LTC3851 LINER [Linear Technology], LTC3851 Datasheet - Page 12

• Manufacturer Part Number: LTC3851
• Description: Synchronous Step-Down Switching Regulator Controller
• Manufacturer: LINER [Linear Technology]

LTC3851
The Typical Application on the fi rst page of this data sheet
is a basic LTC3851 application circuit. The LTC3851 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
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 38V at V
Current Limit Programming
The I
current limit of the controller. When I
maximum current limit threshold of the current compara-
tor is programmed to be 30mV. When I
maximum current limit threshold is 50mV. When I
tied to INTV
set to 75mV.
SENSE
The SENSE
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 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 ac-
curate 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
current.
APPLICATIONS INFORMATION
12
LIM
+
and SENSE
pin is a tri-level logic input to set the maximum
+
CC
and SENSE
SENSE
SENSE
, the maximum current limit threshold is
is chosen based on the required output
–
(if R
Pins
–
IN
SENSE
pins are the inputs to the current
.
is used) and the inductor
LIM
LIM
is grounded, the
is fl oated, the
LIM
is
The current comparator has a maximum threshold, V
determined by the I
threshold sets the maximum peak of the inductor current,
yielding a maximum average output current, I
the maximum peak value less half the peak-to-peak ripple
current, ΔI
the IC and external component values yields:
Inductor DCR Sensing
For applications requiring the highest possible effi ciency,
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
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. R2 also resets the sense capacitor, C1, dur-
ing DCM operation. Check the manufacturer’s data sheet
for specifi cations regarding the inductor DCR, in order
to properly dimension the external fi lter components.
Figure 1. Using a Resistor to Sense Current with the LTC3851
R
SENSE
LTC3851
L
=
. Allowing a margin of 20% for variations in
SENSE
SENSE
INTV
BOOST
0 8
GND
. •
V
SW
BG
TG
CC
PLACED NEAR SENSE PINS
IN
+
–
FILTER COMPONENTS
I
MAX
LIM
V
+
setting. The current comparator
MAX
Δ
I
L
/
2
R
V
SENSE
IN
MAX
V
OUT
, equal to
3851 F01
MAX
3851f
,