Linear Technology 1791A Manual page 3

operAting principles
The demo circuit includes provisions for programming the
two output voltage rails. Resistors R6, R7 and R5, R10 allow
the V and V power rails, respectively, to be reduced
L2 CC2
from their nominal operating voltages. The value of R6 or
R5 should be no greater than 6.04k to minimize errors in
the output voltage caused by the internal voltage dividers.
The formulas presented in Table 2 allow selection of the
appropriate resistor values. In addition the current limit
of each output voltage rail may also be programmed. The
current limit threshold is set by resistors R8 and R9 for
LTM2887 Low EMI Demo Board Radiated Emissions
60
CISPR 22 CLASS B LIMIT
50
DC1791A–A
DC1791A–B
40
30
20
10
0
–10
–20
–30
0 100 200 300 400 500 600 700 800 900 1000
FREQUENCY (MHz)
Figure 1. DC1791A Radiated Emissions
I
VL2
100mA. The current limit may be reduced for either output
by the following equation:
R
Table 2.
DEMO MANUAL DC1791A
DETECTOR = QPEAK
RBW = 120kHz
VBW = 300kHz
SWEEP TIME = 17s
# OF POINTS = 501
DC1791A
and I respectively. The current limit is nominally
VCC2
(R8, R9) = [119.22–(0.894) • (V
IMAX
VOLTAGE RAIL
V
CC2
V
L2
, V )]/I
L2 CC2 LIMIT
RESISTOR TO REDUCE OUTPUT
= 0.6V(1 + R7/R6)
= 0.6V(1 + R10/R5)
dc1791afa
3