MAX8521 datasheet(14/18 Pages) MAXIM

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The output common-mode ripple voltage can be calcu-

lated as follows:

VRIPPLEpk-pk = LIR x ITEC(MAX) (ESR + 1/8 x C x fs)

A 1µF ceramic capacitor with ESR of 10 m

Ω with LIR =

12% and ITEC(MAX) = 1.5A will result in VRIPPLE(P-P) of

24.3mV. For size-constraint application, the capacitor

can be made smaller at the expense of higher ripple

voltage. However, the capacitance must be high

enough so that the LC resonant frequency is less than

1/5 the switching frequency:

where f is the resonant frequency of the output filter.

Differential Mode Filter Capacitor

The differential-mode filter capacitor (C5 in Figure 1) is

used to bypass differential ripple current through the

TEC as the result of unequal duty cycle of each output.

This happens when the TEC current is not at zero. As

TEC current increases from zero, both outputs move

away from the 50% duty-cycle point complementarily.

The common-mode ripple decreases, but the differential

ripple does not cancel perfectly, and there will be a

resulting differential ripple. The maximum value happens

when one output is at 75% duty cycle and the other is at

25% duty cycle. At this operating point, the differential

ripple is equal to 1/2 of the maximum common-mode rip-

ple. The TEC ripple current determines the TEC perfor-

mance, because the maximum temperature differential

that can be created between the terminals of the TEC

depends on the ratio of ripple current and DC current.

The lower the ripple current, the closer to the ideal maxi-

mum. The differential-mode capacitor provides a low-

impedance path for the ripple current to flow, so that the

TEC ripple current is greatly reduced. The TEC ripple

current then can be calculated as follows:

ITEC(RIPPLE) = (0.5 x LIR x ITEC(MAX)) x (ZC5)/(RTEC

+ RSENSE + ZC5)

where ZC5 is the impedance of C5 at twice the switching

frequency, RTEC is the TEC equivalent resistance, and

RSENSE is the current-sense resistor.

Decoupling Capacitor Selection

Decouple each power supply input (VDD, PVDD1,

PVDD2) with a 1µF ceramic capacitor close to the sup-

ply pins. In applications with long distances between

the source supply and the MAX8520/MAX8521, addi-

tional bypassing may be needed to stabilize the input

supply. In such cases, a low-ESR electrolytic or ceramic

capacitor of 100µF or more at VDD is sufficient.

Compensation Capacitor

A compensation capacitor is needed to ensure current-

control-loop stability (see Figure 3). 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:

fBW = Unity-gain bandwidth frequency, less than or

equal to 10% the output filter resonant frequency

gm = Loop transconductance, typically 100µA/V

CCOMP = Value of the compensation capacitor

RTEC = TEC series resistance, use the minimum resis-

tance value

RSENSE = Sense resistor

Setting Voltage and Current Limits

Certain TEC parameters must be considered to guarantee

a robust design. These include maximum positive current,

maximum negative current, and the maximum voltage

allowed across the TEC. These limits should be used to

set the 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/RSENSE 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 VMAXI_. Use resistors in the 10kΩ to 100kΩ range.

VMAXI_ is related to ITEC by the following equations:

VMAXIP = 10(ITECP(MAX) RSENSE)

VMAXIN = 10(ITECN(MAX) RSENSE)

where ITECP(MAX) is the maximum positive TEC current

and ITECN(MAX) is the negative maximum TEC current.

Positive TEC current occurs when CS is less than OS1:

ITEC x RSENSE = OS1 - CS

when ITEC > 0.

ITEC RSENSE = CS - OS1

when ITEC < 0.

COMP

SENSE

TEC

≥

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⎝

⎜

⎞

⎠

⎟ ×

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π()

Smallest TEC Power Drivers for Optical

Modules

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