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| 型号： | LTC3828EUH |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | Model HB Series Horizontal Stacking Banana Plug Patch Cord, Banana Plug Each End Operating Voltage 30VAC/60VDC Max, Current-15A Cont, Operating Temp Max +55 degrees C, Color-Red, Length-24" RoHS Compliant: Yes |
| 中文描述： | 3 A DUAL SWITCHING CONTROLLER, 590 kHz SWITCHING FREQ-MAX, PQCC32 |
| 封装： | 5 X 5 MM, PLASTIC, MO-220WHHD, QFN-32 |
| 文件页数： | 16/32页 |
| 文件大小： | 396K |
| 代理商： | LTC3828EUH |
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16
LTC3828
3828f
APPLICATIOU
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
I
L
decreases with higher induc-
tance or frequency and increases with higher V
IN
:
W
U
U
I
f L
( )( )
V
V
V
L
OUT
OUT
IN
=
1
1
–
Accepting larger values of
I
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
I
L
=0.3(I
MAX
). The maximum
I
L
occurs at the maximum input voltage.
The inductor value also has secondary effects. The transi-
tion to Burst Mode operation begins when the average
inductor current required results in a peak current below
25% of the current limit determined by R
SENSE
. Lower
inductor values (higher
I
L
) 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 decrease.
Inductor Selection
Usually, high inductance is preferred for small current
ripple and low core loss. Unfortunately, increased induc-
tance requires more turns of wire or small air gap of the
inductor, resulting in high copper loss or low saturation
current. Once the value of L is known, the actual inductor
must be selected. There are two popular types of core
material of commercial available inductors.
Ferrite core inductors usually have very low core loss and
are preferred at high switching frequencies, so design
goals can concentrate on copper loss and preventing
saturation. However, ferrite core saturates “hard”, which
means that inductance collapses abruptly when the peak
design current is exceeded. This results in an abrupt
increase in inductor ripple current and consequent output
voltage ripple. One advantage of the LTC3828 is its current
mode control that detects and limits cycle-by-cycle peak
inductor current. Therefore, accurate and fast protection
is achieved if the inductor is saturated in steady state or
during transient mode.
Powder iron inductors usually saturate “soft”, which
means the inductance drops in a linear fashion when the
current increases. However, the core loss of the powder
iron inductor is usually higher than the ferrite inductor. So
design with high switching frequency should pay atten-
tion to the inductor core loss too.
Inductor manufacturers usually provide inductance, DCR,
(peak) saturation current and (DC) heating current ratings
in the inductor data sheet. A good supply design should
not exceed the saturation and heating current rating of the
inductor.
Power MOSFET and D1 Selection
Two external power MOSFETs must be selected for each
controller in the LTC3828: One N-channel MOSFET for the
top (main) switch, and one N-channel MOSFET for the
bottom (synchronous) switch.
The peak-to-peak drive levels are set by the INTV
CC
voltage. This voltage is typically 5V during start-up.
Consequently, logic-level
threshold MOSFETs must be
used in most applications. The only exception is if low
input voltage is expected (V
IN
< 5V); then, sub-logic level
threshold MOSFETs (V
GS(TH)
< 3V) should be used. Pay
close attention to the BV
DSS
specification for the MOSFETs
as well; most of the logic level MOSFETs are limited to 30V
or less.
Selection criteria for the power MOSFETs include the “ON”
resistance R
DS(ON)
, Miller capacitance C
MILLER
, input volt-
age and maximum output current. Miller capacitance,
C
MILLER
, can be approximated from the gate charge curve
usually provided on the MOSFET manufacturers’ data
sheet. C
MILLER
is equal to the increase in gate charge along
the horizontal axis while the curve is approximately flat
divided by the specified change in V
DS
. This result is then
multiplied by the ratio of the application applied V
DS
to the
Gate charge curve specified V
DS
. When the IC is operating
in continuous mode the duty cycles for the top and bottom
MOSFETs are given by:
MainSwitchDutyCycle
V
V
OUT
IN
=
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