LTC3613 Datasheet, PDF (13/36 Pages) Linear Technology – 24V, 15A Monolithic Step Down Regulator

LTC3613
APPLICATIONS INFORMATION
The Typical Application on the first page of this data sheet
is a basic LTC3613 application circuit. The LTC3613 can
be configured to sense the inductor current either through
a series sense resistor, R_SENSE, or through an RC filter
across the inductor (DCR). The choice between the two
current sensing schemes is largely a design trade-off
between cost, power consumption and accuracy. DCR
sensing is becoming popular because it saves expensive
current sensing resistors and is more power efficient,
especially in high current applications. However, cur-
rent sensing resistors provide the most accurate current
limits for the controller. Once the required output voltage
and operating frequency have been determined, external
component selection is driven by load requirements, and
begins with the selection of inductor and current sensing
components. Next, the proper current sense threshold is
programmed using the V_RNG pin. Finally, input and output
capacitors are selected.
Output Voltage Programming and
Differential Output Sensing
The LTC3613 integrates differential output sensing with
output voltage programming, allowing for simple and
seamless design. As shown in Figure 1, the output voltage
is programmed by an external resistor divider from the
regulated output point to its ground reference. The resis-
tive divider is tapped by the V_OSNS+ pin, and the ground
reference is sensed by V_OSNS–. An optional feed-forward
capacitor, C_FF, can be used to improve the transient per-
formance of the regulator system as discussed under
OPTI-LOOP^® Compensation. The resulting output voltage
is given according to the following equation:
V_OUT
=0.6V•
⎛
⎝⎜
1+
R_{F B2}
R_{F B1}
⎞
⎠⎟
More precisely, the V_OUT value programmed in the previous
equation is with respect to the output’s ground reference,
and thus is a differential quantity. For example, if V_OUT
is programmed to 5V and the output ground reference
is at –0.5V, then the output will be 4.5V with respect to
signal ground. The minimum differential output voltage is
limited to the internal reference, 0.6V, and the maximum
differential output voltage is 5.5V.
SW
LTC3613
V_OSNS+
V_OSNS–
C_FF
(OPT)
V_OUT
R_FB2
R_FB1
C_OUT
3613 F01
Figure 1. Setting Output Voltage
The V_OSNS+ pin is high impedance with no input bias cur-
rent. The V_OSNS– pin has about 35μA 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.
These variations are 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 LTC3613’s differential
output sensing can correct for up to ±500mV of variation
in the output’s power and ground lines.
The LTC3613’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 resistive 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 LTC3613 allows for seamless differential output
sensing by sensing the resistively divided feedback volt-
age differentially. This allows for differential sensing in
the full output range from 0.6V to 5.5V. The difference
amplifier of the LTC3613 has a –3dB bandwidth of 8MHz,
high enough to not affect main loop compensation and
transient behavior.
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