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參數(shù)資料
| 型號： | LM3401 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | Hysteretic PFET Controller for High Power LED Drive |
| 中文描述： | 遲滯PFET的控制器的高功率LED驅(qū)動 |
| 文件頁數(shù)： | 7/18頁 |
| 文件大?。?/td> | 324K |
| 代理商： | LM3401 |
W
RSNS
= V
SNS
x I
LED
When selecting a sense resistor, thermal de-rating must also
be taken into consideration.
While R
sets the DC LED current, the AC peak LED cur-
rent will be higher than the DC setting. This peak current must
not be greater than the maximum peak current rating of the
LED, I
. Peak LED current can be calculated from the
equation below:
The LED ripple current, I
LED_RIP
, is described below in the
Hysteresis Adjust section.
HYSTERESIS ADJUST
Adjustable hysteresis (via the HYS pin) provides direct control
over the LED ripple current. The HYS pin also gives some
control over the switching frequency. Although the hysteresis
value can be set after the inductor is selected, a preliminary
value must be set in order to begin the frequency calculation.
The hysteresis window must be set small enough to keep the
peak LED current below its maximum rating, I
LED_max
.
The maximum SNS pin hysteresis can be calculated as
shown:
SNS
HYS_MAX
= (I
LED_max
- I
LED
) x R
SNS
Any SNS
HYS
value between 10mV and this maximum is ac-
ceptable.
The SNS
value is set with a single resistor from the HYS
pin to GND, shown as R2 in the typical application schematic.
The HYS pin voltage, V
, is internally multiplied by
SNS
(0.2 typical) to generate the hysteresis at the SNS
pin, SNS
. The hysteresis setting resistor can be deter-
mined from the following equation:
Where 20 μA is the typical HYS source current, and
SNS
is the resulting SNS pin hysteresis voltage. The hys-
teresis voltage can be set within a range of 10 mV to 100 mV
(50 mV to 500 mV at the HYS pin). The SNS
value defines
both the upper and lower threshold of the SNS pin. For ex-
ample, with a V
setting of 100 mV, SNS
will be 20 mV.
Therefore, the total hysteretic window will be 40 mV, or 20 mV
above and 20mV below the 200 mV reference voltage. This
directly correlates to peak-to-peak inductor and LED ripple
current, approximated by the following equation:
If LED ripple current is a design priority, the preliminary R2
value can be determined using a target LED ripple current as
shown:
A more precise equation for ripple current is given in the In-
ductor Selection section. Once an inductor is selected the
more precise equation should be used to determine the actual
ripple current and LED peak current. Larger hysteresis values
will result in lower switching frequency and higher ripple cur-
rent for a given inductor. Typical examples are shown in
Figure 3
below.
30021422
FIGURE 3. Switching Frequency and Ripple Current vs
Hysteresis
SWITCHING FREQUENCY
Although hysteretic control is a simple control method, the
switching frequency depends on application conditions and
components. If the inductance, input voltage, or LED forward
voltage is changed, there will be a change in the switching
frequency. Therefore, care must be taken to select compo-
nents which will provide the desired frequency range. Usually,
the best approach is to determine a nominal switching fre-
quency for the application and then select the inductor ac-
cordingly. Once the inductor is selected, V
can be adjusted
to set the frequency range more precisely. This design pro-
cess usually involves a few iterations to select appropriate
standard values that will result in the desired frequency and
ripple current.
Switching frequency can be approximately calculated using
the formula:
Where D is the duty cycle, defined as (V
OUT
+ V
DIODE
) / V
IN
,
V
is 200 mV plus the sum of LED forward voltages, and
delay is the sum of the LM3401 propagation delay time and
the PFET delay time. The LM3401 propagation delay is 46 ns
typically (See the Propagation Delay curve). Alternately, the
inductor value can be calculated from a known frequency by
re-arranging the same equation:
Switching frequency for a typical application is shown in
Fig-
ure 4
along with the calculated frequency.
7
www.national.com
L
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