DRV590_16 Datasheet, PDF (10/18 Pages) Texas Instruments – 1.2-A HIGH-EFFICIENCY PWM POWER DRIVER

DRV590
SLOS365A – AUGUST 2001 – REVISED AUGUST 2002
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
the 500-kHz switching frequency. For V_DD = 5 V, the amount of ripple voltage at the TEC element will be
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.
(4)
DIL
+
ǒ_VDD
*
VTEC^ǓDTs
L
D = duty cycle (0.5 worst case)
T_s = 1/f_s = 1/500 kHz
For V_DD = 5 V, V_TEC = 2.5 V, and L = 10 µH, the inductor ripple current is 250 mA. To calculate how much of
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 mΩ), such as ceramic capacitors, equation 5 may be used to estimate the ripple voltage on the capacitor
due to the change in charge.
ǒ Ǔ²
DVC
+
p²
2
(1–D)
fo
fs
VTEC
(5)
D = duty cycle
f_s = 500 kHz
fo
+
2p
1
Ǹ_L
3C
For L = 10 µH and C = 10 µF, the cutoff frequency f_o = 9.2 kHz. For a worst case duty cycle of 0.5 and V_TEC
= 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.
DVC + DIL RESR
(6)
∆_L = inductor ripple current
R_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.
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