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LM2753 Datasheet(PDF) 5 Page - National Semiconductor (TI) |
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LM2753 Datasheet(HTML) 5 Page - National Semiconductor (TI) |
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5 / 8 page 
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 5 |
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