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参数资料
| 型号： | LTC3568IDD#PBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 4 A SWITCHING REGULATOR, 4000 kHz SWITCHING FREQ-MAX, PDSO10 |
| 封装： | 3 X 3 MM, LEAD FREE, PLASTIC, MO-229WEED-2, DFN-10 |
| 文件页数： | 17/18页 |
| 文件大小： | 300K |
| 代理商： | LTC3568IDD#PBF |
LTC3568
3568fa
RT (k)
0
FREQUENCY
(MHz)
0.5
1.5
2.0
2.5
1000
4.5
TA = 25°C
3568 F02
1.0
500
1500
3.0
3.5
4.0
applicaTions inForMaTion
A general LTC3568 application circuit is shown in
Figure 5.Externalcomponentselectionisdrivenbytheload
requirement, and begins with the selection of the inductor
L1. Once L1 is chosen, CIN and COUT can be selected.
OperatingFrequency
Selection of the operating frequency is a tradeoff between
efficiency and component size. High frequency operation
allows the use of smaller inductor and capacitor values.
Operation at lower frequencies improves efficiency by
reducing internal gate charge losses but requires larger
inductance values and/or capacitance to maintain low
output ripple voltage.
The operating frequency, fO, of the LTC3568 is determined
by an external resistor that is connected between the RT
pin and ground. The value of the resistor sets the ramp
current that is used to charge and discharge an internal
timing capacitor within the oscillator and can be calculated
by using the following equation:
R
f
T
O
=
( )
Ω
( )
9 78 1011
1 08
.
.
or can be selected using Figure 2.
The maximum usable operating frequency is limited by
the minimum on-time and the duty cycle. This can be
calculated as:
fO(MAX) ≈ 6.67 (VOUT / VIN(MAX)) (MHz)
The minimum frequency is limited by leakage and noise
coupling due to the large resistance of RT.
InductorSelection
Although the inductor does not influence the operat-
ing frequency, the inductor value has a direct effect on
ripple current. The inductor ripple current
ΔIL decreases
with higher inductance and increases with higher VIN or
VOUT:
Δ =
I
V
f L
V
L
OUT
O
OUT
IN
1
Accepting larger values of
ΔIL allows the use of low induc-
tances, but results in higher output voltage ripple, greater
core losses, and lower output current capability.
A reasonable starting point for setting ripple current is
ΔIL = 0.4 IOUT, where IOUT is the maximum output cur-
rent. The largest ripple current IL occurs at the maximum
input voltage. To guarantee that the ripple current stays
below a specified maximum, the inductor value should be
chosen according to the following equation:
L
V
f
I
V
OUT
O
L
OUT
IN MAX
=
Δ
(
)
1
The inductor value will also have an effect on Burst Mode
operation. The transition from low current operation
begins when the peak inductor current falls below a level
set by the burst clamp. Lower inductor values result in
higher ripple current which causes this to occur at lower
load currents. This causes 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.
Figure2.FrequencyvsRT
InductorCoreSelection
Differentcorematerialsandshapeswillchangethesize/cur-
rent and price/current relationship of an inductor. Toroid or
shieldedpotcoresinferriteorpermalloymaterialsaresmall
anddon’tradiatemuchenergy,butgenerallycostmorethan
powdered iron core inductors with similar electrical char-
acteristics. The choice of which style inductor to use often
depends more on the price vs size requirements and any
radiated field/EMI requirements than on what the LTC3568
requires to operate. Table 1 shows some typical surface
mount inductors that work well in LTC3568 applications.
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