Datasheet

LTC3851-1
12
38511fa
APPLICATIONS INFORMATION
The Typical Application on the fi rst page of this data sheet
is a basic LTC3851-1 application circuit. The LTC3851-1
can be confi gured 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 resis tors and is more power effi cient,
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 require ment, 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 fi rst page can be confi gured for
operation up to 40V at V
IN
.
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 1A. When the SENSE pins ramp up from 0V
to 1.4V, the small base currents fl ow out of the SENSE
pins. When the SENSE pins ramp down from 5V to 1.1V,
the small base currents fl ow into the SENSE pins. The
high impedance inputs to the current comparators allow
accurate DCR sensing. However, care must be taken not
to fl oat 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.
The current comparator has a maximum threshold,
V
MAX
= 50mV. The current comparator threshold sets the
maximum peak of the inductor current, yielding a maximum
average output current, I
MAX
, equal to the peak value less
half the peak-to-peak ripple current, ∆I
L
. Allowing a margin
of 20% for variations in the IC and external component
values yields:
R
V
II
SENSE
MAX
MAX L
=
+
08
2
.•
/∆
Inductor DCR Sensing
For applications requiring the highest possible effi ciency,
the LTC3851-1 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 resis tance 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 specifi cations regarding the inductor DCR, in order
to properly dimension the external fi lter components.
The DCR of the inductor can also be measured using a
good RLC meter.
Figure 1. Using a Resistor to Sense Current with the LTC3851-1
V
IN
V
IN
INTV
CC
BOOST
TG
SW
BG
GND
FILTER COMPONENTS
PLACED NEAR SENSE PINS
SENSE
+
SENSE
–
LTC3851-1
V
OUT
R
SENSE
38511 F01