LTC3838-1_15 Datasheet, PDF (18/52 Pages) Linear Technology – Dual, Fast, Accurate Step-Down DC/DC Controller with Dual Differential Output Sensing

LTC3838-1
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.
V_OUT1+
C_OUT1
R_FB2
R_FB1
V_OUT1–
REMOTELY-SENSED
POWER GROUND 1,
±500mV MAX vs SGND
18
LTC3838-1
V_OUTSENSE1+ V_DFB2+
V_{OUTSENSE1–} V_DFB2–
SGND
R_DFB3 =
R_DFB1//R_DFB2
38381 F01
Figure 1. Setting Output Voltage
R_DFB2
R_DFB1
V_OUT2+
TO PROGRAM
V_OUT2 = 0.6V,
REMOVE R_DFB1 AND
USE R_DFB3 = R_DFB2
C_OUT2
V_OUT2–
REMOTELY-SENSED
POWER GROUND 2,
±200mV MAX vs SGND
For more information www.linear.com/3838-1
38381f