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LM2781TPX Datasheet(PDF) 6 Page - National Semiconductor (TI) |
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LM2781TPX Datasheet(HTML) 6 Page - National Semiconductor (TI) |
6 / 7 page Operation Description (Continued) OUTPUT RESISTANCE Output resistance (R OUT) models the internal non-idealities of a charge pump that result in droop in the output voltage. The effect of output resistance on output voltage is ex- pressed in the following equation: V OUT =-[VIN -(|IOUT|xROUT )] Output resistance is a function of the internal switch resis- tance (R SW) , flying capacitance (C1), flying capacitor equivalent series resistance (ESR C1), output capacitor ESR (ESR Cout), and internal switching frequency (fSW). When the LM2781 operates as a single charge pump (C2 removed), the output resistance can be approximated by the following equation: 1 Fly Cap: R OUT1 ~=2R SW1+[1/(fSW*C1)]+4ESRC1+ESRCout R SW1 is the sum of the ON resistance of the four internal MOSFET switches of Charge Pump 1. With both C1 and C2 in place, the double pumping topology of the LM2781 cuts the output resistance in half: 2 Fly Caps: R OUT2 ~= 0.5xR OUT1 Recommendations for minimizing output resistance include increasing flying capacitance and minimizing output and fly- ing capacitor ESR. CAPACITOR SELECTION Surface-mount multi-layer ceramic capacitors are recom- mended for use with the LM2781. These capacitors are small, inexpensive and have very low equivalent series re- sistance (ESR, < 15m Ω typ.). Tantalum capacitors, OS-CON capacitors, and aluminum electrolytic capacitors generally are not recommended for use with the LM2781 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 LM2781. 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. Capacitors with Y5V and/or Z5U temperature characteristic are generally not recommended for use with the LM2781. These types of capacitors typically have wide capacitance 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 deviation would severely affect performance of the LM2781 circuit. www.national.com 6 |
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