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LM2751SD-B Datasheet(PDF) 10 Page - Texas Instruments |
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LM2751SD-B Datasheet(HTML) 10 Page - Texas Instruments |
10 / 19 page LM2751 SNVS299B – APRIL 2005 – REVISED MAY 2013 www.ti.com APPLICATION INFORMATION CIRCUIT DESCRIPTION The LM2751 is a Switched Capacitor Convertor with gains of 2x and 1.5x. It is capable of continuously supplying up to 150mA at 4.5V or up to 80mA at 5V depending on the output voltage option. The LM2751's fixed frequency pre-regulation maintains the output voltage to within 3% (typ.), making it well suited for driving White LEDs. There are also four user programmable switching frequencies to reduce the quiescent current consumption at light loads. Aside from powering LEDs, the LM2751 is suitable for driving other devices with power requirements up to 150mA. The LM2751 operates over the extended Li-Ion battery range from 2.8V to 5.5V. The LM2751 limits output current to 250mA (typ.) during an output short circuit condition. LED brightness is controlled by applying a PWM (Pulse Width Modulation) signal to the Enable pin (EN). See PWM BRIGHTNESS CONTROL. SOFT START Soft Start is engaged when the device is taken out of Shutdown mode (EN = logic HIGH) or when voltage is supplied simultaneously to the VIN and EN pins. During Soft Start, the voltage on VOUT 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 the regulated output voltage level (4.5V or 5V) in 300µs (typ.). ENABLE MODE The Enable logic pin (EN) disables the part and reduces the quiescent current to 0.77µA (typ.). The LM2751 has an active-high enable pin (LOW = shut down, HIGH = operating) which can be driven with a low-voltage CMOS logic signal (1.5V logic, 1.8V logic, etc). There is an internal 1M Ω pull-down resistor between the EN and GND pins of the LM2751. FREQUENCY MODE SELECT The LM2751 switching frequency is user programmable via two logic input pins, CS0 and CS1. Both logic input pins have active-high logic (LOW = un-selected, HIGH = selected) and can be driven with a low-voltage CMOS logic signal (1.5V logic, 1.8V logic, etc). There are no internal pull-down or pull-up resistors between the CSx and GND pins of the LM2751. The CSO and CS1 can be controlled independently or with the same logic signal. The selectable switching frequencies are 9.5kHz, 37kHz, 300kHz, 725kHz. The switching frequency is programmed according to Table 1. Table 1. Frequency Modes CS0 CS1 Frequency 0 0 37kHz 0 1 300kHz 1 0 9.5kHz 1 1 725kHz VOUT REGULATION The LM2751 uses pre-regulation to regulate the output voltage to 4.5V or 5.0V depending on the voltage option. Pre-regulation uses the voltage present at VOUT to limit the gate drive of the switched capacitor charge pump. This regulation is done before the voltage is gained up by the charge pump, giving rise to the term "pre- regulation". Pre-regulation helps to reduce input current noise and large input current spikes normally associated with switched capacitor charge pumps. The LM2751 switched capacitor charge pump has gains of 2x and 1.5x. When the input voltage to the device is greater than 3.58V (typ.), the LM2751 operates in a gain of 1.5x. When the input voltage falls below 3.5V (typ.), the device switches to a gain of 2x. 10 Submit Documentation Feedback Copyright © 2005–2013, Texas Instruments Incorporated Product Folder Links: LM2751 |
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