lm2781tpx National Semiconductor Corporation, lm2781tpx Datasheet - Page 6

• Manufacturer Part Number: lm2781tpx
• Description: Ultra-low Ripple Switched Capacitor Voltage Inverter
• Manufacturer: National Semiconductor Corporation

www.national.com
Operation Description
OUTPUT RESISTANCE
Output resistance (R
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
Output resistance is a function of the internal switch resis-
tance (R
equivalent series resistance (ESR
(ESR
LM2781 operates as a single charge pump (C2 removed),
the output resistance can be approximated by the following
equation:
1 Fly Cap: R
R
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
Recommendations for minimizing output resistance include
increasing flying capacitance and minimizing output and fly-
ing capacitor ESR.
OUT
SW1
Cout
= - [V
is the sum of the ON resistance of the four internal
), and internal switching frequency (f
SW
IN
) , flying capacitance (C1), flying capacitor
OUT1
OUT2
- ( |I
~ = 2R
OUT
~ = 0.5xR
OUT
| x R
SW1
) models the internal non-idealities
OUT
+[1/(f
OUT1
)]
SW
C1
*C1)]+4ESR
), output capacitor ESR
(Continued)
SW
C1
). When the
+ESR
Cout
6
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,
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
±
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.
15% over -55˚C to 125˚C; X5R:
±
10%), hold their value over temperature (X7R:
<
15mΩ typ.). Tantalum capacitors, OS-CON
±
15% over -55˚C to 85˚C),