DS2109
022698 4/10
DETAILED PIN DESCRIPTION Table 1
PIN SYMBOL FCN DESCRIPTION
1 PDI I Power Down Internal. For Bus Ground Sensing Circuit connect to pin 22
of internal SCSI connector, see Figure 4.
2–6,
10–13,
16–19,
24–27
R1...R13
R15...18
T Signal Termination. 110 ohm termination. Connect to SCSI bus signal
lines. For Bus Current Sensing Circuit connect to data, parity, and control
lines, except SCSI–RST line, see Figure 6.
7, 8, 21 HS–GND P Heat Sink Ground. Internally connected to the mounting pad. Should be
either grounded or electronically isolated from other circuitry.
9 PDO O Power Down Output. Connect to DS21S07A for Wide SCSI applica-
tions, see Figure 8.
14 TPWR P Termination Power. Connect to the SCSI TERMPWR line. Bypass with
a 2.2 µF capacitor, see Figures 4 and 6.
15 V
REF
O Reference Voltage. 2.85 volt reference; must be decoupled with a 4.7 µF
capacitor, see Figures 4 and 6.
20 PDE I Power Down External. For Bus Ground Sensing Circuit, connect to pin
36 of external SCSI connector, see Figure 4.
22 TCS I Termination Current Sense. Used to sense current on the SCSI bus.
For Bus Current Sensing Circuit, connect to SCSI signal line –RST, see
Figure 6.
23 R14 T Signal Termination. 110 ohm termination. Connect to SCSI bus signal
line. For Bus Current Sensing Circuit connect to SCSI controller chip, see
Figure 6.
28 GND P Ground. Signal ground; 0.0 volts.
ACTIVE TERMINATION
The voltage regulator circuitry (bandgap reference and
class AB power amplifier) produces a precise laser–
trimmed 2.85 volt level and is capable of sourcing 24 mA
into each of the terminating resistors when the signal
line is low (active). When the external driver for a given
signal line turns off, the active terminator will pull that
signal line to 2.85 volts (quiescent state). When used
with an active negation driver, the power amp can sink
22 mA per line while keeping the voltage reference in
regulation; the terminating resistors maintain their 110Ω
value over the entire voltage range. To maintain the spe-
cified regulation, a 4.7 µF capacitor is required between
the V
REF
pin and ground. A high frequency cap (0.1 µF
ceramic recommended) can also be placed on the V
REF
pin in applications that use fast rise/fall time drivers. The
power down capacitance on terminating resistors
R1–R13 and R15–R18 is <4 pF; R14 is slightly higher
due to the bus current sensing circuitry.
The DS2109 can be removed from the SCSI bus by
using either of two automatic methods, Bus Ground
Sensing or Bus Current Sensing.
As with all analog circuitry, the TERMPWR lines should
be bypassed locally. A 2.2 µF capacitor is recom-
mended between TPWR and ground and placed as
close as possible to the DS2109. The DS2109 should
be placed as close as possible to the connector to mini-
mize signal and power trace length, thereby resulting in
less input capacitance and reflections which can
degrade the bus signals.
BUS GROUND SENSING
If internal and external SCSI devices are connected to
the SCSI bus, the DS2109 will disconnect from the SCSI
bus according to the circuit in Figure 3. To utilize this
automatic disconnect method of bus sensing, configure
the DS2109 as shown in Figure 4. The PDI
(Power
Down Internal) pin should be connected to pin 22 of the
internal SCSI connector, and the PDE (Power Down
External) connected to pin 36 of the external SCSI con-
nector.
