MAX8520 View Datasheet(PDF) - Maxim Integrated

Part Name

Description

Manufacturer

MAX8520

Smallest TEC Power Drivers for Optical Modules

Maxim Integrated 

Smallest TEC Power Drivers for Optical

Modules

Detailed Description

The MAX8520/MAX8521 TEC drivers consist of two

switching buck regulators that operate together to

directly control the TEC current. This configuration cre-

ates a differential voltage across the TEC, allowing bi-

directional TEC current for controlled cooling and

heating. Controlled cooling and heating allow accurate

TEC temperature control to within 0.01°C. The voltage at

CTLI directly sets the TEC current. An external thermal-

control loop is typically used to drive CTLI. Figures 1

and 2 show examples of the thermal-control-loop circuit.

Ripple Cancellation

Switching regulators like those used in the MAX8520/

MAX8521 inherently create ripple voltage on the output.

The dual regulators in the MAX8520/MAX8521 switch

in-phase and provide complementary in-phase duty

cycles so ripple waveforms at the TEC are greatly

reduced. This feature suppresses ripple currents and

electrical noise at the TEC to prevent interference with

the laser diode.

Switching Frequency

For the MAX8521, FREQ sets the switching frequency

of the internal oscillator. With FREQ = GND, the oscilla-

tor frequency is set to 500kHz. The oscillator frequency

is 1MHz when FREQ = VDD.

For the MAX8520, connect a resistor (REXT in Figure 2)

from FREQ to GND. Choose REXT = 60kΩ for 1MHz

operation, and REXT = 150kΩ for 500KHz operation. For

any intermediary frequency between 500kHz and

1MHz, use the following equation to find the value of

REXT value needed for VDD = 5V:

REXT

=

90

×

⎛

⎝⎜

1

fs

−

1⎞

3 ⎠⎟

where REXT is the resistance given in kΩ, and fs is the

desired frequency given in MHz. Note that for VDD <

5V, the frequency is reduced slightly, to the extent of

about 7% when VDD reaches 3V. This should be taken

into consideration when selecting the value for REXT at

known supply voltage.

Voltage and Current-Limit Setting

Both the MAX8520 and MAX8521 provide control of the

maximum differential TEC voltage. Applying a voltage

to MAXV limits the maximum voltage across the TEC.

The voltage at MAXIP and MAXIN sets the maximum

positive and negative current through the TEC. These

current limits can be independently controlled.

Table 1. TEC Connection for Figure 1

TEC Connection

Heating Mode

Cooling Mode

Thermistor

PTC

NTC

Table 2. TEC Connection for Figure 2

TEC Connection

Heating Mode

Cooling Mode

Thermistor

NTC

PTC

Current Monitor Output

ITEC provides a voltage output proportional to the TEC

current (ITEC). See the Functional Diagram for more

detail:

VITEC = 1.5V +(8  (VOS1-VCS))

Reference Output

The MAX8520/MAX8521 include an on-chip voltage ref-

erence. The 1.50V reference is accurate to 1% over

temperature. Bypass REF with 0.1µF to GND. REF can

be used to bias an external thermistor for temperature

sensing as shown in Figures 1 and 2.

Thermal and Fault-Current Protection

The MAX8520/MAX8521 provide fault-current protec-

tion in either FETs by turning off both high-side and

low-side FETs when the peak current exceeds 3A in

either FETs. In addition, thermal-overload protection

limits the total power dissipation in the chip. When the

device’s die junction temperature exceeds +165°C, an

on-chip thermal sensor shuts down the device. The

thermal sensor turns the device on again after the junc-

tion temperature cools down by +15°C.

Design Procedures

Duty-Cycle Range Selection

By design, the MAX8520/MAX8521 are capable of

operating from 0% to 100% duty cycle, allowing both

LX outputs to enter dropout. However, as the LX pulse

width narrows, accurate duty-cycle control becomes

difficult. This can result in a low-frequency noise

appearing at the TEC output (typically in the 20kHz to

50kHz range). While this noise is typically filtered out by

the low thermal-loop bandwidth, for best result, operate

the PWM with a pulse width greater than 200ns. For

500kHz application, the recommended duty-cycle

range is from 10% to 90%. For 1MHz application, it is

from 20% to 80%.

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