參數(shù)資料
型號(hào)： LTC3851IUD-1#TRPBF
廠商： LINEAR TECHNOLOGY CORP
元件分類： 穩(wěn)壓器
英文描述： SWITCHING CONTROLLER, PQCC16
封裝： 3 X 3 MM, LEAD FREE, PLASTIC, MO-220WEED-6, QFN-16
文件頁(yè)數(shù)： 5/28頁(yè)
文件大?。?/td> 337K
代理商： LTC3851IUD-1#TRPBF
LTC3851-1
13
38511f
APPLICATIONS INFORMATION
Figure 2. Current Mode Control Using the Inductor DCR
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in constant fre-
quency architectures by preventing sub-harmonic 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 duty cycles >40%. However, the
LTC3851-1 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 efciency 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
ΔIL decreases with higher
inductance or frequency and increases with higher VIN:
ΔI
fL
V
L
OUT
IN
=
1
–
Accepting larger values of
ΔIL 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
ΔIL = 0.3(IMAX). The maximum
ΔIL 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 RSENSE. Lower
inductor values (higher
ΔIL) will cause this to occur at
lower load currents, which can cause a dip in efciency 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 efciency 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
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
LTC3851-1 controller: one N-channel MOSFET for the top
(main) switch, and one N-channel MOSFET for the bottom
(synchronous) switch.
VIN
INTVCC
BOOST
TG
SW
BG
GND
INDUCTOR
DCR
L
SENSE+
SENSE–
LTC3851-1
VOUT
38511 F02
R1
R2
*PLACE C1 NEAR SENSE+, SENSE– PINS
C1*
R1||R2 C1 =
RSENSE(EQ) = DCR
L
DCR
R2
R1 + R2
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