Datasheet

LTC3861
17
3861fa
For more information www.linear.com/LTC3861
applicaTions inFormaTion
Output Voltage Programming and Differential Output
Sensing
The LTC3861 integrates differential output sensing with
output voltage programming, allowing for a simple and
seamless design. As shown in Figure 3, the output voltage
is programmed by an external resistor divider from the
regulated output point to its ground reference. The resis
-
tive divider is tapped by the VSNSP pin, and the ground
reference
is sensed by VSNSN. An optional feedforward
capacitor, C
FF
, can be used to improve the transient
performance. The
resulting output voltage is given ac-
cording to the following equation:
VV
R
R
OUT
FB
FB
=+
⎛
⎝
⎜
⎞
⎠
⎟
06 1
2
1
.•
More precisely, the V
OUT
value programmed in the
previous equation is with respect to the output’s ground
reference, and thus, is a differential quantity. The minimum
differential output voltage is limited to the internal
reference, 0.6V, and the maximum differential output
voltage is V
CC
– 0.5V.
The VSNSP pin is high impedance with no input bias
current. The VSNSN pin has about 7.5μA of current flowing
out of the pin. Differential output sensing allows for more
accurate output regulation in high power distributed
systems having large line losses. Figure 4 illustrates the
potential variations in the power and ground lines due
to parasitic elements. These variations are exacerbated
in multiapplication systems with shared ground planes.
Without differential output sensing, these variations
directly reflect as an error in the regulated output voltage.
The LTC3861’s differential output sensing scheme is
distinct from conventional schemes. In conventional
schemes, the regulated output and its ground reference are
directly sensed with a difference amplifier whose output
is then divided down with an external resistive divider
and fed into the error amplifier input. This conventional
scheme is limited by the common mode input range of
the difference amplifier and typically limits differential
sensing to the
lower range of output voltages.
The
LTC3861 allows for seamless differential output
sensing by sensing the resistively divided feedback voltage
differentially. This allows for differential sensing in the
full output range from 0.6V to V
CC
– 0.5V. The difference
Figure 3. Setting Output Voltage
R
FB2
VSNSP
LTC3861
VSNSN
C
OUT
C
FF
(OPT)
3861 F03
V
OUT
R
FB1
R
FB1
MB
R
FB2
MT
L
C
IN
V
IN
C
OUT1
C
OUT2
3861 F04
I
LOAD
OTHER CURRENTS FLOWING IN
SHARED GROUNDPLANE
POWER TRACE
PARASITICS
±V
DROP(PWR)
+
–
GROUND TRACE
PARASITICS
±V
DROP(GND)
I
LOAD
LTC3861
VSNSP VSNSN
Figure 4: Differential Output Sensing Used to Correct Line Loss Variations
in a High Power Distributed System with a Shared Ground Plane