Datasheet

LTC3838-1
18
38381f
For more information www.linear.com/3838-1
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
references to program the output voltages. On chan-
nel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, add a 3rd resistor with value equal to the two
voltage-divider resistors in parallel (or simply add two
parallel resistors
equal to each of the two voltage divider
resistors). By regulating the tapped (differential) feedback
voltages to the internal reference 0.6V, the resulting output
voltages are:
V
OUT1
+
– V
OUT1
–
= 0.6V • (1 + R
DFB2
/R
FB1
)
and
V
OUT2
+
– V
OUT2
–
= 0.6V • (1 + R
DFB2
/R
FB1
)
The minimum (differential) V
OUT1
is limited to the internal
reference 0.6V. To program V
OUT1
= 0.6V, remove R
FB1
(effectively R
FB1
= ∞), and/or short out R
FB2
(effectively
R
FB2
= 0). To program V
OUT2
= 0.6V, R
DFB1
can be removed,
and the R
DFB3
= R
DFB1
//R
DFB2
uses the same value as
R
DFB2
, as effectively R
DFB1
= ∞.
The maximum output voltages on both channels can be
set up to 5.5V, as limited by the maximum voltage that
can be applied on the SENSE pins. For example, if V
OUT1
is programmed to 5.5V and the output ground reference
is sitting at 0.5V with respect to SGND, then the absolute
value of the output will be 6V with respect
to SGND, which
is the absolute maximum voltage that can be applied on
the SENSE pins.
The V
OUTSENSE1
+
and V
DFB2
+
are high impedance pins with
no input bias current other than leakage in the nA range. The
V
OUTSENSE1
–
pin has about 25µA of current flowing out of
the pin. The V
DFB2
–
pin has about 6μA flowing 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 LTC3838-1 differential
output sensing
can correct for up to ±500mV of common-
mode deviation in the output’s power and ground lines on
channel 1, and ±200mV on channel 2.
The LTC3838-1’s differential output sensing schemes are
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 difference amplifier
and typically limits differential sensing to the lower range
of output voltages.
Figure 1. Setting Output Voltage
R
DFB2
V
DFB2
+
LTC3838-1
V
DFB2
–
SGND
C
OUT2
C
OUT1
38381 F01
V
OUT2
+
V
OUT1
+
V
OUT1
–
V
OUT2
–
R
DFB1
R
DFB3
=
R
DFB1
//R
DFB2
R
FB2
R
FB1
REMOTELY-SENSED
POWER GROUND 1,
±500mV MAX vs SGND
REMOTELY-SENSED
POWER GROUND 2,
±200mV MAX vs SGND
V
OUTSENSE1
+
V
OUTSENSE1
–
TO PROGRAM
V
OUT2
= 0.6V,
REMOVE R
DFB1
AND
USE R
DFB3
= R
DFB2