SSM2402/SSM2412

REV. A

–7–

APPLICATIONS INFORMATION

FUNCTIONAL SECTIONS

Each half of the SSM2402/SSM2412 are made up of three major

functional blocks:

1. “T” Switch

2. Ramp Generator

Generates a ramp voltage on command of the Control Input

(see Figure 1). A LOW-to-HIGH TTL input at Control

Input initiates a ramp that goes from approximately –7 V to

+7 V in 12 ms. Conversely, a HIGH-to-LOW TTL transi-

tion at Control Input will cause a downward ramp from ap-

proximately +7 V to –7 V in 12 ms for the SSM2402, and

4 ms for the SSM2412. The Ramp Generator also supplies

the +3 V and –3 V reference levels for Switch Control.

3. Switch Control

The ramp from the Ramp Generator section is applied to two

block. (See Simplified Schematic). One amplifier is refer-

enced to –3 V and the other is referenced to +3 V. Switch

Control Outputs are:

—Main Switch Control—Drives two 0.25 mA current

sources that control the inverting inputs of each op amp.

When ON, the current sources cause a gate-to-source volt-

age of approximately 2.5 V which is sufficient to turn off

Control are OFF, each op amp acts as a unity-gain fol-

—Shunt Switch Control—Controls the Shunt Switch of

the “T” configuration.

SWITCH OPERATION

Unlike conventional analog switches, the SSM2402/SSM2412

are designed to ramp on and off gradually over several millisec-

onds. The soft transition prevents popping or clicking in audio

systems. Transients are minimized in active filters when the

SSM2402/SSM2412 are used to switch component values.

To see how the SSM2402/SSM2412 switches work, first con-

sider an OFF-to-ON transition. The Control Input is initially

LOW and the Ramp Output is at approximately –7 V. The

holds them OFF.

The Shunt Switch Control is negative which holds the shunt

Figure 1. Ramp Generator

Figure 2. Switch Control

When the Control Input goes from LOW to HIGH, the Ramp

Generator slews in the positive direction as shown in Figure 2.

When the ramp goes more positive than –3 V, the Shunt Switch

“T” are OFF.