LTC3252

9

3252f

Further input noise reduction can be achieved by filtering

the input through a very small series inductor as shown in

Figure 5. A 10nH inductor will reject the fast input tran-

sients caused by the blanking period, thereby presenting

a nearly constant load to the input supply. For economy the

10nH inductor can be fabricated on the PC board with

about 1cm (0.4") of PC board trace.

discussing the specified capacitance value. For example,

over rated voltage and temperature conditions, a 4.7

µF,

10V, Y5V ceramic capacitor in a 0805 case may not

provide any more capacitance than a 1

µF, 10V, X7R

available in the same 0805 case. In fact, over bias and

temperature range, the 1

µF, 10V, X7R will provide more

capacitance than the 4.7

µF, 10V, Y5V. The capacitor

manufacturer’s data sheet should be consulted to deter-

mine what value of capacitor is needed to ensure mini-

mum capacitance values are met over operating

temperature and bias voltage.

Below is a list of ceramic capacitor manufacturers and

how to contact them:

AVX

www.avxcorp.com

Kemet

www.kemet.com

Murata

www.murata.com

Taiyo Yuden

www.t-yuden.com

Vishay

www.vishay.com

Layout Considerations

Due to the high switching frequency and transient currents

produced by the LTC3252 careful board layout is neces-

sary for optimal performance. A true ground plane and

short connections to all capacitors will improve perfor-

mance and ensure proper regulation under all conditions.

Figure 7 shows the suggested layout configuration. Note

the exposed paddle of the package is ground (GND) and

must be soldered to the PCB ground.

very high edge rate wave forms. The large dv/dt on these

pins can couple energy capacitively to adjacent printed

circuit board runs. Magnetic fields can also be generated

if the flying capacitors are not close to the LTC3252 (i.e.,

the loop area is large). To decouple capacitive energy

transfer, a Faraday shield may be used. This is a grounded

PC trace between the sensitive node and the LTC3252

pins. For a high quality AC ground, it should be returned to

a solid ground plane that extends all the way to the

LTC3252. Keep the FB traces away from or shielded from

the flying capacitor traces or degraded performance could

result.

OPERATIO

(Refer to Simplified Block Diagram)

Figure 5. 10nH Inductor Used for

Additional Input Noise Reduction

3252 F05

LTC3252

GND

4.7

µF

SUPPLY

10nH

Flying Capacitor Selection

Warning: A polarized capacitor such as tantalum or alumi-

num should never be used for the flying capacitors since

their voltages can reverse upon start-up of the LTC3252.

Ceramic capacitors should always be used for the flying

capacitors.

The flying capacitors control the strength of the charge

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

necessary for the flying capacitor to have at least 0.4

µF of

capacitance over operating temperature with a 2V bias

(see the Ceramic Capacitor Selection Guidelines). If 100mA

or less of current is required from an output then its asso-

ciated flying capacitor minimum can be reduced to 0.15

µF.

Ceramic Capacitor Selection Guidelines

Capacitors of different materials lose their capacitance

with higher temperature and voltage at different rates. For

example, a ceramic capacitor made of X7R material will

retain most of its capacitance from – 40

°Cto85°Cwhereas

a Z5U or Y5V style capacitor will lose considerable

capacitance over that range (60% to 80% loss typical).

Z5U and Y5V capacitors may also have a very strong

voltage coefficient causing them to lose an additional

60% or more of their capacitance when the rated voltage

is applied. Therefore, when comparing different capaci-

tors it is often more appropriate to compare the amount

of achievable capacitance for a given case size rather than