IC-HTG datasheet(13/40 Pages) ICHAUS

13 / 40 page

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iC-HTG

POWER CW LASER DIODE DRIVER

Rev A1, Page 13/40

CONTROL MODES AND LASER DIODE/LED TYPES

The iC-HTG has no integrated driver transistor. Ex-

ternal power transistor (P-channel or N-channel) can

be driven, connecting the gate of the transistor to pins

VRP or VRN. Another possibility is to control a DC/DC

converter directly, providing the best power efficiency.

The on-chip regulator has an offset-cancelling feature,

which can be controlled with register bit EOC (see table

below).

EOC

Addr. 0x10; bit 4

R/W 1

Regulator offset compensation disabled

Regulator offset compensation enabled

Table 9: Enable offset compensation

The iC-HTG can be configured in two main control

modes: laser power control (APC) and laser current

control (ACC). The control mode is selected by setting

EACC register bit.

EACC

Addr. 0x10; bit 0

R/W 0

APC mode enabled (laser power control)

ACC mode enabled (laser current control)

Table 10: Select APC or ACC

CI capacitor

For most applications, a CI capacitor of at least 220 pF

is recommended in order to ensure the stability of the

regulation. The exact amount of capacitance needed

depends on many factors such as PCB layout, output

transistor, laser diode, and current range. The CI ca-

pacitor can be used in APC and ACC.

ACC mode

In ACC mode, the laser current is controlled. ACC

mode is selected by setting EACC register bit to ’1’. In

this mode, an external shunt resistor (RMC) is used

to monitor the laser diode current. The voltage drop

across this shunt resistor serves as feedback to the

laser current control loop. To insert the current voltage

drop in the control loop, both pins of the shunt resistor

must be connected to the pins MCH and MCL. The volt-

age drop MCx = V(MCH)-V(MCL) needs to be positive.

This voltage drop is internally amplified by a factor of

2, 5, 10 or 50. This factor can be selected using the

so that the value of the voltage drop multiplied by the

amplification factor does not exceed the higher value

of the reference generated with the 10-bit logarithmic

D/A converter. This value is typically 1 V. More about

this is on page 20

CGAIN(1:0)

Addr. 0x15; bit 7:6

R/W 00

Amplification set to x2

Amplification set to x5

Amplification set to x10

Amplification set to x50

Table 11: MCx voltage drop amplification

Depending on the output configuration and the position

of RMC in the current path, the voltage between pins

MCH and MCL will be in between 0 and 5 V or in the

range from VBL - 5 V to VBL. The MCx voltage range

can be set with the register bit MCVR.

MCVR

Addr. 0x13; bit 2

R/W 0

MCx Voltage Range is 0 to 5V

MCx Voltage Range is VBL-5V to VBL

Table 12: MCx voltage range

In ACC mode, the register EPNNP is ignored.

Some examples of connecting RMC using N-channel

and P-channel transistor are shown in Figure 3.

CURRENTCHANNEL

LASERREGULATOR

POWERCHANNEL

x5,x10,x50

..VBL-5V

ANIN

RMC

DCO

RMC

MCH

DCO

MCH

ANIN

MCL

VRN

VRP

VRN

VRP

8Bit

VBL

..5V

255

CIL

VRN

VBL

DCO

VRN

DCO

ANIN

MCL

VBL

VRP

MCH

ANIN

MCL

CIL

MCH

VRP

CIL

VBL

Figure 3: Operation in ACC mode with N-channel

or P-channel output transistor. EACC=1,

MCVR=0 and MCVR=1

By using the output transistor like in Figure 3, as a

source follower, the system has increased stability, and

the CI capacitor can be smaller. This is recommended

for analog modulation with pin MOD. There are many

other configurations possible depending on laser type,

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