HC5549 Datasheet, PDF (9/13 Pages) Intersil Corporation – Low Power SLIC with Battery Switch

HC5549
Since the current relationships are different for constant
current versus constant voltage, the region of device
operation is critical to valid power dissipation calculations.
Reverse Active
Overview
The reverse active mode (RA, 011) provides the same
functionality as the forward active mode. On hook
transmission, DC loop feed and voice transmission are
supported. Loop supervision is provided by either the switch
hook detector (E0 = 1) or the ground key detector (E0 = 0).
The device may be operated from either high or low battery.
During reverse active the Tip and Ring DC voltage
characteristics exchange roles. That is, Ring is typically 4V
below ground and Tip is typically 4V more positive than
battery. Otherwise, all feed and voice transmission
characteristics are identical to forward active.
Silent Polarity Reversal
Changing from forward active to reverse active or vice versa
is referred to as polarity reversal. Many applications require
slew rate control of the polarity reversal event. Requirements
range from minimizing cross talk to protocol signalling.
The device uses an external low voltage capacitor, C_POL, to
set the reversal time. Once programmed, the reversal time
will remain nearly constant over various load conditions. In
addition, the reversal timing capacitor is isolated from the AC
loop, therefore loop stability is not impacted.
The internal circuitry used to set the polarity reversal time is
shown below.
I₁
POL
75kΩ
C_POL
I₂
FIGURE 8. REVERSAL TIMING CONTROL
During forward active, the current from source I1 charges the
external timing capacitor C_POL and the switch is open. The
internal resistor provides a clamping function for voltages on
the POL node. During reverse active, the switch closes and
I2 (roughly twice I1) pulls current from I1 and the timing
capacitor. The current at the POL node provides the drive to
a differential pair which controls the reversal time of the Tip
and Ring DC voltages.
C_POL = ^∆-₇---₅-^t--ⁱ⁰--^m--₀---_0e--
(EQ. 30)
Where ∆time is the required reversal time. Polarized
capacitors may be used for C_POL. The low voltage at the
POL pin and minimal voltage excursion ±0.75V, are well
suited to polarized capacitors.
Power Dissipation
The power dissipation equations for forward active operation
also apply to the reverse active mode.
Ringing
Overview
The ringing mode (RNG, 100) provides the low side return
path for externally supplied battery backed ringing. The
ringing signal must be injected through a relay at the ring
terminal. The device should be operated from the low battery
voltage during this mode to minimize the overall power
dissipation during ringing. Current flowing through the Tip
terminal will provide the necessary ring trip information.
Ringing Bias Input
The ringing bias input, VRB, is a high impedance input. The
VRB input is only selected during the ringing mode. The gain
from the VRB input to the Tip output is typically 40V/V. The
following equation shows the relationship of the Tip output
voltage to the VRB input voltage.
V_TIP= ^V-----^B2----^L-- + 40 × VRB
(EQ. 31)
A positive DC voltage at VRB is required to shift the Tip
output voltage towards ground to provide the low side ringing
return path. Tying the logic input F2 to VRB provides the
positive voltage to offset Tip during ringing. A voltage divider
is suggested to provide control the actual voltage applied to VRB.
Logic Control
Ringing patterns consist of silent intervals. The ringing to
silent pattern is called the ringing cadence. During the silent
portion of ringing, the device can be programmed to any
other operating mode. The most likely candidates are low
power standby or forward active. Depending on system
requirements, the low or high battery may be selected.
Loop supervision is provided with the ring trip detector. The
ring trip detector senses the change in loop current when the
phone is taken off hook. The loop detector full wave rectifies
the ringing current, which is then filtered with external
components RRT and CRT. The resistor RRT sets the trip
threshold and the capacitor CRT sets the trip response time.
Most applications will require a trip response time less than
150 milliseconds.
Three very distinct actions occur when the devices detects a
ring trip. First, the DET output is latched low. The latching
mechanism eliminates the need for software filtering of the
detector output. The latch is cleared when the operating
mode is changed externally. Second, the VRS input is
disabled, removing the Tip biasing signal from the line. Third,
the device is internally forced to the forward active mode.
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