Datasheet

Ť
H
LP
Ť
dB
+ –40 log
ǒ
f
s
f
o
Ǔ
f
o
+
1
2p LC
Ǹ
f
s
+ 500 kHz (DRV593 or DRV594 switching frequency)
DI
L
+
ǒ
V
O
–V
TEC
Ǔ
DT
s
L
D + duty cycle (0.5 worst case)
T
s
+ 1ńf
s
+ 1ń500 kHz
DV
C
+
p
2
2
ǒ
1–D
Ǔ
ǒ
f
o
f
s
Ǔ
2
V
TEC
f
o
+
1
2p LC
Ǹ
f
s
+ 500 kHz
D + duty cycle
DV
C
+ DI
L
R
ESR
DI
L
+ inductor ripple current
R
ESR
+ filter capacitor ESR
DRV593
DRV594
www.ti.com SLOS401C –OCTOBER 2002–REVISED JULY 2010
For the DRV593 and DRV594, 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 5 may then be simplified to give the following magnitude Equation 6. These equations
assume the use of the filter in Figure 22.
(6)
If L=10 mH and C=10 mF, 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 DRV593 and DRV594, this 5.4 mA corresponds to 0.11% ripple
current, causing less than 0.0001% reduction of the maximum temperature differential of the TEC element (see
Equation 4).
LC FILTER IN THE TIME DOMAIN
The ripple current of an inductor may be calculated using Equation 7:
(7)
For V
O
= 5 V, V
TEC
= 2.5 V, and L = 10 mH, 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 mF) with very low equivalent series resistance (ESR, less than
10 mΩ, such as ceramic capacitors, the following Equation 8 may be used to estimate the ripple voltage on the
capacitor due to the change in charge:
(8)
For L = 10 mH and C = 10 mF, the cutoff frequency, f
o
, is 15.9 kHz. For worst case duty cycle of 0.5 and
V
TEC
=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 1.5Ω, 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 mF) with relatively high ESR (greater than 100 mΩ), such as electrolytic
capacitors, the ESR dominates over the charging/discharging of the capacitor. The following simple Equation 9
may be used to estimate the ripple voltage:
(9)
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Product Folder Link(s): DRV593 DRV594