Wavelength Electronics QCL OEM Series, Datasheet And Operating Manual

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www.teamWavelength.com

15

QCL OEM SERIES LOW-NOISE DRIVER

SOFT-CLAMP CURRENT LIMIT

The QCL driver employs a soft-clamping current limit that 

begins to act at a current lower than the calculated absolute 

current limit.  We recommend that setpoint is below 85% of 

the current limit, because the noise level introduced into the 

QCL rises when operating close to the current limit.

Normally,  the  LIM_MON  voltage  is  calculated  using  the 

transfer function shown in 

Table 3

. Using the transfer 

function to set the LIM_MON voltage sets the 

absolute

current limit, which is the current that will not be exceeded 

under any circumstance.

Because of the soft-clamp circuit, the actual current where 

the limit circuit begins to act is 

offset 

from the calculated  

absolute current limit. The Limit Offset Current is calculated 

as follows, where X

LIM _ MON 

is the current limit transfer function 

for the QCL driver model you are using (see 

Table 3

):

Limit Offset Current = X

LIM_MON

 * (V

LIM_MON

 – 0.4)   (A)

If  the  output  current  is  between  the  Limit  Offset  Current 

and the Absolute Current Limit, the output response to the 

analog input setpoint is nonlinear. The result is that the drive 

current will asymptotically approach the absolute current 

limit,  and  will  never  exceed  it  even  if  the  analog  input 

setpoint voltage far exceeds the current limit voltage.

Figure 9

 illustrates the behavior of the soft-clamp current 

limit. In this case the analog input voltage setpoint signal is 

a triangle wave, with the amplitude set so that the driver will 

attempt to overdrive the current limit.

Figure 9.   Soft Clamping Current Limit Behavior

REPLACE THE INTERNAL FUSES

The QCL driver is equipped with two internal fuses. If the 

QCL driver will not power on and you have verified that the 

power supplies are wired and working properly, check and 

replace these two fuses. See 

Figure 8

 for the fuse location. 

The fuses are 5 A, 5 x 20 mm, SLO-BLO fuses.

In  an  ESD-safe  environment,  remove  the  eight  Phillips 

screws around the base of the unit. Then remove the 

baseplate—the fit is intentionally tight and will require some 

effort.

LOCAL ↔ REMOTE

PWR-
PWR+
EN-
EN+

Figure 8.   Location of Internal Fuses

Replace  the  fuses,  then  reinstall  the  chassis  cover  and 

tighten the eight screws.