HC5549_14 Datasheet, PDF (7/13 Pages) Intersil Corporation – Low Power SLIC with Battery Switch

HC5549
voltage exceeds the MTU reference of -49V (typically), the
Ring terminal will be clamped by the internal reference. The
same Ring relationships apply when operating from the low
battery voltage. For high battery voltages (VBH) less than or
equal to the internal MTU reference threshold:
V_RING = V_BH + 4
(EQ. 12)
Loop Current
During LPS, the device will provide current to a load. The
current path is through resistors and switches, and will be
function of the off hook loop resistance (R_LOOP). This
includes the off hook phone resistance and copper loop
resistance. The current available during LPS is determined
by Equation 13.
I_LOOP = (– 1 – (–49)) ⁄ (600 + 600 + R_LOOP)
(EQ. 13)
Internal current limiting of the standby switches will limit the
maximum current to 20mA.
Another loop current related parameter is longitudinal
current capability. The longitudinal current capability is
reduced to 10mA_RMS per pin. The reduction in longitudinal
current capability is a result of turning off the Tip and Ring
amplifiers.
On Hook Power Dissipation
The on hook power dissipation of the device during LPS is
determined by the operating voltages and quiescent currents
and is calculated using Equation 14.
P_LPS= V_BH × I_BHQ + V_BL × I_BLQ + V_CC × I_CCQ
(EQ. 14)
The quiescent current terms are specified in the electrical
tables for each operating mode. Load power dissipation is
not a factor since this is an on hook mode. Some
applications may specify a standby current. The standby
current may be a charging current required for modern
telephone electronics.
Standby Current Power dissipation
Any standby line current, I_SLC, introduces an additional
power dissipation term P_SLC. Equation 15 illustrates the
power contribution is zero when the standby line current is
zero.
P_SLC= I_SLC × ( V_BH – 49 + 1 + I_SLCx1200)
(EQ. 15)
If the battery voltage is less than -49V (the MTU clamp is
off), the standby line current power contribution reduces to
Equation 16.
P_SLC= I_SLC × ( V_BH + 1 + I_SLCx1200)
(EQ. 16)
Most applications do not specify charging current
requirements during standby. When specified, the typical
charging current may be as high as 5mA.
Forward Active
Overview
The forward active mode (FA, 001) is the primary AC
transmission mode of the device. On hook transmission, DC
loop feed and voice transmission are supported during forward
active. 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 for on-
hook transmission and low battery for loop feed.
On-Hook Transmission
The primary purpose of on hook transmission will be to
support caller ID and other advanced signalling features.
The transmission over load level while on hook is 3.5V_PEAK.
When operating from the high battery, the DC voltages at Tip
and Ring are MTU compliant. The typical Tip voltage is -4V
and the Ring voltage is a function of the battery voltage for
battery voltages less than -60V as shown in Equation 17.
V_RING = V_BH + 4
(EQ. 17)
Loop supervision is provided by the switch hook detector at
the DET output. When DET goes low, the low battery should
be selected for DC loop feed and voice transmission.
Feed Architecture
The design implements a voltage feed current sense
architecture. The device controls the voltage across Tip and
Ring based on the sensing of load current. Resistors are
placed in series with Tip and Ring outputs to provide the
current sensing. The diagram below illustrates the concept.
R_B
R_A
V_IN
V_OUT
R_CS
-
+
R_L
R_C
-
+
K_S
FIGURE 3. VOLTAGE FEED CURRENT SENSE DIAGRAM
By monitoring the current at the amplifier output, a negative
feedback mechanism sets the output voltage for a defined
load. The amplifier gains are set by resistor ratios (R_A, R_B,
R_C) providing all the performance benefits of matched
resistors. The internal sense resistor, R_CS, is much smaller
than the gain resistors and is typically 20Ω for this device.
The feedback mechanism, K_S, represents the amplifier
configuration providing the negative feedback.
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FN4539.3