DRV591 datasheet(12/23 Pages) TI2

12 / 23 page

DRV591

SLOS389A – NOVEMBER 2001– REVISED MAY 2002

www.ti.com

LC FILTER IN THE FREQUENCY DOMAIN

The transfer function for a 2nd order low-pass filter (Figures

17 and 18) is shown in equation (2):

w) +

– w

) 1

0 +

w + DRV591 switching frequency

+ quality factor

For the DRV591, the differential output switching

frequency is typically selected to be 500 kHz. The resonant

frequency for the filter is typically chosen to be at least one

order of magnitude lower than the switching frequency.

Equation (2) may then be simplified to give the following

magnitude equation (3). These equations assume the use

of the filter in Figure 17.

LP dB +

–40 log

fo +

p LC

fs + 500 kHz (DRV591 switching frequency)

If L=10

µH and C=10 µF, the cutoff frequency is 15.9 kHz,

which corresponds to –60 dB of attenuation at the 500 kHz

switching frequency. For VDD = 5 V, the amount of ripple

voltage at the TEC element is approximately 5 mV.

The average TEC element has a resistance of 1.5

Ω, so the

ripple current through the TEC is approximately 3.4 mA. At

the 3-A maximum output current of the DRV591, this 5.4

mA corresponds to 0.11% ripple current, causing less than

0.0001%

reduction

the

maximum

temperature

differential of the TEC element (see equation 1).

LC FILTER IN THE TIME DOMAIN

The ripple current of an inductor may be calculated using

equation (4):

L +

–V

TEC

DTs

+ duty cycle (0.5 worst case)

Ts + 1 fs + 1 500 kHz

For VO = 5 V, VTEC = 2.5 V, and L = 10 µH, the inductor

ripple current is 250 mA. To calculate how much of that

ripple current flows through the TEC element, however,

the properties of the filter capacitor must be considered.

For relatively small capacitors (less than 22

µF) with very

low equivalent series resistance (ESR, less than 10 m

Ω),

such as ceramic capacitors, the following equation (5) may

be used to estimate the ripple voltage on the capacitor due

to the change in charge:

C +

1–D

TEC

fo +

p LC

fs + 500 kHz

+ duty cycle

For L = 10

µH and C = 10 µF, the cutoff frequency, fo, is 15.9

kHz. For worst case duty cycle of 0.5 and VTEC=2.5 V, the

ripple voltage on the capacitors is 6.2 mV. The ripple

current may be calculated by dividing the ripple voltage by

the TEC resistance of 1.5

Ω, resulting in a ripple current

through the TEC element of 4.1 mA. Note that this is

similar to the value calculated using the frequency domain

approach.

For larger capacitors (greater than 22

µF) with relatively

high ESR (greater than 100 m

Ω), such as electrolytic

capacitors, the ESR dominates over the charging-

discharging of the capacitor. The following simple equation

(6) may be used to estimate the ripple voltage:

C + D

ESR

L +

inductor ripple current

ESR +

filter capacitor 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.

SWITCHING FREQUENCY CONFIGURATION:

OSCILLATOR COMPONENTS ROSC AND

COSC AND FREQ OPERATION

The onboard ramp generator requires an external resistor

and capacitor to set the oscillation frequency. The

frequency may be either 500 kHz or 100 kHz by selecting

the proper capacitor value and by holding the FREQ pin

either low (500 kHz) or high (100 kHz). Table 1 shows the

values required and FREQ pin configuration for each

switching frequency.

Table 1. Frequency Configuration Options

SWITCHING

FREQUENCY

ROSC

COSC

FREQ

500 kHz

120 k

Ω

220 pF

LOW (GND)

100 kHz

120 k

Ω

1 nF

HIGH (VDD)

(2)

(3)

(4)

(5)

(6)

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