參數(shù)資料
型號: LM3407MYX
廠商: NATIONAL SEMICONDUCTOR CORP
元件分類: 顯示驅(qū)動器
英文描述: 350 mA, Constant Current Output Floating Buck Switching Converter for High Power LEDs
中文描述: LED DISPLAY DRIVER, PDSO8
封裝: PLASTIC, MSOP-8
文件頁數(shù): 9/16頁
文件大小: 321K
代理商: LM3407MYX
30046620
FIGURE 2. LM3407 Switching Waveforms
The switching frequency and duty ratio of the converter equal:
By comparing the area of V
and V
over the ON period,
an error signal is generated. Such a comparison is function-
ally equivalent to comparing the middle level of I
to V
during the ON-period of a switching cycle. The error signal is
fed to a PWM comparator circuit to produce the PWM control
pulse to drive the internal power N-MOSFET.
Figure 3
shows
the implementation of the PWM switching signal. The error
signal is fed to a PWM comparator circuit to produce the PWM
control pulse to drive the internal power N-MOSFET.
Figure
3
shows the implementation of the PWM switching signal.
In closed loop operation, the difference between V
and
V
is reflected in the changes of the switching duty cycle of
the power switch. This behavior is independent of the induc-
tance of the inductor and input voltage because for the same
set of I
* R
ISNS
, ON time, and switching period, there exists
only one V
shows two sets of current sense sig-
nals named V
and V
that have identical frequencies
and duty cycles but different shapes of trapezoidal wave-
forms, each generating identical PWM signals.
30046623
FIGURE 3. Pulse-Level Transformation
When V
is higher than V
, the peak value of V
, the
switching duty cycle of the power switch will be reduced to
lower V
. When V
is lower than the peak value of V
,
the switching duty cycle of the power switch will be increased
to raise V
. For example, when I
is decreased, V
will
become lower than V
. In order to maintain output current
regulation, the switching duty cycle of the power switch will
be increased and eventually push up V
until V
MSL
equals
V
REF
. Since in typical floating buck regulators V
is equal
to I
* R
, true average output current regulation can be
achieved by regulating V
.
Figure 5
shows the waveforms
of V
ISNS
and V
RP
under closed loop operation.
9
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