)

I

I

2

(

V

I

V

P

P

E

Q

OUT

IN

OUT

OUT

IN

OUT

˜

u

u

COUT

C1

1

SW

SW

OUT

ESR

ESR

4

C

F

1

R

2

R

˜

u

˜

Reg

2×

V '

2×V '

LM2775-Q1

Output Resistance Model

13

LM2775-Q1

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increases. To prevent droop in output voltage, the voltage drop across the regulator is reduced, V' increases, and

regulator, V' equals the input voltage, and the output voltage is near the edge of regulation. Additional output

current causes the output voltage to fall out of regulation, and the LM2775-Q1 operation is similar to a basic

open-loop doubler. As in a voltage doubler, increase in output current results in output voltage drop proportional

to the output resistance of the doubler. The out-of-regulation LM2775-Q1 output voltage can be approximated by:

(1)

Again, Equation 1 only applies at low input voltage and high output current where the LM2775-Q1 is not

regulating. See Output Current vs. Output Voltage curves in the Typical Characteristics section for more details.

Figure 20. LM2775-Q1 Output Resistance Model

A more complete calculation of output resistance takes into account the effects of switching frequency, flying

capacitance, and capacitor equivalent series resistance (ESR) (see Equation 2).

(2)

Switch resistance component (3

Ω typical) dominates the output resistance equation of the LM2775-Q1. With a

2-MHz typical switching frequency, the 1/(F×C) component of the output resistance contributes only 0.5

Ω to the

total output resistance. Increasing the flying capacitance only provides minimal improvement to the total output

current capability of the LM2775-Q1. In some applications it may be desirable to reduce the value of the flying

capacitor below 1 µF to reduce solution size and/or cost, but this should be done with care so that output

resistance does not increase to the point that undesired output voltage droop results. If ceramic capacitors are

used, ESR will be a negligible factor in the total output resistance, as the ESR of quality ceramic capacitors is

typically much less than 100 mΩ.

8.2.2.2 Efficiency

Charge-pump efficiency is derived in Equation 3 and Equation 4 (supply current and other losses are neglected

for simplicity):

(3)

If one includes the quiescent current drawn by the LM2775-Q1 to operate, the following can be derived :

(4)

the LM2775-Q1 device, G = 2.

8.2.2.3 Power Dissipation

(5)

Power dissipation increases with increased input voltage and output current, up to 1.35 W at the ends of the

amount power/heat so the LM2775-Q1 does not overheat is a demanding thermal requirement for a small

surface-mount package. When soldered to a PCB with layout conducive to power dissipation, the excellent

thermal properties of the WSON package enable this power to be dissipated from the LM2775-Q1 with little or no

derating, even when the circuit is placed in elevated ambient temperatures.