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LTC3852

3852f

Further input noise reduction can be achieved by powering

VIN1 through a very small series inductor as shown in

Figure 12. A 10nH inductor will reject the fast current

notches, thereby presenting a nearly constant current

load to the input power supply. For economy, the 10nH

inductor can be fabricated on the PC board with about

1cm (0.4") of PC board trace.

APPLICATIONS INFORMATION

LTC3852

0.22μF

2.2μF

VIN1

GND1

1cm OF PCB TRACE

10nH

VIN

3852 F12

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

charge pump. 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 1μF of capacitance for the ﬂying

capacitor.

Ceramic Capacitors

Ceramic capacitors of different materials lose their capac-

itance with higher temperature and voltage at different rates.

For example, a capacitor made of X5R or 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 poor voltage coefﬁcient causing them

to lose 60% or more of their capacitance when the rated

voltage is applied. Therefore when comparing different

capacitors, it is often more appropriate to compare the

amount of achievable capacitance for a given case size

rather than discussing the speciﬁed capacitance value. For

example, over rated voltage and temperature conditions,

a 1μF 10V Y5V ceramic capacitor in a 0603 case may not

Figure 12. 10nH Inductor Used for

Additional Input Noise Reduction

provide any more capacitance than a 0.22μF 10V X7R

capacitor available in the same 0603 case. In fact, for the

charge pump, these capacitors can be considered roughly

equivalent. The capacitor manufacturer’s data sheet should

be consulted to ensure the desired capacitance at all temp-

eratures and voltages.

Table 1 shows a list of ceramic capacitor manufacturers

and how to contact them:

Table 1.

AVX

www.avx.com

Kemet

www.kemet.com

Murata

www.murata.com

Taiyo Yuden

www.t-yuden.com

TDK

www.component.tdk.com

Vishay

www.vishay.com

Efﬁciency Considerations

The percent efﬁciency of a switching regulator is equal to

the output power divided by the input power times 100. It

is often useful to analyze the individual loss components

to determine what limits the efﬁciency and which change

would produce the biggest improvement. The efﬁciency

can be expressed as:

% Efﬁciency = 100% – (L1 + L2 + L3 + ...)

where L1, L2, etc. are the individual losses as a percentage

of input power.

Although all dissipative elements in the circuit produce

losses, there are ﬁve main sources of power loss in

LTC3851 circuits: 1) I2R losses, 2) transition losses in the

top MOSFET, 3) gate charge losses within the controller

due to the input capacitance of the power MOSFETs,

4) the DC bias current of the controller (VIN2), and 5) the

efﬁciency of the charge pump.

1. I2R losses are predicted from the DC resistances of the

fuse (if used), top and bottom MOSFET on-resistances,

the inductor DCR and the current sense resistor (if used).

In continuous conduction mode (CCM), the average

output current ﬂows through the inductor (L) and sense

resistor (RSENSE), but is “chopped” between the top

and bottom MOSFETs. Since the two MOSFETs rarely

have the same RON, an effective MOSFET resistance

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