LTC3851_15 Datasheet, PDF (12/28 Pages) Linear Technology – Synchronous Step-Down Switching Regulator Controller

LTC3851
APPLICATIONS INFORMATION
The Typical Application on the first page of this data sheet
is a basic LTC3851 application circuit. The LTC3851 can
be configured to use either DCR (inductor resistance)
sensing or low value resistor sensing. The choice of 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, 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 the inductor
value. Next, the power MOSFETs and Schottky diodes are
selected. Finally, input and output capacitors are selected.
The circuit shown on the first page can be configured for
operation up to 38V at V_IN.
Current Limit Programming
The I_LIM pin is a tri-level logic input to set the maximum
current limit of the controller. When I_LIM is grounded, the
maximum current limit threshold of the current compara-
tor is programmed to be 30mV. When I_LIM is floated, the
maximum current limit threshold is 50mV. When I_LIM is
tied to INTV_CC, the maximum current limit threshold is
set to 75mV.
SENSE⁺ and SENSE^– Pins
The SENSE⁺ and SENSE^– pins are the inputs to the current
comparators. The common mode input voltage range of
the current comparators is 0V to 5.5V. Both SENSE pins
are high impedance inputs with small base currents of
less than 1μA. When the SENSE pins ramp up from 0V
to 1.4V, the small base currents flow out of the SENSE
pins. When the SENSE pins ramp down from 5V to 1.1V,
the small base currents flow into the SENSE pins. The
high impedance inputs to the current comparators allow
accurate DCR sensing. However, care must be taken not
to float these pins during normal operation.
Low Value Resistors 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.
12
V_IN
INTV_CC
BOOST
TG
LTC3851
SW
BG
GND
SENSE⁺
SENSE^–
V_IN
R_{SENSE VOUT}
FILTER COMPONENTS
PLACED NEAR SENSE PINS
3851 F01
Figure 1. Using a Resistor to Sense Current with the LTC3851
The current comparator has a maximum threshold, V_MAX,
determined by the I_LIM setting. The current comparator
threshold sets the maximum peak of the inductor current,
yielding a maximum average output current, I_MAX, equal to
the maximum 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_MAX
I_MAX + ΔI_L/2
Inductor DCR Sensing
For applications requiring the highest possible efficiency,
the LTC3851 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 winding
resistance of the copper, which can be less than 1mΩ for
today’s low value, high current inductors. If the external
R1||R2 • C1 time constant is chosen to be exactly equal
to the L/DCR time constant, the voltage drop across the
external capacitor is equal to the voltage drop across
the inductor DCR multiplied by R2/(R1 + R2). Therefore,
R2 may be used to scale the voltage across the sense
terminals when the DCR is greater than the target sense
resistance. Check the manufacturer’s data sheet for
specifications regarding the inductor DCR, in order to
properly dimension the external filter components. The
DCR of the inductor can also be measured using a good
RLC meter.
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