LTC3854_15 Datasheet, PDF (10/28 Pages) Linear Technology – Small Footprint, Wide VIN Range Synchronous Step-Down DC/DC Controller

LTC3854
APPLICATIONS INFORMATION
The LTC3854 can be configured to use either DCR (inductor
winding resistance) sensing or low value resistor sensing.
The choice of the two current sensing schemes is largely
a design tradeoff between cost, power consumption, and
accuracy. DCR sensing is becoming popular because it
eliminates expensive current sensing resistors and is
more power efficient, especially in high current applica-
tions. However, current sensing resistors provide the most
accurate current limits for the controller. Other external
component selection is driven by the load requirement,
and begins with the selection of R_SENSE (if R_SENSE is used)
and inductor value. Next, the power MOSFETs and Schottky
diodes are selected. Finally, input and output capacitors are
selected. The Typical Application shown on the first page
can be configured for operation up to 38V on V_IN.
SENSE+ and SENSE– Pins
The SENSE+ and SENSE– pins are the inputs to the cur-
rent comparator. The common mode input voltage range
of the current comparator is 0V to 5.5V. Both SENSE pins
are high impedance inputs with small input bias currents
of less than 1μA. When the SENSE pins ramp up from 0V
to 1.4V, small bias currents flow out of the SENSE pins.
When the SENSE pins ramp down from 5.5V to 1.1V, the
small bias currents flow into the SENSE pins. The high
impedance inputs to the current comparator allow accurate
DCR sensing.
Using a Sense Resistor for Current Sensing
A typical sensing circuit using a discrete resistor is shown
in Figure 1. R_SENSE is chosen based on the required output
current.
The current comparator has a maximum threshold of 50mV.
The input common mode range of the current comparator
is 0V to 5.5V. The current comparator threshold sets the
peak of the inductor current, yielding a maximum aver-
age output current I_MAX equal to the peak value less half
the peak-to-peak ripple current, ΔI_L. Allowing a margin
of 20% for variations in the IC and external component
values yields:
R_SENSE
=
0.8
•
V_{SENSE(MAX )}
I_MAX
+
∆I_L
2
Inductor DCR Sensing
For applications requiring the highest possible efficiency,
the LTC3854 is capable of sensing the voltage drop across
the inductor DCR, as shown in Figure 2. The DCR of the
inductor represents the small amount of DC copper wind-
ing resistance, which can be less than 1mΩ for today’s
low value, high current inductors. When the external
R1||R2•C1 time constant is chosen to be equal to the
L/DCR time constant, the voltage drop across the external
V_IN
INTV_CC
BOOST
TG
LTC3854 _SW
BG
GND
SENSE⁺
SENSE^–
R_SENSE
V_OUT
FILTER COMPONENTS
PLACED NEAR SENSE PINS
3854 F01
Figure 1. Using a Resistor to Sense Current with the LTC3854
3854fb
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