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LM2754 датащи(PDF) 9 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor. Click here to check the latest version.
номер детали LM2754
подробное описание детали  800mA Switched Capacitor Flash LED Driver with Time-Out Protection
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производитель  NSC [National Semiconductor (TI)]
домашняя страница  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM2754 датащи(HTML) 9 Page - National Semiconductor (TI)

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Application Information (Continued)
signal on the TX pin, the LED current level returns to the
Flash current level set by R
SET2. The TX pin responds to the
typical logic High (>1.2V) and logic Low (<0.4V) signal
levels. Flash Timeout is not active during the TX mode
operation.
SEL PIN
Connecting the SEL pin to a logic Low (<0.4V) signal places
the device in normal operation, with all 4 current sinks active.
To accommodate Flash LED modules with only 3 LEDs,
place a logic High (>1.2V) signal on the SEL pin to disable
the current sink D
4. If only 3 current sinks are used, the
200mA per current sink recommendation still applies, and
the maximum Flash current will be 600mA. Connect D
4 to
V
OUT when the logic in the SEL pin is High. Optional use of
the SEL pin is to reduce the LED current used for Torch or
Flash by 25% for high battery load conditions.
SETTING LED CURRENTS
The current through the LEDs connected to D
1-4 can be set
simply by connecting an appropriately sized resistor (R
SETx)
between the I
SET1 pin of the LM2754 and GND for Torch
mode and the I
SET2 pin and GND for Flash Mode. The LED
currents are proportional to the current that flows out of the
I
SETx pin and are a factor of approximately 800 times greater
than the I
SETx current. The feedback loop of an internal
amplifier sets the voltage of the I
SET pin to 1.25V (typ.). The
statements above are simplified in the equations below:
I
Dx = 800 x(VSET /RSET)
R
SET = 800 x (1.25V / IDx)
The maximum recommended current through each current
sink is 100mA during Torch mode and 200mA during Flash
mode. Maximum recommended total Flash current with all 4
current sinks used is 800mA (max 200mA per current sink).
Using the part in conditions where the junction temperature
might rise above the rated maximum requires that the oper-
ating ranges and/or conditions be de-rated. The printed cir-
cuit board also must be carefully laid out to account for high
thermal dissipation in the part.
PARALLEL DX OUTPUTS FOR INCREASED CURRENT
DRIVE
Outputs D
1-4 may be connected together to drive a one or
two LEDs at higher currents. In applications using a single
LED, all four parallel current sinks of equal value drive the
single LED. For this type of configuration, the LED current
should be programmed so that the current through each of
the outputs is 25% of the total desired LED current. For
example, if 200mA is the desired drive current for the single
LED, R
SET should be selected such that the current through
each of the current sink inputs is 50mA. Similarly, if two
LEDs are to be driven by pairing up the D
1-4 inputs (i.e D1-2,
D
3-4), RSET should be selected such that the current through
each current sink input is 50% of the desired LED current.
Connecting the outputs in parallel does not affect internal
operation of the LM2754 and has no impact on the Electrical
Characteristics and limits previously presented. The avail-
able diode output current, maximum diode voltage, and all
other specifications provided in the Electrical Characteristics
table apply to this parallel output configuration, just as they
do to the standard 4-LED application circuit.
Maximum recommended LED current for any configuration
is 200mA per current sink, and 800mA total. For situations
where only 3 current sinks will be used for the application,
see the SEL PIN operation section.
CAPACITOR SELECTION
The LM2754 requires 4 external capacitors for proper opera-
tion. Surface-mount multi-layer ceramic capacitors are rec-
ommended. These capacitors are small, inexpensive and
have very low equivalent series resistance (ESR <20m
typ.). Tantalum capacitors, OS-CON capacitors, and alumi-
num electrolytic capacitors are not recommended for use
with the LM2754 due to their high ESR, as compared to
ceramic capacitors.
For most applications, ceramic capacitors with X7R or X5R
temperature characteristic are preferred for use with the
LM2754. These capacitors have tight capacitance tolerance
(as good as ±10%) and hold their value over temperature
(X7R: ±15% over -55˚C to 125˚C; X5R: ±15% over -55˚C to
85˚C).
Capacitors with Y5V or Z5U temperature characteristic are
generally not recommended for use with the LM2754. Ca-
pacitors with these temperature characteristics typically
have wide capacitance tolerance (+80%, -20%) and vary
significantly over temperature (Y5V: +22%, -82% over -30˚C
to +85˚C range; Z5U: +22%, -56% over +10˚C to +85˚C
range). Under some conditions, a nominal 1µF Y5V or Z5U
capacitor could have a capacitance of only 0.1µF. Such
detrimental deviation is likely to cause Y5V and Z5U capaci-
tors to fail to meet the minimum capacitance requirements of
the LM2754.
The voltage rating of the output capacitor should be 10V or
more. For example, a 10V 0603 4.7µF output capacitor (TDK
C1608X5R1A475) is acceptable for use with the LM2754, as
long as the capacitance on the output does not fall below a
minimum of 3µF in the intended application. All other capaci-
tors should have a voltage rating at or above the maximum
input voltage of the application and should have a minimum
capacitance of 1µF.
POWER EFFICIENCY
Efficiency of LED drivers is commonly taken to be the ratio of
power consumed by the LEDs (P
LED) to the power drawn at
the input of the part (P
IN). With a 1x/1.5x/2x charge pump,
the input current is equal to the charge pump gain times the
output current (total LED current). The efficiency of the
LM2754 can be predicted as follows:
P
LED =NxVLED xILED
P
IN =VIN xIIN
P
IN =VIN x (GainxNxILED +IQ)
E=(P
LED ÷PIN)
For a simple approximation, the current consumed by inter-
nal circuitry (I
Q) can be neglected, and the resulting effi-
ciency will become:
E=V
LED ÷(VIN x Gain)
Neglecting I
Q will result in a slightly higher efficiency predic-
tion, but this impact will be negligible due to the value of I
Q
being very low compared to the typical Torch and Flash
current levels (100-800mA). It is also worth noting that effi-
ciency as defined here is in part dependent on LED voltage.
Variation in LED voltage does not affect power consumed by
the circuit and typically does not relate to the brightness of
the LED. For an advanced analysis, it is recommended that
power consumed by the circuit (V
IN xIIN) be evaluated rather
than power efficiency.
www.national.com
9


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