SI3232 datasheet(29/128 Pages) SILABS

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Si3232

Preliminary Rev. 0.96

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 VBLO supply.

When BATSEL is pulled high, the VBHI 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 (VBLO) 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, VBLO, to the high battery, VBHI, due to an

off-hook to on-hook transition. The BATLTH RAM

location specifies the threshold at which the Si3232 will

switch from VBHI to VBLO 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 = [(2

πf 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

Mnemonic

Bits

Programmable

Range

Resolution

(LSB Size)

High Battery Detect Threshold

BATHTH

BATHTH[14:0]

0 to 160.173 V*

628 mV

(4.907 mV)

Low Battery Detect Threshold

BATLTH

BATLTH[14:0]

0 to 160.173 V*

628 mV

(4.907 mV)

Ringing Battery Switch

RLYCON

GPO

Toggle

N/A

Battery Select Indicator

RLYCON

BSEL

Toggle

N/A

Battery Switching LPF

BATLPF

BATLPF[15:3]

0 to 4000

N/A

*Note: The usable range for BATHTH and BATLTH is limited to the

VBHI voltage.

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