Internal/External Power Supply Considerations - Keysight Technologies 34945A Configuration Manual

13 | Keysight | 34945A, L4445A, and L4490A/L4491A - Configuration Guide
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 (continu-
ous) 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 Keysight 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]
= 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.
+ [200 mA]
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