum,
ceramic, or any capacitors in the design.
Health Monitor Algorithm
The health check algorithm sinks 10mA into the STR pin
for a time determined by I2C register HMON_TSET. It
then sinks 50mA for 200us. It monitors the voltage on
the STR pin, and if the voltage drops more than the
percentage set by I2C register HMON_THR, it asserts
nIRQ low and sets the fault bit. The IC’s specific CMI
option sets the default HMON_TSET and HMON_THR
settings. After the HMON_TSET time, the boost turns
back on to recharge the storage capacitor.
Output Voltage UVLO Setting
The health check function can be enabled and disabled
by the DIS_HEALTH_CHK bit (0x1Fh [5]). It also can be
forced to perform a one-shot health check by the
FORCE_HLTHCHK bit (0x0Ah [1]). Forcing a single
health check is valid even when the DIS_HEALTH_CHK
bit =1. If set in continuous mode, the IC performs a
health check every 4 minutes. This timing can be
configured for every 8 minutes or 16 minutes by bits
SCALE_HCHK _2X and SCALE_HCHK _4X in register
0x27h [1:0].
The storage capacitor power good signal indicates that
the STR voltage is above the UVLO threshold. The
UVLO threshold is shared with the storage capacitor
health check, and is set by the I2C register HMON_THR
[3:0]. The default value is determined by the IC’s
specific CMI option.
Table 6: Storage Capacitor PG Threshold
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