Datasheet

LTC3852
16
3852f
APPLICATIONS INFORMATION
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in constant
frequency architectures by preventing sub-harmonic
oscillations at high duty cycles. It is accomplished inter nally
by adding a compensating ramp to the inductor current
signal. Normally, this results in a reduction of maximum
inductor peak cur rent for duty cycles >40%. However, the
LTC3852 uses a novel scheme that allows the maximum
inductor peak current to remain unaffected throughout
all duty cycles.
Inductor Value Calculation
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use of
smaller inductor and capacitor values. A higher frequency
generally results in lower effi ciency because of MOSFET
gate charge losses. In addition to this basic trade-off, the
effect of inductor value on ripple current and low current
operation must also be considered.
The inductor value has a direct effect on ripple current.
The inductor ripple current DI
L
decreases with higher
inductance or frequency and increases with higher V
IN
:
ΔI
L
=
1
f•L
V
OUT
1–
V
OUT
V
IN
⎛
⎝
⎜
⎞
⎠
⎟
V
IN2
V
IN
INTV
CC
BOOST
TG
SW
BG
GND2
INDUCTOR
DCRL
SENSE
+
SENSE
–
LTC3852
V
OUT
3852 F03
R1**
R2
*PLACE C1 NEAR SENSE
+
, SENSE
–
PINS
**PLACE R1 NEAR INDUCTOR
C1*
R1||R2 • C1 =
R
SENSE(EQ)
= DCR
L
DCR
R2
R1 + R2
Accepting larger values of DI
L
allows the use of low
inductances, but results in higher output voltage ripple
and greater core losses. A reasonable starting point for
setting ripple current is DI
L
= 0.3(I
MAX
). The maximum
DI
L
occurs at the maximum input voltage.
The inductor value also has secondary effects. The tran-
sition to Burst Mode operation begins when the average
inductor current required results in a peak current below
≈10% of the current limit determined by R
SENSE
. Lower
inductor values (higher DI
L
) will cause this to occur at
lower load currents, which can cause a dip in effi ciency in
the upper range of low current operation. In Burst Mode
operation, lower inductance values will cause the burst
frequency to increase.
Inductor Core Selection
Once the value for L is known, the type of inductor must
be selected. High effi ciency converters generally cannot
afford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite or molypermalloy
cores. Actual core loss is independent of core size for a
fi xed inductor value, but it is very dependent on inductance
selected. As inductance increases, core losses go down.
Unfortunately, increased inductance requires more turns
of wire and therefore copper losses will increase.
Ferrite designs have very low core loss and are preferred
at high switching frequencies, so design goals can con-
centrate on copper loss and preventing saturation. Ferrite
core material saturates “hard,” which means that induc-
tance collapses abruptly when the peak design current is
exceeded. This results in an abrupt increase in inductor
ripple current and consequent output voltage ripple. Do
not allow the core to saturate!
Power MOSFET and Schottky Diode (Optional)
Selection
Two external power MOSFETs must be selected for the
LTC3852 controller: one N-channel MOSFET for the top
(main) switch, and one N-channel MOSFET for the bottom
(synchronous) switch.
Figure 3. Current Mode Control Using the Inductor DCR