LTC3839 Datasheet, PDF (16/50 Pages) Linear Technology – Fast, Accurate, 2-Phase, Single-Output Step-Down DC/DC Controller

LTC3839
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, an external resistor divider is
used from the regulated output to its ground references
to program the output voltage. The resistive divider is
tapped by the V_OUTSENSE+ pin and the ground reference
is remotely sensed by the V_{OUTSENSE–} pin. By regulating
the differential feedback voltages to the internal reference
0.6V, the resulting output voltage is:
V_OUT – V_{OUTSENSE–} = 0.6V • (1 + R_FB2/R_FB1)
For example, if V_OUT 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
voltage is limited to the internal reference 0.6V, and the
maximum is 5.5V.
LTC3839
V_OUTSENSE+
V_{OUTSENSE–}
V_OUT
R_FB2
R_FB1
C_OUT
3839 F01
Figure 1. Setting Output Voltage
The V_OUTSENSE+ pin is a high impedance pin with no in-
put bias current other than leakage in the nA range. The
V_{OUTSENSE–} pin has about 30μA 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 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 LTC3839’s differential
output sensing can correct for up to ±500mV of variation
in the output’s power and ground lines.
The LTC3839’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 difference amplifier
and typically limits differential sensing to the lower range
of output voltages.
The LTC3839 allows for seamless differential output sens-
ing 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 enough so that
it will not affect main loop compensation and transient
behavior.
To avoid noise coupling into the feedback voltage, the
resistor dividers should be placed close to the V_OUTSENSE+
and V_{OUTSENSE–} pins. Remote output and ground traces
should be routed together as a differential pair to the
remote output. For best accuracy, these traces to the
remote output and ground should be connected as close
as possible to the desired regulation point.
Switching Frequency Programming
The choice of operating frequency is a trade-off between
efficiency and component size. Lowering the operating fre-
quency improves efficiency by reducing MOSFET switching
losses but requires larger inductance and/or capacitance
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