V-Tec C5-AT25/D4 Instrukcja Użytkownika Pobierz pdf (Strona 15)

Take care not to exceed the positive or negative cur-

rent limit on the TEC. Refer to the manufacturer’s data

sheet for these limits.

Setting Max TEC Voltage

Apply a voltage to the MAXV pin to control the maximum

differential TEC voltage. MAXV can vary from 0 to REF.

The voltage across the TEC is four times V

MAXV

and

can be positive or negative:

OS1

- V

OS2

| = 4 x V

MAXV

or V

, whichever is lower

Set V

MAXV

with a resistor-divider between REF and

GND using resistors from 10kΩ to 100kΩ. V

MAXV

can

vary from 0 to REF.

Control Inputs/Outputs

Output Current Control

The voltage at CTLI directly sets the TEC current. CTLI

is typically driven from the output of a temperature con-

trol loop. The transfer function relating current through

the TEC (I

TEC

) and V

CTLI

is given by:

TEC

= (V

CTLI

- V

REF

) / (10 ✕ R

SENSE

)

where V

REF

is 1.50V and:

ITEC = (V

OS1

- V

) / R

SENSE

CTLI is centered around REF (1.50V). I

TEC

is zero when

CTLI = 1.50V. When V

CTLI

> 1.50V, the current flow is

from OS2 to OS1. The voltages on the pins relate as

follows:

OS2

> V

OS1

> V

The opposite applies when V

CTLI

< 1.50V current flows

from OS1 to OS2:

OS2

< V

OS1

< V

Shutdown Control

The MAX8520/MAX8521 can be placed in a power-saving

shutdown mode by driving SHDN low. When the

MAX8520/MAX8521 are shut down, the TEC is off (OS1

and OS2 decay to GND) and supply current is reduced to

2mA (typ).

ITEC Output

ITEC is a status output that provides a voltage proportional

to the actual TEC current. V

ITEC

= REF when TEC current

is zero. The transfer function for the ITEC output is:

ITEC

= 1.50 + 8 ✕ (V

OS1

– V

)

Use ITEC to monitor the cooling or heating current

through the TEC. For stability, keep the load capaci-

tance on ITEC to less than 150pF.

Applications Information

The MAX8520/MAX8521 typically drive a TEC inside a

thermal-control loop. TEC drive polarity and power are

regulated based on temperature information read from a

thermistor or other temperature-measuring device to

maintain a stable control temperature. Temperature sta-

bility of ±0.01°C can be achieved with carefully selected

external components.

There are numerous ways to implement the thermal loop.

Figures 1 and 2 show designs that employ precision op

amps, along with a DAC or potentiometer to set the con-

trol temperature. The loop may also be implemented dig-

itally, using a precision A/D to read the thermistor or

other temperature sensor, a microcontroller to implement

the control algorithm, and a DAC (or filtered-PWM signal)

to send the appropriate signal to the MAX8520/MAX8521

CTLI input. Regardless of the form taken by the thermal-

control circuitry, all designs are similar in that they read

temperature, compare it to a set-point signal, and then

send an error-correcting signal to the MAX8520/

MAX8521 that moves the temperature in the appropriate

direction.

PC Board Layout and Routing

High switching frequencies and large peak currents

make PC board layout a very important part of design.

Good design minimizes excessive EMI and voltage

gradients in the ground plane, both of which can result

in instability or regulation errors. Follow these guide-

lines for good PC board layout:

1) Place decoupling capacitors as close to the IC pins

as possible.

2) Keep a separate power ground plane, which is con-

nected to PGND1 and PGND2. PV

1, PV

PGND1, and PGND2 are noisy points. Connect

decoupling capacitors from PV

s to PGNDs as

directly as possible. Output capacitors C2 and C7

returns are connected to PGND plane.

3) Connect a decoupling capacitor from V

to GND.

Connect GND to a signal ground plane (separate

from the power ground plane above). Other V

decoupling capacitors (such as the input capacitor)

need to be connected to the PGND plane.

4) Connect GND and PGND_ pins together at a single

point, as close as possible to the chip.

5) Keep the power loop, which consists of input

capacitors, output inductors, and capacitors, as

compact and small as possible.

MAX8520/MAX8521

Smallest TEC Power Drivers for

Optical Modules

______________________________________________________________________________________ 15

1 2 ... 10 11 12 13 14 15 16 17 18 19

MAX8520/MAX8521 1
Optical Modules 1

Komentarze do niniejszej Instrukcji

Brak uwag

V-Tec C5-AT25/D4 Instrukcja Użytkownika Strona 15

Komentarze do niniejszej Instrukcji