LTC1751 查看數據表（PDF） - Linear Technology

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LTC1751

Micropower, Regulated Charge Pump DC/DC Converters

Linear Technology 

LTC1751/LTC1751-3.3/LTC1751-5

APPLICATIO S I FOR ATIO

Flying Capacitor Selection

Warning: A polarized capacitor such as tantalum or

aluminum should never be used for the flying capacitor

since its voltage can reverse upon start-up of the LTC1751.

Low ESR ceramic capacitors should always be used for

the flying capacitor.

The flying capacitor controls the strength of the charge

pump. In order to achieve the rated output current, it is

necessary to have at least 0.6µF of capacitance for the

flying capacitor. Capacitors of different materials lose their

capacitance with higher temperature and voltage at differ-

ent rates. For example, a ceramic capacitor made of X7R

material will retain most of its capacitance from – 40°C to

85°C, whereas, a Z5U or Y5V style capacitor will lose

considerable capacitance over that range. Z5U and Y5V

capacitors may also have a very strong voltage coefficient

causing them to lose 50% or more of their capacitance

when the rated voltage is applied. The capacitor

manufacturer’s data sheet should be consulted to deter-

mine what value of capacitor is needed to ensure 0.6µF at

all temperatures and voltages.

Generally an X7R ceramic capacitor is recommended for

the flying capacitor with a minimum value of 1µF. For very

low load applications, it may be reduced to 0.01µF-0.68µF.

A smaller flying capacitor delivers less charge per clock

cycle to the output capacitor resulting in lower output

ripple. The output ripple is reduced at the expense of

maximum output current and efficiency.

The theoretical minimum output resistance of a voltage

doubling charge pump is given by:

ROUT(MIN)

≡

2VIN – VOUT

IOUT

=

1

fC

Where f if the switching frequency and C is the value of the

flying capacitor. (Using units of MHz and µF is convenient

since they cancel each other.) Note that the charge pump

will typically be weaker than the theoretical limit due to

additional switch resistance. However, for light load appli-

cations, the above expression can be used as a guideline

in determining a starting capacitor value.

Below is a list of ceramic capacitor manufacturers and

how to contact them:

AVX

Kemet

Murata

Taiyo Yuden

Vishay

www.avxcorp.com

www.kemet.com

www.murata.com

www.t-yuden.com

www.vishay.com

Output Ripple

Low frequency regulation mode ripple exists due to the

hysteresis in the sense comparator and propagation

delays in the charge pump control circuits. The amplitude

and frequency of this ripple are heavily dependent on the

load current, the input voltage and the output capacitor

size. For large VIN the ripple voltage can become substan-

tial because the increased strength of the charge pump

causes fast edges that may outpace the regulation cir-

cuitry. In some cases, rather than bursting, a single

output cycle may be enough to boost the output voltage

into or possibly beyond regulation. In these cases the

average output voltage will climb slightly. For large input

voltages a larger output capacitor will ensure that burst-

ing always occurs, thus mitigating possible DC problems.

Generally the regulation ripple has a sawtooth shape

associated with it.

A high frequency ripple component may also be present

on the output capacitor due to the charge transfer action

of the charge pump. In this case, the output can display a

voltage pulse during the output-charging phase. This

pulse results from the product of the charging current and

the ESR of the output capacitor. It is proportional to the

input voltage, the value of the flying capacitor and the ESR

of the output capacitor.

For example, typical combined output ripple for an

LTC1751-5 with VIN = 3V under maximum load is

75mVP-P with a low ESR 10µF output capacitor. A smaller

output capacitor and/or larger output current load will

result in higher ripple due to higher output voltage slew

rates.

8

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