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LM2794 Datasheet(PDF) 11 Page - Texas Instruments |
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LM2794 Datasheet(HTML) 11 Page - Texas Instruments |
11 / 21 page LM2794/95 3.0V GND BRGT DKX DK2 DK1 *Optional Independent Shutdown 5mA (3/2) VIN = approx. 4.5V POUT RSET D4 D3 D2 D1 124: CIN 1 …F VIN C1- C1+ C2- C2+ CPOUT 1 …F C2 1 …F C1 1 …F 15mA each = 60 mA Total Keypad LEDs LM2794, LM2795 www.ti.com SNVS168K – MAY 2004 – REVISED MAY 2004 Table 4. LED Current (continued) RSET Values BRGT 2.67K Ω 1.33K Ω 909 Ω 665 Ω 0.5V 1.4mA 2.9mA 4.2mA 5.7mA 1.0V 2.1mA 4.3mA 6.3mA 8.6mA 1.5V 2.9mA 5.8mA 8.4mA 11.5mA 2.0V 3.6mA 7.2mA 10.5mA 14.4mA 2.5V 4.3mA 8.7mA 12.7mA 17.3mA 3.0V 5.0mA 10.1mA 14.8mA 20.2mA CHARGE PUMP OUTPUT (POUT) The LM2794/5 charge pump is an unregulated switched capacitor converter with a gain of 1.5. The voltage at the output of the pump (the POUT pin) is nominally 1.5 × VIN. This rail can be used to deliver additional current to other circuitry. Figure 3 shows how to connect additional LEDs to POUT. A ballast resistor sets the current through each LED, and LED current matching is dependent on the LED forward voltage matching. Because of this, LEDs driven by POUT are recommended for functions where brightness matching is not critical, such as keypad backlighting. Since POUT is unregulated, driving LEDs directly off POUT is usually practical only with a fixed input voltage. If the input voltage is not fixed (Li-Ion battery, for example), using a linear regulator between the POUT pin and the LEDs is recommended. National Semiconductor's LP3985-4.5V low-dropout linear regulator is a good choice for such an application. The voltage at POUT is dependent on the input voltage supplied to the LM2794/5, the total LM2794/5 output current, and the output resistance (ROUT) of the LM2794/5 charge pump. Output resistance is a model of the switching losses of the charge pump. Resistances of the internal charge pump switches (MOS transistors) are a primary component of the LM2794/5 output resistance. Typical LM2794/5 output resistance is 3.0 Ω. For worst- case design calculations, using an output resistance of 3.5 Ω is recommended. (Worst-case recommendation accounts for parameter shifts from part-to-part variation and applies over the full operating temperature range). Figure 3. Keypad LEDs Connected to POUT Output resistance results in droop in the POUT voltage proportional to the amount of current delivered by the pump. The POUT voltage is an important factor in determining the total output current capability of an application. Taking total output current to be the sum of all DX output currents plus the current delivered through the POUT pin, the voltage at POUT can be predicted with the following equations: ITOTAL = ID1 + ID2 + ID3 + ID4 + IPOUT (1) VPOUT = 1.5 × VIN − ITOTAL × ROUT (2) Copyright © 2004, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM2794 LM2795 |
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