Datasheet

LTC3838
18
3838fa
APPLICATIONS INFORMATION
Once the required output voltage and operating frequency
have been determined, external component selection is
driven by load requirement, and begins with the selec-
tion of inductors and current sense method (either sense
resistors R
SENSE
or inductor DCR sensing). Next, power
MOSFETs are selected. Finally, input and output capaci-
tors are selected.
Output Voltage Programming
As shown in Figure 1, external resistor dividers are used
from the regulated outputs to their respective ground refer-
ences to program the output voltages. On Channel 1, the
resistive divider is tapped by the V
OUTSENSE1
+
pin, and the
ground reference is remotely sensed by the V
OUTSENSE1
–
pin, this voltage is sensed differentially. On Channel 2, the
resistive divider is tapped by the V
FB2
pin, with respect to
signal ground at the SGND pin. By regulating the tapped
(differential) feedback voltages to the internal reference
0.6V, the resulting output voltages are:
V
OUT1
– V
OUTSENSE1
–
= 0.6V • (1 + R
FB2
/R
FB1
)
and
V
OUT2
= 0.6V • (1 + R
FB2
/R
FB1
)
For example, if V
OUT1
is programmed to 5V and the out-
put ground reference is sitting at –0.5V with respect to
SGND, then the absolute value of the output will be 4.5V
with respect to SGND. The minimum (differential) output
voltages are limited to the internal reference 0.6V, and the
maximum are 5.5V.
The V
OUTSENSE1
+
pin is a high impedance pin with no
input bias current other than leakage in the nA range. The
V
OUTSENSE1
–
pin has about 30µA of current flowing out
of the pin. The V
FB2
pin is quasi-high impedance pin with
minimum bias current out of the pin.
Differential output sensing allows for more accurate output
regulation in high power distributed systems having large
line losses. Figure 2 illustrates the potential variations in
the power and ground lines due to parasitic elements.
The variations may be exacerbated in multi-application
systems with shared ground planes. Without differential
output sensing, these variations directly reflect as an error
in the regulated output voltage. The LTC3838 Channel 1’s
differential output sensing can correct for up to ±500mV
of variation in the output’s power and ground lines.
The LTC3838 Channel 1’s differential output sensing
scheme is distinct from conventional schemes where the
regulated output and its ground reference are directly
sensed with a difference amplifier whose output is then
divided down with an external resistor divider and fed
into the error amplifier input. This conventional scheme
is limited by the common mode input range of the differ-
ence amplifier and typically limits differential sensing to
the lower range of output voltages.
The LTC3838’s Channel 1 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 5.5V. The difference
amplifier (DIFFAMP) has a bandwidth of 8MHz, high
R
FB2
V
OUTSENSE1
+
/V
FB2
LTC3838
V
OUTSENSE1
–
/SGND
C
OUT
3838 F01
V
OUT
R
FB1
Figure 1. Setting Output Voltage