DRV590

SLOS365A – AUGUST 2001 – REVISED AUGUST 2002

10

www.ti.com

APPLICATION INFORMATION

LC filter in the frequency domain (continued)

If L = 10

µH and C = 10 µF, the resonant frequency is 9.2 kHz, which corresponds to –69 dB of attenuation at

approximately 1.7 mV.

The average TEC element has a resistance of 1.5

Ω, so the ripple current through the TEC is approximately

1.13 mA. At the 1-A maximum output current of the DRV590, this 1.13 mA corresponds to 0.113% ripple current,

causing less than 0.0001% reduction of the maximum temperature differential of the TEC element (see

equation 1).

LC filter in the time domain

The ripple current of an inductor can be calculated using equation 4.

DI

L +

V

DD *

V

TEC

DTs

L

D = duty cycle (0.5 worst case)

that ripple current will flow through the TEC element, however, the properties of the filter capacitor must be

considered.

For relatively small capacitors (less than 10

µF) with very low equivalent series resistance (ESR, less than

10 m

Ω), such as ceramic capacitors, equation 5 may be used to estimate the ripple voltage on the capacitor

due to the change in charge.

DV

C +

2

(1–D)

fo

fs

2

V

TEC

fo +

1

2

p L

3C

D = duty cycle

For L = 10

= 2.5, the ripple voltage on the capacitors is 2 mV. The ripple current may be simply calculated by dividing the

ripple voltage by the TEC resistance of 1.5

Ω, resulting in a ripple current through the TEC element of 1.33 mA.

Note that this is similar to the value calculated using the frequency domain approach.

For larger capacitors (greater than 10

µF) with relatively high ESR (greater than 100 mΩ), such as electrolytic

capacitors, the ESR drop dominates over the charging-discharging of the capacitor. Equation 6 can be used

to estimate the ripple voltage.

DV

C + D

I

L

R

ESR

For a 100-

µF electrolytic capacitor, an ESR of 0.1 Ω is common. If the 10-µH inductor is used, delivering 250 mA

of ripple current to the capacitor (as calculated above), then the ripple voltage is 25 mV. This is over ten times

that of the 10-

µF ceramic capacitor, as ceramic capacitors typically have negligible ESR.

(4)

(5)

(6)