Linear Technology DC2518A Demo Manual page 15

ADVANCED DEMO BOARD OPERATIONS
+
LT3090
–15V V V
IN OUT
Figure 13. Negative Voltage Sensing
To minimize these errors, one can choose REFP as the
positive reference supply and use 0.1% tolerance resistors.
The reference is accurate to within a few millivolts of the
nominal 1.232V value. To reduce the error introduced by
the I current, divider resistor values must be cho-
SENSEM
sen to have a low Thevenin resistance while not drawing
more than 100μA from the REFP pin. The divided down rail
voltage is sensed across resistor R1. The resistors divide
the –12V down by 10k/(10k+140k) or 15. This is reflected
in the scale factor in the LTpowerPlay GUI.
To eliminate REFP and V
SENSEM
channel (CH3) uses a ground-referenced op-amp circuit
to measure the negative rail voltage. See Figure 14. The
LTC6015 op-amp has input common mode range down
to GND. Attaching a resistor from the negative supply to
the inverting input and corresponding feedback resistor
allows the circuit to "touch" highly negative nodes while
providing a single-ended low impedance output to a
V input. Components R2/C2 set the low frequency
SENSEP
anti-aliasing filter to 1.59kHz. The time constant 100μs
establishes the response time to over- and under-voltage
conditions on the –48V output.
+
C1
LT8580
10 F
+12V V V
IN OUT
Figure 14. Output Voltage Sensing for Negative Rail
REFP
+1.232V
V
SENSEP
R1
LTC2975
10k
V
88 A
SENSEM
R2
DC2518A F13
C1
140k
10 F
–12V
pin current errors, the –48V
C2, 10nF
R2, 10k
+12V
–
V
SENSEP
+
R1
LTC2975
100k
V
SENSEM
–48V
DEMO MANUAL DC2518A
Output Current Sensing Schemes
The output current on each channel of the DC2518A demo
board uses a 50mΩ sense resistor. This board uses a
variety of sensing schemes: IMON, current sense ampli-
fier, and op-amp with voltage-translating transistor. Each
of the channels allows the user to try different sensing
methods. Zero ohm jumpers select between one or the
other method so that you can understand the trade-offs
of cost versus accuracy.
Each channel's output has a pre-load resistor that is
soldered in place. The primary reason for the pre-load is
to quickly discharge the output when sequencing off/on.
The board comes with IOUT_OC_FAULT limits set to 0.4A
for the +12V/–12V rails and 0.2A for the +48V/–48V rails.
A deglitched fault response is set to mask OC when the
channels start up.
The LTC6101 and LT6105 current sense amplifiers are
used on the +12V (CH0), –12V (CH1), and +48V (CH2).
LT3090
–15V V
IN
Figure 15. Output Current Sensing for Negative Rail
The LT6105 is a low side current sense amplifier and is
able to tolerate voltages up to 36V. For the –48V rail, an
alternative solution is needed. The –48V rail (CH3) uses
an LTC2054 and NFET to provide the current sensing and
level shift to a single-ended signal that drives the ISNSP
and ISNSM pins. The circuit implemented on the DC2518
is that shown on the front page of the LTC2054 data sheet.
DC2518A F14
+3.3V
+
V
V
V
OUT
SENSEP
LT6105
R3
1k
V
SENSEM
R1 R2
100Ω 100Ω
R
SNS
LOAD
0.05Ω
V
OUT
–12V
LOAD
CURRENT
LTC2975
DC2518A F15
dc2518af
15