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LM2751SD-B bảng dữ liệu(PDF) 10 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
tên linh kiện LM2751SD-B
Giải thích chi tiết về linh kiện  Regulated 2X, 1.5X Switched Capacitor White LED Driver
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LM2751SD-B bảng dữ liệu(HTML) 10 Page - National Semiconductor (TI)

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Application Information (Continued)
The voltage rating of the output capacitor should be 10V or
more. All other capacitors should have a voltage rating at or
above the maximum input voltage of the application.
DRIVING WHITE LEDS
The desired LED current is set by placing a resistor (R) in
series with each LED, and is determined by the equation:
I
LED =(VOUT -VLED)÷R
In the equation above, I
LED is the current that flows through
a particular LED, and V
LED is the forward voltage of the LED
at the given current. The output voltage (V
OUT)ofthe
LM2751 is tightly regulated to 4.5V or 5V depending on the
output voltage option. However, LED forward voltage varies
from LED to LED, and LED current will vary accordingly.
Mismatch of LED currents will result in brightness mismatch
from one LED to the next. Therefore it is suggested that LED
groups with tightly controlled I-V characteristics ("Binned"
LEDs) be used. LEDs with looser tolerance can be used in
applications where brightness matching is not critical, such
as in keypad or general backlighting. The typical and maxi-
mum diode forward voltage depends highly on the manufac-
turer and their technology.
PWM BRIGHTNESS CONTROL
Perceived LED brightness can be adjusted using a PWM
control signal on the Enable pin of the LM2751, to turn the
voltage output ON and OFF at a rate faster than perceptible
by the eye. When this is done, the total brightness perceived
is proportional to the duty cycle (D) of the PWM signal (D =
the percentage of time that the LED is on in every PWM
cycle). A simple example: if the LEDs are driven at 15mA
each with a PWM signal that has a 50% duty cycle, per-
ceived LED brightness will be about half as bright as com-
pared to when the LEDs are driven continuously with 15mA.
For linear brightness control over the full duty cycle adjust-
ment range, the PWM frequency (f) should be limited to
accommodate the turn-on time (typ. T
ON = 300µs) of the
device.
D x (1/f) > T
ON
f
MAX =DMIN ÷TON
The minimum recommended PWM frequency is 100Hz. Fre-
quencies below this may be visibly noticeable as flicker or
blinking. The maximum recommended PWM frequency is
1kHz. Frequencies above this may cause noise in the au-
dible range.
THERMAL PROTECTION
When the junction temperature exceeds 150˚C (typ.), inter-
nal thermal protection circuitry disables the device. This
feature protects the LM2751 from damage due to excessive
power dissipation. The device will recover and operate nor-
mally when the junction temperature falls below 140˚C (typ.).
It is important to have good thermal conduction with a proper
layout to reduce thermal resistance.
POWER EFFICIENCY
Charge-Pump efficiency is derived in the following two ideal
equations (supply current and other losses are neglected for
simplicity):
I
IN =GxIOUT
E=(V
OUT xIOUT)÷(VIN xIIN)=VOUT ÷(GxVIN)
In the equations, G represents the charge pump gain. Effi-
ciency is at its highest asGxV
IN approaches VOUT. Refer to
the efficiency graph in the Typical Performance Character-
istics section for the detailed efficiency data.
POWER DISSIPATION
The power dissipation (P
DISSIPATION) and junction tempera-
ture (T
J) can be approximated with the equations below. PIN
is the product of the input current and input voltage, P
OUT is
the power consumed by the load connected to the output,
T
Ais the ambient temperature, and
θ
JA is the junction-to-
ambient thermal resistance for the LLP-10 package. V
IN is
the input voltage to the LM2751, V
VOUT is the voltage at the
output of the device, and I
OUT is the total current supplied to
the load connected to V
OUT.
P
DISSIPATION =PIN -POUT
=(V
IN xIIN)−(VVOUT xIOUT)
T
J =TA +(PDISSIPATION x
θ
JA)
The junction temperature rating takes precedence over the
ambient temperature rating. The LM2751 may be operated
outside the ambient temperature rating, so long as the junc-
tion temperature of the device does not exceed the maxi-
mum operating rating of 115˚C. The maximum ambient tem-
perature rating must be derated in applications where high
power dissipation and/or poor thermal resistance causes the
junction temperature to exceed 115˚C.
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