LP3942YQ datasheet(19/20 Pages) NSC

19 / 20 page

Charge Pump Application

Information (Continued)

have a capacitance of only 0.1 µF. Such detrimental devia-

tion is likely to cause these Y5V and Z5U of capacitors to fail

to meet the minimum capacitance requirements of the

LP3942. The table below lists some leading ceramic capaci-

tor manufacturers.

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 CAPACITORS

The input capacitor (CIN) is used as a reservoir of charge,

helping to quickly transfer charge to the flying capacitor

during the charge phase (

Φ1 ) of operation. The input

capacitor helps to keep the input voltage from drooping at

the start of the charge phase, when the flying capacitor is

first connected 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 60 mA and 120 mA, a minimum input capacitor of

2.2 µF is required. For applications where the maximum load

current is 60 mA or less, 1.0 µF of input capacitor is suffi-

cient. Failure to provide enough capacitance on the LP3942

input can result in poor part performance, often consisting of

output voltage droop, excessive output voltage ripple and/or

excessive input voltage ripple.

FLYING CAPACITOR (C1 and C2)

The flying capacitor (CFLY) transfers charge from the input

to the output, providing the voltage boost of the charge

pump. A polarized capacitor (tantalum, aluminum electro-

lytic, etc.) must not be used here, as the capacitor will be

reverse-biased upon start-up of the LP3942. The size of the

flying capacitor and its ESR affect output current capability

when the input voltage of the LP3942 is low, most notable for

input voltages below 3.4V. These issues were discussed

previously in the Output Current Capability section. For

most applications,a1µF X7R/X5R ceramic capacitor is

recommended for the flying capacitor. When considering

also voltage roll-off, minimum capacitance value of 700 nF

should be available in all voltage conditions.

OUTPUT CAPACITOR

The output capacitor of the LP3942 plays an important part

in determining the characteristics of the output signal of the

LP3942, 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 LP3942

output. In addition to these issues previously discussed, the

output capacitor of the LP3942 also affects control-loop sta-

bility of the part. Instability typically results in the switching

frequency effectively reducing by a factor of two, giving

excessive output voltage droop and/or increased voltage

ripple on the output and the input. Minimum output capaci-

tance of 3.0 µF is required.

POWER EFFICIENCY AND POWER DISSIPATION ON

1.5x MODE

Efficiency of the LP3942 mirrors that of an unregulated

switched capacitor converter followed by a linear regulator.

The simplified power model of the LP3942, in Figure 2, will

be used to discuss power efficiency and power dissipation.

In calculating power efficiency, output power (P

OUT) is easily

determined as the product of the output current and the 4.5V

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 3/2-multiplier must be 1.5 times the

output current. Adding a correction factor for operating qui-

escent current (IQ, 1.2 mA typ) gives an approximation for

total input current which, when combined with the other input

and output parameter(s), yields the following equation for

efficiency:

Because efficiency is inversely proportional to input voltage,

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

voltage of 3.4V, efficiency of the LP3942 is greater than 80%

OUT

≥ 40 mA). The average efficiency for an input voltage

range spanning the input voltage range (3.4V-to-4.2V) is

75% (I

out = 120 mA). At 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 circumstances. Low efficiency equates

to high power dissipation, however, which could become an

issue worthy of attention.

LP3942’s charge pump power dissipation (PD) is calculated

simply by subtracting output power from input power:

D =PIN –POUT =[VIN x (1.5·IOUT +IQ)] – [VOUT xIOUT]

Power dissipation increases with increased input voltage

and output current, up to 450 mW at the ends of the oper-

ating ratings (V

IN = 5.5V, IOUT = 120 mA). Internal power

dissipation self-heats the device. Dissipating this amount

power/ heat so the LP3942 does not overheat is a demand-

ing 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 LP3942 with

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

evated ambient temperatures.

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