Datasheet

REV. –14–
ADN8830
Inductor Selection
In addition to the external transistors, the PWM amplifier requires
an inductor and a capacitor at its output to filter the switched
output waveform. Proper inductor selection is important to
achieve the best efficiency. The duty cycle of the PWM sets the
OUT A output voltage and is
D
OUT A
V
DD
=
(22)
The average current through the inductor is equal to the TEC
current. The ripple current through the inductor, I
L
, varies
with the duty cycle and is equal to
ΔI
VD D
Lf
L
DD
CLK
=
××
()
×
1–
(23)
where f
CLK
is the clock frequency as set by the resistor R
FREQ
at
Pin 26 or an external clock frequency. Refer to the Setting the
Switching Frequency section for more information. Selecting a
faster switching frequency or a larger value inductor will reduce
the ripple current through the inductor. The waveform of the
inductor current is shown in Figure 13.
TIME
I
TEC
INDUCTOR CURRENT (A)
ΔI
L
1
f
CLK
T =
Figure 13. Current Waveform Through Inductor
It is important to select an inductor that can tolerate the maxi-
mum possible current that could pass through it. Most TECs
are specified with a maximum voltage and current for proper
and reliable operation. The maximum instantaneous inductor
current can be found as
II I
L MAX TEC MAX L,,
.=+×05 Δ
(24)
where I
L
can be found from Equation 23 with the appropriate
duty cycle calculated from Equation 22 with OUT A = V
TEC, MAX
.
Design Example 3
A TEC is specified with a maximum current of 1.5 A and maxi-
mum voltage of 2.5 V. The ADN8830 will be operating from a
3.3 V supply voltage with a 200 kHz clock and a 4.7 μH inductor.
The duty cycle of the PWM amplifier at 2.5 V is calculated to be
75.8%. Using Equation 23, the inductor ripple current is found
to be 664 mA. From Equation 24, the maximum inductor current
will be 1.82 A and should be considered when selecting the
inductor. Notice that increasing the clock frequency to 1 MHz would
reduce I
L, MAX
to 1.56 A.
Design Example 4
Using the same TEC as above, the ADN8830 will be powered
from 5.0 V instead. Here, the duty cycle is 50%, which happens
to be the worst-case duty cycle for inductor current ripple. Now
DIL equals 1.33 A with a 200 kHz clock, and I
L, MAX
is 2.83 A.
Reducing the inductor ripple current is another compelling
reason to operate the ADN8830 from a 3.3 V supply instead.
Table II lists some inductor manufacturers and part numbers
along with some key specifications. The column I
MAX
refers to the
maximum current at which the inductor is rated to remain linear.
Although higher currents can be pushed through the inductor,
efficiency and ripple voltage will be dramatically degraded.
This is by no means a complete list of manufacturers or inductors
that can be used in the application. More information on these
inductors is available at thei r websites. Note the trade-offs
between inductor height, maximum current, and series resistance.
Smaller inductors cannot handle as muèH current and therefore
require higher clock speeds to reduce their ripple current. They
also have higher series resistance, which can lower the overall
efficiency of the ADN8830.
PWM Output Filter Requirements
The switching of Q1 and Q2 creates a pulse width modulated
(PWM) square wave from 0 V to V
DD
. This square wave must
be filtered sufficiently to create a steady voltage that will drive
the TEC. The ripple voltage across the TEC is a function of the
inductor ripple current, the L-C filter cutoff frequency, and the
equivalent series resistance (ESR) of the filter capacitor. The
equivalent circuit for the PWM side is given in Figure 14.
Table II. Partial List of Inductors and Key Specifications
Inductance (␮H) I
MAX
(A) R
S, TYP
(m⍀)Height (mm) Part Number Manufacturer Website
4.71.1 200 1 LPO1704-472M Coilcraft www.coi lcraft.com
4.71.59 55 2 A918CY-4R7M Toko www.toko.com
4.73.948 2.8 UP2.8B-4R7 Cooper www.cooperet.com
4.71.5 90 3 DO1608C-472 Coilcraft www. coilcraft.com
4.71.32 56 3 CDRH4D28 4R7 Sumida www.sumida.com
4.77.512 4.5 892NAS-4R7M Toko www.toko.com
4.7* 5.418 5.2 DO3316P-472 Coilcraft www.coilcraft.com
10 2.780 2.8 UP2
.8B-100 Cooper www.cooperet.com
15 8 32 8 DO5022P-153HC Coilcraft www.coilcraft.com
47 4.586 7.1 DO5022P-473 Coilcraft www.coilcraft.com
*Recommend inductor in typical application circuit Figure 1.
D