SSM-2404 Analog Devices, SSM-2404 Datasheet - Page 7

• Manufacturer Part Number: SSM-2404
• Description: Quad Audio Switch
• Manufacturer: Analog Devices

REV. B
DETAILED SWITCH OPERATION
A simplified circuit schematic with the functional sections is
shown in Figure 21. The TTL interface has an internally
regulated 5 V to ensure TTL logic levels regardless of the supply
voltage. The logic threshold is with respect to the DGND pin,
which can be offset. For example, if DGND is connected to the
negative supply, then the SSM2404 will operate with negative
rail logic. The interface shifts the control logic down to the
negative supply and inverts it to drive N1.
N1 in combination with C1 and the 100 nA current source
provides the break-before-make operation of the switch. When
the switch is on, N1 is off and C1 is charged up to the positive
rail. However, when the SW CONTROL is turned off, then the
gate of N1 is pulled high. This turns N1 on, providing a low
impedance path to quickly discharge C1 to the negative rail,
which quickly “breaks” the switch. On the other hand, when the
SW CONTROL goes high again, the gate of N1 is pulled low,
turning it off. This leaves C1 to be slowly charged up to the
positive rail by the 100 nA current source. The difference in the
discharge and charging times ensures break-before-make
operation, even from device to device.
The voltage on C1 is inverted by P1 to drive the ramp generator
differential pair, consisting of P2, P3 and N2, N3. This dif-
ferential pair steers the 100 nA of tail current to either charge or
discharge C2. As discussed above, when the switch is on, C1 is
charged up to the positive rail. P1 inverts this, putting a low
voltage equivalent to the negative supply on the gate of P2. The
BIAS voltage is approximately equal to the midpoint of the two
supply voltages. Thus, when P2 is pulled down, it is turned on
and P3 is off. All of the 100 nA flows through N2 and is mir-
rored by N3. Thus, the 100 nA discharges C2 through N3.
When C2 is pulled low, the inverter turns N4 on by pulling its
gate high, and the second inverter turns P4 on. To turn the
switch off the gate of P2 is pulled above the BIAS so that all
100 nA charges C2 through P3. This is then inverted to turn off
N4 and P4.
The internal ramp has rise and fall times on the order of a few
milliseconds which is sped up by the inverters. As the gate
CONTROL
DGND
SW
INTERFACE
TTL
Figure 21. Simplified Schematic
BREAK-BEFORE-MAKE
100nA
N1
C1
15pF
P1
N2
RAMP GENERATOR
P2
P3
100nA
N3
C2
15pF
V+
BIAS
V–
–1
N4
–1
SW1 A
SW1 B
P4
–7–
voltages of N4 and P4 are changing, the ON resistance of each
switch is ramping from its OFF state to 28
The actual rise and fall times are shown in Figures 18 and 19
for a 5 k
typical switches, minimizing the SSM2404’s charge injection
and giving it “clickless” performance.
DOUBLE-POLE DOUBLE-THROW SWITCH
The SSM2404 is ideal as a one-chip solution for a stereo
switch. The schematic in Figure 22 shows the typical configura-
tion. This circuit will select one of two stereo sources, channel
A or B. The switch controls for the left and right input of each
channel are tied together so that both will be turned on or off
simultaneously. An inverter is inserted between the channel A
and B controls so that only one logic signal is needed. The out-
puts can be configured many different ways, such as an invert-
ing or noninverting amplifier stage, and the 10 k load resistors
are added to improve the OFF-isolation. The performance of
this stereo switch is equivalent to each individual switch, yield-
ing a high quality audio switch that is virtually transparent to
the signal.
VIRTUAL GROUND SWITCHING
The SSM2404 was built on a CMOS process with a 24 V
operating limit for the total supply voltage across the part. This
leads to a corresponding limit on the analog voltage range. How-
ever, to achieve larger signal swings, the SSM2404 should be
configured in the virtual ground mode. As shown in Figure 23,
the output of the SSM2404 is connected to the inverting input
of an amplifier. Since the noninverting input is grounded, the
SSM2404 will also be biased at ground, and large voltage
swings on the circuit’s input will not significantly change the
voltage on the switch. The only limitation is that the current
through the switch needs to be less than 10 mA, and the voltage
range is limited only by the op amp and its supply voltages.
Figure 22. Double-Pole, Double-Throw Stereo Switch
SWA/SWB
load. These times are significantly slower than
L
L
R
R
INA
INB
INA
INB
10
15
11
20
16
6
5
1
4
SW1 CONTROL
SW4 CONTROL
SW2 CONTROL
SW3 CONTROL
DGND
SSM2404
SW2
SW3
SW1
SW4
V+
V–
14
AGND
17
18
12
19
13
8
3
2
9
SWA/SWB
10k
10k
SSM2404
and vice versa.
0
1
R
L
OUT
OUT
SELECTED
CHANNEL
B
A