SI3232 Datasheet, PDF (29/128 Pages) Silicon Laboratories – DUAL PROGRAMMABLE CMOS SLIC WITH LINE MONITORING

Si3232
4.5. Automatic Dual Battery Switching
The Si3232 and Si3200 provide the ability to switch
between several user-provided battery supplies to aid
thermal management. This method is required during
the ringing to off-hook and on-hook to off-hook state
transitions.
During the on-hook operating state, the Si3232 must
operate from the ringing battery supply in order to
quickly provide the desired ringing signal when
required. Once an off-hook condition has been
detected, the Si3232 must transition to the lower battery
supply (typically –24 V, in order to reduce power
dissipation during the active state). The low current
consumed by the Si3232 during the on-hook state
results in very little power dissipation while being
powered from the ringing battery supply, which can
have an amplitude as high as 100 V depending on the
desired ringing amplitude.
The BATSEL pins serve to switch between the two
battery voltages based on the operating state and the
TIP-RING voltage. Figure 12 illustrates the chip
connections required to implement an automatic dual
battery switching scheme. When BATSEL is pulled low,
the desired channel is powered from the V_BLO supply.
When BATSEL is pulled high, the V_BHI source will
supply power to the desired channel.
The BATSEL pins for both channels are controlled using
the BATSEL bit of the RLYCON register and should be
programmed to automatically switch to the lower battery
supply (V_BLO) whenever an off-hook condition is
sensed.
Two thresholds are provided to enable battery switching
with hysteresis. The BATHTH RAM location specifies
the threshold at which the Si3232 will switch from the
low battery, V_BLO, to the high battery, V_BHI, due to an
off-hook to on-hook transition. The BATLTH RAM
location specifies the threshold at which the Si3232 will
switch from V_BHI to V_BLO due to a transition from the on-
hook or ringing state to the off-hook state or because
the overhead during active off-hook mode is sufficient to
feed the subscriber loop using a lower battery voltage.
The low-pass filter coefficient is calculated using the
equation below and is entered into the BATLPF RAM
location.
BATLPF = [(2πf x 4096)/800] x 2³
Where f = the desired cutoff frequency of the filter
The programmable range of the filter is from 0 (blocks
all signals) to 4000h (unfiltered). A typical value of 10
(0A10h) is sufficient to filter out any unwanted ac
artifacts while allowing the dc information to pass
through the filter.
Table 17 provides the register and RAM locations used
for programming the battery switching functions.
Table 17. Register and RAM Locations used for Battery Switching
Parameter
High Battery Detect Threshold
Register/RAM
Mnemonic
BATHTH
Register/RAM
Bits
BATHTH[14:0]
Low Battery Detect Threshold
BATLTH
BATLTH[14:0]
Ringing Battery Switch
RLYCON
GPO
Battery Select Indicator
RLYCON
BSEL
Battery Switching LPF
BATLPF
BATLPF[15:3]
*Note: The usable range for BATHTH and BATLTH is limited to the V_BHI voltage.
Programmable
Range
0 to 160.173 V^*
0 to 160.173 V^*
Toggle
Toggle
0 to 4000
Resolution
(LSB Size)
628 mV
(4.907 mV)
628 mV
(4.907 mV)
N/A
N/A
N/A
Preliminary Rev. 0.96
29