Datasheet

Design Procedure
Inductor Selection
Small surface-mount inductors are ideal for use with the
MAX1978/MAX1979. Select the output inductors so that
the LC resonant frequency of the inductance and the
output capacitance is less than 1/5 the selected switch-
ing frequency. For example, 3.0µH and 1µF have a res-
onance at 92kHz, which is adequate for 500kHz
operation.
¡
where:
f
LC
= resonant frequency of output filter.
Capacitor Selection
Filter Capacitors
Decouple each power-supply input (V
DD
, PV
DD
1, and
PV
DD
2) with a 10µF ceramic capacitor close to the sup-
ply pins. If long supply lines separate the source sup-
ply from the MAX1978/MAX1979, or if the source
supply has high output impedance, place an additional
22µF to 100µF ceramic capacitor between the V
DD
power plane and power ground. Insufficient supply
bypassing can result in supply bounce and degraded
accuracy.
Compensation Capacitor
Include a compensation capacitor to ensure current-
power control-loop stability. Select the capacitor so that
the unity-gain bandwidth of the current-control loop is
less than or equal to 10% the resonant frequency of the
output filter:
where:
f
BW
= unity-gain bandwidth frequency
g
m
= loop transconductance, typically 100µA/V
C
COMP
= value of the compensation capacitor
R
TEC
= TEC series resistance
R
SENSE
= sense resistor
Setting Voltage and Current Limits
Consider TEC parameters to guarantee a robust
design. These parameters include maximum positive
current, maximum negative current, and the maximum
voltage allowed across the TEC. These limits should be
used to set MAXIP, MAXIN, and MAXV voltages.
Setting Max Positive and Negative TEC Current
MAXIP and MAXIN set the maximum positive and nega-
tive TEC currents, respectively. The default current limit
is ±150mV / R
SENSE
when MAXIP and MAXIN are con-
nected to REF. To set maximum limits other than the
defaults, connect a resistor-divider from REF to GND to
set V
MAXI_
. Use resistors in the 10kΩ to 100kΩ range.
V
MAXI_
is related to ITEC by the following equations:
V
MAXIP
= 10 (I
TECP(MAX)
 R
SENSE
)
V
MAXIN
= 10 (I
TECN(MAX)
 R
SENSE
)
where I
TECP(MAX)
is the maximum positive TEC current
and I
TECN(MAX)
is the maximum negative TEC current.
Positive TEC current occurs when CS is less than OS1:
I
TEC
 R
SENSE
= CS - OS1 when I
TEC
< 0.
I
TEC
 R
SENSE
= OS1 - CS when I
TEC
> 0.
C
g
f
R
RR
COMP
m
BW
SENSE
SENSE TEC
≥
⎛
⎝
⎜
⎞
⎠
⎟
×
×
×+
⎛
⎝
⎜
⎞
⎠
⎟
24
2π ( )
f
LC
LC
=
1
2π
Integrated Temperature
Controllers for Peltier Modules
MAX1978
MAX1979
x50
FB+
FB-
BFB-
AOUT
AIN+
AIN-
10kΩ
1µF
20kΩ
1%
80.6kΩ
1%
69.8kΩ
1%
105kΩ
1%
REF
REF
V
SETPOINT
Figure 3. Thermistor Voltage Monitor
MAX1978/MAX1979
Maxim Integrated
15