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參數(shù)資料
| 型號(hào)： | LM3595 |
| 廠商： | National Semiconductor Corporation |
| 元件分類： | LED驅(qū)動(dòng)器 |
| 英文描述： | The LM3595 is a parallel white-LED driver that is capable of driving four white LEDs. |
| 中文描述： | 該LM3595是平行的白光LED驅(qū)動(dòng)器能夠驅(qū)動(dòng)4個(gè)白光LED的能力。 |
| 文件頁數(shù)： | 8/11頁 |
| 文件大?。?/td> | 233K |
| 代理商： | LM3595 |
Circuit Description
The LM3595 is a parallel white-LED driver with four matched
current outputs. This part is intended to be driven off of an
existing power supply rail or other regulated voltage source
found within the surrounding system. The matched current
regulators each have a 100:1 current ratio between the
OUTx outputs and the I
SET
currents. The mirrors control the
current through the LEDs without the use of external ballast
resistors. With a total of 100mA of total output current avail-
able, the LM3595 is easily capable of supplying 25mA per
each of the four outputs through the proper selection of the
R
SET
resistor. LED brightness control can be achieved on
the LM3595 with a PWM signal and/or an analog voltage.
Application Information
ENABLE / SHUTDOWN
When the voltage on the active-high-logic enable pin is low,
the LM3595 will be in shutdown. While disabled, the LM3595
typically draws 0.1μA. When the EN pin is unconnected, the
part automatically goes into shutdown due to an internal
1M
pull-down resistor that is tied between EN and GND.
OUTPUT CURRENT CAPABILITY
The LM3595 is capable of providing up to 25mA of current to
each of the four outputs given an input voltage of 3.0V to
5.5V. The outputs have a typical current matching of 0.5%
between adjacent sources. An external resistor can be used
to set the output current, as approximated with the following
the equation:
R
SET
=100 x (1.22V / I
OUTX
) with BR-ON = 0V
In order for the output currents to be regulated properly,
sufficient headroom voltage (V
HR
) must be present. The
headroom voltage refers to the minimum amount of voltage
that must be present across the current source in order to
ensure the desired current is realizable. To ensure the de-
sired current is obtained, apply the following equations to
find the minimum input voltage required:
V
IN
- V
OUTX
≥
V
HR
V
is the diode forward voltage, and V
HR
is defined by
the following equation:
V
HR
= k
HR
x (0.95 x I
OUTX
)
I
is the desired diode current, and k
, typically
23mV/mA in the LM3595, is a proportionality constant that
represents the ON-resistance of the internal current mirror
transistors. For worst-case design calculations, using a k
of 30mV/mAis recommended. (Worst-case recommendation
accounts for parameter shifts from part-to-part variation and
applies over the full operating temperature range). Changes
in headroom voltage from one output to the next, possible
with LED forward voltage mismatch, will result in different
output currents and LED brightness mismatch. Thus, oper-
ating the LM3595 with insufficient headroom voltage across
all current sources should be avoided.
TABLE 1. I
OUT
, R
SET
and V
HR-MIN
Voltage (BR-ON = 0V), k
HR
= 30 mV/mA (worst-case)
I
OUT
10mA
15mA
25mA
R
SET
12.4k
8.25k
4.88k
V
HEADROOM
285mV
430mV
715mV
PWM BRIGHTNESS CONTROL
Brightness control can be implemented by pulsing a signal at
the EN pin. When a PWM signal is used to drive the EN pin
of the LM3595, the BR-ON pin should be driven low. The
R
value should be selected using the R
equation
when the analog brightness control feature is disabled. LED
brightness is proportional to the duty cycle (D) of the PWM
signal. For linear brightness control over the full duty cycle
adjustment range, the PWM frequency (f) should be limited
to accommodate the turn-on time (T
ON
= 50μs) of the de-
vice.
D x (1/f)
>
T
ON
f
MAX
= D
MIN
÷ T
ON
If the PWM frequency is much less than 100Hz, flicker may
be seen in the LEDs. For the LM3595, zero duty cycle will
turn off the LEDs and a 50% duty cycle will result in an
average I
being half of the programmed LED current. For
example, if R
is set to program 15mA, a 50% duty cycle
will result in an average I
of 7.5mA. I
being half the
programmed LED current. R
should be chosen not to
exceed the maximum current delivery capability of the de-
vice.
ANALOG BRIGHTNESS CONTROL
The LM3595 allows for analog brightness control using the
BR-ON, and BRGT pin. To enable this function, the BR-ON
pin must be set to a voltage greater or equal to 0.45*V
.
With analog brightness enabled, the voltage delivered to pin
BRGT can be used to adjust the output currents. Use the
maximum desired current (I
) and the maximum
analog voltage to be used in the adjustment (V(BR)
(max)
) to
size R
SET
according to the following equation:
Table 2
shows the current through each LED for the LM3595
with various BRGT and RSET values.
When analog brightness is used, applying 0V to the BRGT
pin does not turn the LEDs off completely. There is a small
amount of leakage current (100μA) that typically causes a
small amount of light to be emitted from the LED. In order to
completely turn off the LEDs, drive the EN pin low to disable
the part.
L
www.national.com
8
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| LM359J | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Dual, High Speed, Programmable, Current Mode Norton Amplifiers |
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