DocID14937 Rev 4

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TS2007FC

Application information

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4.4

Low frequency response

If a low frequency bandwidth limitation is required, it is possible to use input coupling

capacitors. In the low frequency region, the input coupling capacitor Cin has a greater effect.

Cin and the input impedance Zin form a first-order high-pass filter with a -3 dB cutoff

frequency (see Table 5 to Table 9).

where FCL = cutoff frequency

So, for a desired cutoff frequency we can calculate Cin as follows:

where FCL is expressed in Hz, Zin in Ω, and Cin in F.

The input impedance, Zin, is for the whole power supply voltage range, typically 75 kΩ.

There is also a tolerance around the typical value (see Table 5 to Table 9). The tolerance of

FCL can also be calculated:

•

•

4.5

Circuit decoupling

A power supply capacitor, referred to as CS, is needed to correctly bypass the TS2007FC.

The TS2007FC has a typical switching frequency of 280 kHz and an output fall and rise time

of less than or equal to 5 ns. Due to these very fast transients, careful decoupling is

mandatory.

A 1 µF ceramic capacitor is enough, but it must be located very close to the TS2007FC in

order to avoid any extra parasitic inductance created by a long track wire. Parasitic loop

inductance, in relation with di/dt, introduces overvoltage that decreases the global efficiency

of the device and may cause, if this parasitic inductance is too high, a TS2007FC

breakdown. For filtering low frequency noise signals on the power line, it is recommended to

use a capacitor of at least 1 µF.

In addition, even if a ceramic capacitor has an adequate high frequency ESR (equivalent

series resistance) value, its current capability is also important. A 0603 size is a good

compromise, particularly when a 4

Ω load is used.

Another important parameter is the rated voltage of the capacitor. A 1 µF/6.3 V capacitor

used at 5 V, can loose about 50% of its value. With a power supply voltage of 5 V, the

decoupling value, instead of 1 µF, can be reduced to 0.5 µF. As CS has particular influence

on the THD+N in the medium to high frequency region, this capacitor variation becomes

decisive. In addition, less decoupling means higher overshoots which can be problematic if

they reach the power supply AMR value (6 V).

F

CL

1

2

π Z

in

C

in

⋅⋅

⋅

------------------------------------

=

C

in

1

2

π Z

in

F

CL

⋅⋅

⋅

--------------------------------------

=

F

CLmax

1.103 F

CL

⋅

=

F

CLmin

0.915 F

CL

⋅

=