LTC1875EGN Просмотр технического описания (PDF) - Linear Technology
Номер в каталоге
Компоненты Описание
производитель
LTC1875EGN Datasheet PDF : 20 Pages
| |||
LTC1875
U
OPERATIO
Slope Compensation and Inductor Peak Current
Slope compensation is required in order to prevent sub-
harmonic oscillation at high duty cycles. It is accom-
plished by internally adding a compensating ramp to the
inductor current signal at duty cycles in excess of 40%. As
a result, the maximum inductor peak current is reduced for
duty cycles > 40%. This is shown in the decrease of the
inductor peak current as a function of duty cycle graph in
Figure 3.
2200
2000
VIN = 3V
1800
1600
1400
1200
1000
800
0
20
40
60
80 100
DUTY CYCLE (%)
1875 F03
Figure 3. Maximum Inductor Peak Current vs Duty Cycle
APPLICATIO S I FOR ATIO
The basic LTC1875 application circuit is shown on the first
page of this data sheet. External component selection is
driven by the load requirement and begins with the selec-
tion of L followed by CIN and COUT.
Inductor Value Calculation
The inductor selection will depend on the operating fre-
quency of the LTC1875. The internal nominal frequency is
550kHz, but can be externally synchronized from 350kHz
to 750kHz.
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use
of smaller inductor and capacitor values. However, oper-
ating at a higher frequency results in lower efficiency
because of increased switching losses.
The inductor value has a direct effect on ripple current. The
ripple current ∆IL decreases with higher inductance or
frequency and increases with higher input voltages.
∆IL
=
1
(f)(L)
VOUT
1–
VOUT
VIN
(1)
Accepting larger values of ∆IL allows the use of smaller
inductors, but results in higher output voltage ripple.
A reasonable starting point for setting ripple current is
∆IL = 0.3(IMAX).
The inductor value also has an effect on Burst Mode
operation. The transition to low current operation begins
when the inductor current peaks fall to approximately
500mA. Lower inductor values (higher ∆IL) will cause this
to occur at lower load currents, which can cause a dip in
efficiency in the upper range of low current operation. In
Burst Mode operation, lower inductance values will cause
the burst frequency to increase.
Inductor Selection
The inductor should have a saturation current rating
greater than the peak inductor current set by the current
comparator of LTC1875. Also, consideration should be
given to the resistance of the inductor. Inductor conduc-
tion losses are directly proportional to the DC resistance
of the inductor. Manufacturers sometimes provide maxi-
mum current ratings based on the allowable losses in the
inductor.
Suitable inductors are available from Coilcraft, Coiltron-
ics, Dale, Sumida, Toko, Murata, Panasonic and other
manufacturers.
1875f
10
Share Link: 