Datasheet

LTC3854
11
3854fb
APPLICATIONS INFORMATION
Figure 2. Buck Regulator Using DCR Current Sense
capacitor is equal to the voltage drop across the inductor
DCR • R2/(R1+R2). R2 may be used to scale the voltage
across the same terminals when the DCR is greater than
the target sense resistance. Check the manufacturer’s
datasheet 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
precision RLC meter.
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in constant-fre-
quency architectures by preventing subharmonic oscil-
lations at high duty cycles. It is accomplished internally
by adding a compensating ramp to the inductor current
signal. Normally, this results in a reduction of maximum
inductor peak current for high duty cycles. However, the
LTC3854 uses a novel scheme that allows the maximum
inductor peak current to remain unaffected throughout
all duty cycles.
3854 F02
LTC3854
V
IN
RUN/SS TG
BOOST
SW
BG
GND
INTV
CC
ITH
FB
SENSE
–
SENSE
+
M1
M2
C
B
D
BOOST
C
IN
C
VINT
C
C2
C
C
R
C
C
SS
R
FB1
L DCR
R1
C
OUT
C1
V
OUT
R
FB2
V
IN
R2
INDUCTOR
Inductor Value Calculation
The inductor value has a direct effect on ripple current.
The inductor ripple current
∆I
L
decreases with higher
inductance or frequency and increases with higher V
IN
.
L
MIN
=
1
∆I
L
• f
SW
• V
OUT
1−
V
OUT
V
IN(MAX)






Accepting larger values of ∆I
L
allows the use of low value
inductors, but results in a higher output voltage ripple
and greater core losses. A reasonable starting point for
setting ripple current is I
L
= 0.4 • (I
MAX
). The maximum
∆I
L
occurs at the maximum input voltage.
Option 1: DCR within desired range
R1• C1=
L
DCR
(R2 not used)
Option 2: DCR > desired R
SENSE
R1||R2 • C1 =
L
DCR
(at 20°C)
R
SENSE
(EQ) = DCR(MAX) •
R2
R1+ R2