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LM2753 Datasheet(PDF) 5 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor. Click here to check the latest version.
Part No. LM2753
Description  High Power Switched Capacitor Voltage Convertor/Flash LED Driver
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Maker  NSC [National Semiconductor (TI)]
Homepage  http://www.national.com
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LM2753 Datasheet(HTML) 5 Page - National Semiconductor (TI)

   
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Application Information
CIRCUIT DESCRIPTION
The LM2753 is a Switched Capacitor Doubler with a regu-
lated 5V output. It is capable of continuously supplying up to
200mA at 5V to a load connected to V
OUT. This device uses
Pulse Frequency Modulation and a Multi-Level Switch Array
to regulate and maintain the output voltage. For higher load
currents, such as during Flash operation, the output voltage
is allowed to droop to supply the necessary current. Although
there is no current limit on this device, the device will auto-
matically default to a gain of 1 when the output is brought
below the input voltage. This configuration limits the input
current to about 300mA (typ.). The operating range for the
LM2753 is over the extended Li-Ion battery range from 2.7V
to 5.5V.
Aside from powering Flash LEDs, the LM2753 is suitable for
driving other devices with power requirements up to 200mA.
White LEDs can also be connected to this device to backlight
a cellular phone keypad and display. The LED brightness
can be controlled by applying a PWM (Pulse Width Modula-
tion) signal to the Enable pin (EN) during "Torch" mode, or to
the Flash pin during "Flash" mode. (see PWM BRIGHT-
NESS CONTROL PROCEDURES section).
SOFT START
Soft Start is engaged when the device is taken out of Shut-
down mode (EN = logic HIGH) or when voltage is supplied
simultaneously to the VIN and EN pins. During Soft Start, the
voltage on V
OUT will ramp up in proportion to the rate that the
reference voltage is being ramped up. The output voltage is
programmed to rise from 0V to 5V in 640µs (typ.).
ENABLE MODE
The Enable logic pin (EN) disables the part and reduces the
quiescent current to 0.1µA (typ.). The LM2753 has an active-
high enable pin (LOW = shut down, HIGH = operating). The
LM2753 EN pin can be driven with a low-voltage CMOS logic
signal (1.5V logic, 1.8V logic, etc). There is an internal
300k
Ω pull-down resistor between the EN and GND pins of
the LM2753.
FLASH MODE
The Flash logic pin (Flash) controls the internal FET con-
nected between the V
OUT and IOUT pins on the LM2753. The
LM2753 has an active-HIGH Flash pin (LOW = shut down,
HIGH = operating). A logic HIGH signal must be present on
the EN pin before a logic HIGH signal is place on the Flash
input pin. The EN and Flash input pins can be connected
together and controlled with the same logic signal. The
turn-on time for I
OUT in this configuration will be limited by
the turn-on time of the device. The turn-on time for the
internal FET is typically 10ns when the device is already on
(EN signal HIGH, V
OUT at 5V). The LM2753 Flash pin can be
driven with a low-voltage CMOS logic signal (1.5V logic,
1.8V logic, etc). There is an internal 300k
Ω pull-down resis-
tor between the Flash and GND pins of the LM2753.
CAPACITOR SELECTION
The LM2753 requires 3 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,
≤15mΩ
typ.). Tantalum capacitors, OS-CON capacitors, and alumi-
num electrolytic capacitors are generally not recommended
for use with the LM2753 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
LM2753. These capacitors have tight capacitance tolerance
(as good as ±10%), hold their value over temperature (X7R:
±15% over −55˚C to 125˚C; X5R: ±15% over −55˚C to
85˚C), and typically have little voltage coefficient when com-
pared to other types of capacitors. However selecting a
capacitor with a voltage rating much higher than the voltage
it will be subjected to, will ensure that the capacitance will
stay closer to the capacitor’s nominal value. Capacitors with
Y5V or Z5U temperature characteristic are generally not
recommended for use with the LM2753. Capacitors with
these temperature characteristics typically have wide ca-
pacitance tolerance (+80%, −20%), vary significantly over
temperature (Y5V: +22%, −82% over −30˚C to +85˚C range;
Z5U: +22%, −56% over +10˚C to +85˚C range), and have
poor voltage coefficients. 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 capacitors to fail to meet the minimum capacitance
requirements of the LM2753. Table 1 lists suggested capaci-
tor suppliers for the typical application circuit.
TABLE 1. Ceramic Capacitor Manufacturers
Manufacturer
Contact
TDK
www.component.tdk.com
Murata
www.murata.com
Taiyo Yuden
www.t-yuden.com
FLASH LED SELECTION
The LM2753 provides a 5V (typ.) fixed voltage to drive a
Flash LED with a continuous current up to 200mA (typ.). At
LED currents above 200mA (typ.), the output of the LM2753
is allowed to droop to deliver the desired current to the Flash
LED. This droop limits the maximum forward voltage and in
turn the maximum current that can be supplied to a given
LED. LEDs should be chosen such that the LED forward
voltage at the desired maximum LED current does not ex-
ceed the output voltage of the LM2753 when loaded down
with that same current. It is suggested that the selected
LEDs be binned due to the relatively high forward voltage
tolerance of Flash LEDs. The typical and maximum diode
forward voltage depends highly on the manufacturer and
their technology. Table 2 lists several suggested manufac-
turers.
TABLE 2. Flash LED Selection
Manufacturer
Contact
Agilent
www.agilent.com/semiconductors
AOT
www.aot.com.tw
Citizen
www.c-e.co.jp/e/
Lumiled
www.lumileds.com
Nichia
www.nichia.com
Osram
www.osram-os.com
Panasonic
www.panasonic.co.jp/semicon/
Seoul Semiconductor
en.seoulsemicon.co.kr
www.national.com
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