LTC3838-2_15 Datasheet, PDF (20/56 Pages) Linear Technology – Dual, Fast, Accurate Step-Down DC/DC Controller with xternal Reference Voltage and Dual Differential Output Sensing

LTC3838-2
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. 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_FB2/R_FB1)
The minimum (differential) V_OUT1 is limited to the inter-
nal reference 0.6V, when R_FB1 is removed (effectively
R_FB1 = ∞), and/or R_FB2 is shorted (effectively R_FB2 = 0).
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). Note the external reference V_REF2 is sensed dif-
ferentially through the EXTV_REF2 pin and the 3rd resistor:
V_OUT2+ – V_OUT2– = (V_REF2+ – V_REF2–) • (1 + R_DFB2/R_DFB1)
The minimum (differential) V_OUT2 is limited to the (dif-
ferential) external reference V_REF2. To program V_OUT2 =
V_REF2, 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.
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 a current of around
(V_DFB2+ – V_DFB2–)/50k 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 LTC3838-2’s
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 LTC3838-2’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
20
V_OUT2+
LTC3838-2
V_OUTSENSE1+ V_DFB2+
V_{OUTSENSE1–} V_DFB2–
EXTV_REF2
R_DFB2
R_DFB1
TO PROGRAM V_OUT2 = V_REF2,
C_{OUT2 REMOVE RDFB1AND USE}
R_DFB3 = R_DFB2
V_REF2+
+–
V_REF2–
R_DFB3 =
R_DFB1//R_DFB2
V_OUT2–
REMOTELY-SENSED
POWER GROUND 2,
REMOTELY-SENSED
±200mV MAX vs SGND
EXTERNAL REFERENCE GROUND
38382 F01
Figure 1. Setting Output Voltage
38382f
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