Datasheet

LTC3852
15
3852f
The Typical Application on the fi rst page of this data sheet
is a basic LTC3852 application circuit. The LTC3852 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 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.
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 2. R
SENSE
is chosen based on the required
output current. For simplicity, the charge pump section
is omitted.
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
APPLICATIONS INFORMATION
half the peak-to-peak ripple current, DI
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
V
IN2
V
IN
INTV
CC
BOOST
TG
SW
BG
GND2
FILTER COMPONENTS
PLACED NEAR SENSE PINS
SENSE
+
SENSE
–
LTC3852
V
OUT
R
SENSE
3852 F02
Inductor DCR Sensing
For applications requiring the highest possible effi ciency,
the LTC3852 is capable of sensing the voltage drop across
the inductor DCR, as shown in Figure 3. The DCR of the
inductor represents the small amount of DC winding
resis tance of the copper, which can be less than 1mW 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 2. Using a Resistor to Sense Current with the LTC3852