Manufacturer

Contact Information

TDK

www.component.tdk.com

AVX

www.avx.com

Murata

www.murata.com

Taiyo-Yuden

www.t-yuden.com

Vishay-Vitramon

www.vishay.com

INPUT CAPACITOR

The input capacitor (C

helping to quickly transfer charge to the flying capacitor

during the charge phase (

φ1) of operation. The input capaci-

tor helps to keep the input voltage from drooping at the start

of the charge phase, when the flying capacitor is first con-

nected to the input, and helps to filter noise on the input pin

that could adversely affect sensitive internal analog circuitry

biased off the input line. As mentioned above, an X7R/X5R

ceramic capacitor is recommended for use. For applications

where the maximum load current required is between 60mA

and 120mA, a minimum input capacitance of 2.0µF is re-

quired. For applications where the maximum load current is

60mA or less, 1.0µF of input capacitance is sufficient. Failure

to provide enough capacitance on the LM2750 input can

result in poor part performance, often consisting of output

voltage droop, excessive output voltage ripple and/or exces-

sive input voltage ripple.

A minimum voltage rating of 10V is recommended for the

input capacitor. This is to account for DC bias properties of

ceramic capacitors. Capacitance of ceramic capacitors re-

duces with increased DC bias. This degradation can be quite

age rating of the capacitor.

FLYING CAPACITOR

The flying capacitor (C

the output, providing the voltage boost of the doubler. A

polarized capacitor (tantalum, aluminum electrolytic, etc.)

must not be used here, as the capacitor will be reverse-

biased upon start-up of the LM2750. The size of the flying

capacitor and its ESR affect output current capability when

the input voltage of the LM2750 is low, most notable for input

voltages below 3.0V. These issues were discussed previ-

ously in the Output Current Capability section. For most

applications, a 1µF X7R/X5R ceramic capacitor is recom-

mended for the flying capacitor.

OUTPUT CAPACITOR

The output capacitor of the LM2750 plays an important part

in determining the characteristics of the output signal of the

LM2750, many of which have already been discussed. The

ESR of the output capacitor affects charge pump output

resistance, which plays a role in determining output current

capability. Both output capacitance and ESR affect output

voltage ripple. For these reasons, a low-ESR X7R/X5R ce-

ramic capacitor is the capacitor of choice for the LM2750

output.

In addition to these issues previously discussed, the output

capacitor of the LM2750 also affects control-loop stability of

the part. Instability typically results in the switching fre-

quency effectively reducing by a factor of two, giving exces-

sive output voltage droop and/or increased voltage ripple on

the output and the input. With output currents of 60mA or

less, a minimum capacitance of 1.0µF is required at the

output to ensure stability. For output currents between 60mA

and 120mA, a minimum output capacitance of 2.0µF is

required.

A minimum voltage rating of 10V is recommended for the

output capacitor. This is to account for DC bias properties of

ceramic capacitors. Capacitance of ceramic capacitors re-

duces with increased DC bias. This degradation can be quite

age rating of the capacitor.

POWER EFFICIENCY AND POWER DISSIPATION

Efficiency of the LM2750 mirrors that of an unregulated

switched capacitor converter followed by a linear regulator.

The simplified power model of the LM2750, in Figure 3, will

be used to discuss power efficiency and power dissipation.

In calculating power efficiency, output power (P

determined as the product of the output current and the 5.0V

output voltage. Like output current, input voltage is an

application-dependent variable. The input current can be

calculated using the principles of linear regulation and

switched capacitor conversion. In an ideal linear regulator,

the current into the circuit is equal to the current out of the

circuit. The principles of power conservation mandate the

ideal input current of a voltage doubler must be twice the

output current. Adding a correction factor for operating qui-

escent current (I

input current which, when combined with the other input and

output parameter(s), yields the following equation for effi-

ciency:

Comparisons of LM2750 efficiency measurements to calcu-

lations using the above equation have shown the equation to

be a quite accurate approximation of actual efficiency. Be-

cause efficiency is inversely proportional to input voltage, it

is highest when the input voltage is low. In fact, for an input

voltage of 2.9V, efficiency of the LM2750 is greater than 80%

(I

OUT

≥ 40mA) and peak efficiency is 85% (I

The average efficiency for an input voltage range spanning

the Li-Ion range (2.9V-to-4.2V) is 70% (I

higher input voltages, efficiency drops dramatically. In Li-Ion-

powered applications, this is typically not a major concern,

as the circuit will be powered off a charger in these circum-

stances. Low efficiency equates to high power dissipation,

however, which could become an issue worthy of attention.

LM2750 power dissipation (P

tracting output power from input power:

P

Power dissipation increases with increased input voltage

and output current, up to 772mW at the ends of the operating

ratings (V

tion self-heats the device. Dissipating this amount power/

heat so the LM2750 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 LLP pack-

age enable this power to be dissipated from the LM2750 with

little or no derating, even when the circuit is placed in el-

evated ambient temperatures.

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