LTC3854_15 Datasheet, PDF (11/28 Pages) Linear Technology – Small Footprint, Wide VIN Range Synchronous Step-Down DC/DC Controller

APPLICATIONS INFORMATION
V_IN
C_SS
C_C
R_C
R_FB1
R_FB2
V_IN
RUN/SS
TG
ITH
BOOST
C_C2
LTC3854
SW
INTV_CC
FB
SENSE^–
BG
SENSE⁺ GND
C_IN
M1
C_B
D_BOOST
C_VINT
M2
INDUCTOR
L
DCR
C1
R1
R2
LTC3854
V_OUT
C_OUT
3854 F02
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.
Inductor Value Calculation
The inductor value has a direct effect on ripple current.
The inductor ripple current ΔI_L decreases with higher
inductance or frequency and increases with higher V_IN.
L_MIN
=
∆I_L
1
• f_SW
• V_OUT

 ^1−
V_OUT
V_IN(MAX
)


Accepting larger values of ΔI_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
ΔI_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 (at 20°C)
DCR
R_SENSE
(EQ)
=
DCR(MAX)
•
R2
R1+ R2
3854fb
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