LM2780TP datasheet(6/7 Pages) NSC

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:

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

LM2780 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

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 LM2780 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 LM2780. 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 LM2780 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

LM2780. 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 LM2780.

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 LM2780

circuit.

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