User`s guide

11
Internal/External Power
Supply Considerations
As shown in Figures 2 and 3, the channel
drive source for the master 34945EXT
is provided through the internal +24 V
power supply of the 34945A, L4445A, and
L4490A/L4491A. If necessary, external
supplies can be used. For either case, the
supply must meet the quiescent (contin-
uous) and switching current requirements
of the switches in your application.
Internal Supply
When using the internal +24 V supply
of the 34945A or L4445A, the quiescent
current draw must not exceed 100 mA,
with a switching pulse current that does
not exceed 200 mA. The duration of the
switching pulse must be < 15 ms with a
25% duty cycle. The remainder of the duty
cycle (75%) allows recovery time for the
supply to provide the next pulse.
For the L4490A/L4491A, the quiescent and
switching currents from the supply must
not exceed the supply’s specification of
600 mA continuous.
When current limits are exceeded,
overcurrent conditions can occur which
shuts down the internal supply. When this
happens, you must remove the device/
condition causing the overcurrent and
reset or cycle power on the instrument.
The quiescent current requirement of
the 34945EXTs themselves is accounted
for by the internal supply rating and is
not included in quiescent current and
switching current calculations. Only the
specified currents (quiescent, switching) of
the switch must be considered.
As an example, the Agilent 87104/87106A,
B, C coaxial switches specify the following:
Supply current (switching: pulse width = 15 ms:Vcc = 24 VDC): 200 mA Nom
Supply current (quiescent): 50 mA Max
Using the internal +24 V supply,
the current required for actuating a
87104/87106 switch is:
[87104/87106 quiescent current] + [87104/87106 switching current]
[50 mA] + [200 mA]
= 250 mA (15 ms pulse, 50 mA quiescent)
The number of switches available and their
varying power requirements often make
it difficult to determine if the capacity of
the 34945A, L4445A, or L4490A/L4491A
internal +24 V supply will be exceeded. If
overcurrent conditions occur repeatedly
the use of an external power supply may
be required. Refer to the product user
manuals for more information.