HIP9022AM Intersil Corporation, HIP9022AM Datasheet - Page 8

• Manufacturer Part Number: HIP9022AM
• Description: Dual High Speed Laser Driver
• Manufacturer: Intersil Corporation

Circuit Block Descriptions
Laser Drive Circuitry
In Figure 3, the gate of the external current source Power
FET, Q2 is driven via the Operational Transconductance
Amplifier (OTA), A2 on the IC. The voltage on the current
sense resistor, R
monitored by a X10 gain of the feedback amplifier, A1. The
stability of the current loop is established with an external
0.1 F capacitor to ground at the gate of the Power FET. The
sampled voltage range is 0 to 0.5V when the proper value of
sense resistor, R
OTA, A2 compares the X10 gain signal to a 0 to 5V reference
signal from an on-chip Sample and Hold (First S/H) circuit.
The Q2 drain current (Laser Drive current), I
where V
voltage reference signal.
The S/H reference for the Laser Drive Current current is
updated with other multiplexed S/H circuits from a serial bus
and an off-chip D/A converter. Laser constant current is fully
controllable by the multiplex analog S/H bus, allowing accurate
calibration of the laser output and corrections as the laser ages.
In Figure 1, the laser drive current from Q2 is digitally switched
to either flow through or is shunted around the laser diode by
switching the external Shunt Power FET, Q3 on or off. The gate
of the Shunt FET is switched between two voltages (Upper and
Lower) which are provide by 2nd and 3rd S/H circuits. These
Shunt FET gate drive levels are fully programmable via the
multiplexed analog S/H bus. By adjusting these levels to
account for the laser power supply, the Shunt FET threshold
and channel resistance; minimum Shunt FET gate drive power
levels can be established. The Upper and Lower gate voltage
driver circuits are two high current OTA amplifiers with two filter
capacitors. The upper voltage is programmable in the 0V to
V
programmable in the range of -4V to V
accomplished by an optional on-chip voltage inverter circuit.
The input to amplifier A3 is either direct from the S/H input or
inverted by amplifier A5.
The maximum laser on-off switching speeds are dependent
on the selection of Shunt FETs. A Harris dual
complementary MOSFET, RF3V49092 or RF3S49092 has
been designed specifically for this application. With the
constant current set at 0.8A, a typical laser switching speed
of 20ns has been measured.
Thermal Compensation
A 4th S/H circuit is used to set the amplitude of an optional
thermal compensation signal which can be used to
modulate the constant laser current source as a two pole
filtered effect of the laser on-off data. This feature may be
disabled when it is not required. This circuit is designed to
compensate for the temperature variations in the laser as
the laser is turned on and off. The bypass capacitors at the
Thermal Comparator (CTCx-10K, CTCx-27K) represent the
respective poles for the filter.
I
DL
CC
=
range at the input of amplifier A4. The lower voltage is
-------------------------
V
R
IN(DL)
IN DL
S
10
is the programmed V
S
S
in the source of the Power FET is
is chosen (typically 0.25 for 2A). The
4-8
IN
CC
for the First S/H
. The -4V extension is
DL
is:
(EQ. 1)
HIP9022
A laser cools after it has been off for a period of time and is
more efficient when it is turned-on. Compensation for the
increased efficiency is made by slightly reducing the
current level of the constant current source FET. The level
will be reduced by a programmable amount of 0 to 5% of
full scale. The programmable amount is fixed by the level of
compensation to S/H addresses 7 and 8 (see Table 1). The
percent of modulation (change) in drive current is
calculated as follows:
For example, if we control the Laser Drive current with 2V
programmed with address 1 and 2 for the First S/H’s, given
that V
I
If 2V is programmed to addresses 7 and 8 as Thermal
Compensation, V
Mod.% = (2/2) x 5% = 5%.
In Figure 3, the correction is applied from the output of the
Thermal Compensation circuit (where the current is
2V/20k = 0.1mA) to the input of amplifier A2. The 0.1mA is
forced into the 1k resistor (and the low Z output of A1) to
increase the voltage at the inverting input of A2 by 0.1V or 5%
of the +V
by the 0 to V
Input Data
Modulation
DL
12k
30k
FIGURE 3. LASER CONSTANT CURRENT SOURCE DRIVER
= V
IN(DL)
IN(DL)
IN(DL)
S/H SYS
CC
= 2V and R
/(R
WITH OVER CURRENT DETECTION AND
THERMAL COMPENSATION
input (2V) to A2. The modulation input is limited
=
input range of S/H maximum V
1k
S
IN(TC)
V
---------------------
V
THERMAL
CURRENT
x 10) = 2/(0.25 x 10) = 0.8A.
VOLT. TO
+
-
COMP.
IN TC
IN DL
A2
20k
X10
+
-
A1
CURRENT
VOLT. TO
S
for the 4th S/H’s, then,
27k
COMP.
+
O. C.
-
= 0.25 . Then, from EQ. 1,
5
10k
55
54
53
49
51
52
OC2
CC2
XTEN2+
XTEN2-
CTC2-27K
CTC2-10K
0.1 F
0.02 F
IN
.
0.1 F
0.25
Q2
(EQ. 2)
R
S