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LMH6525 Fiches technique(PDF) 11 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor. Click here to check the latest version.
No de pièce LMH6525
Description  Four-hannel Laser Diode Driver with Dual Output
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Fabricant  NSC [National Semiconductor (TI)]
Site Internet  http://www.national.com
Logo NSC - National Semiconductor (TI)

LMH6525 Fiches technique(HTML) 11 Page - National Semiconductor (TI)

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Application Section
CIRCUIT DESCRIPTION
General & Spec
The LMH6525/6526 is a 4-channel-input, dual-output laser
driver. The dual outputs are meant to drive two different laser
diodes, one for CD reading and writing and one for DVD
reading and writing. The part has an oscillator that can be set
for both amplitude and frequency. The oscillator has four
input pins for setting both the amplitude and frequency by
connecting external resistors to ground. The part operates at
5V and is capable to deliver a minimum total output current
of 500 mA.
INPUTS
Current-Setting Inputs
The 4 input channels are transconductance-type inputs. This
means the output current of the channel is proportional to the
current (not voltage) sourced into the input pin. That is why
these pins are designated by the letter “I” to indicate the
current input nature of the pin. The read channel current-
setting pin is “I
R”, the Channel 2 current-setting pin is “I2”
and so on. Using a transconductance-type input eliminates
the high-impedance inputs associated with a voltage input
amplifier. The lower input impedances of the input nodes
lowers the susceptibility of the part to EMI/RFI. The Read
Channel (I
R) and Channel 3 (I3) and 4 (I4) current-setting
inputs have a gain of 150. The Channel 2 input (I2) has a
current gain of 300. Sourcing one milliampere into the pins
I
R, I3 or I4, will result in 150 mA at the output for each
Channel, while 1 mA into I2 will result in 300 mA at the output
for Channel 2. These currents of 150 mA and 300 mA are the
maximum allowable currents per channel. The total allow-
able output current from all the channels operating together
exceeds 500 mA.
Channel Enable Inputs
Each of the four channels has one (read) or two enable
inputs that allow the channel to be turned on or off. The read
channel enable (ENR) is a single-ended TTL/CMOS com-
patible input. A single-ended signal is adequate for this chan-
nel because the read channel is generally enabled the entire
time the drive is reading or writing. The three write/erase
channels need to be operated much faster so these channel
enables are LVDS (Low Voltage Differential Signal) inputs.
Each channel has two inputs, such as EN2 and EN2B.
Following the standard an LVDS output consists of a current
source of 3.5 mA, and this current produces across the
internal termination resistor of 100
Ω in the LMH6525 or
LMH6526 a voltage of 350 mV. The polarity of the current
through the resistor can change very quickly thus switching
the channel current on or off. The bias level of the LVDS
signal is about 1.2V, so the operating levels are 175 mV
above and below this bias level. The ENxB inputs act as the
not input so if the other input is at logical ‘1’ state and the not
input at ‘0’ state the channel is activated. The internal 100
resister provides a proper termination for the LVDS signals,
saving space and simplifying layout and assembly.
Control Inputs
There are two other control inputs (next to the oscillator
enable which is covered in the next section). There are the
global chip Enable and output select pin SELA or SELB.
Setting the Enable pin to a level above 2V will enable the
part. This means the supply current raises from sleep mode
value to the normal operating values. The SELA or SELB
input (TTL/ CMOS levels) controls which output is active.
When at logical ‘1’ state the output indicated by it’s name is
active. The mode of this pin also controls the oscillator
circuitry which means that the appropriate setting resistors
become active as described in the next section.
Oscillator Inputs
The oscillator section can be switched on or off by a LVDS
signal for the LMH6525 and by a TTL/ CMOS signal for the
LMH6526. When switched on the oscillator will modulate the
output current. The settings of the frequency and amplitude
are done by 4 resistors, two for every channel. R
FA and RFB
pins set the oscillator frequency for the A and B outputs
respectively. The R
AA and RAB pins set the oscillator ampli-
tude for the A and B channels respectively. These 4 inputs
work by having current drawn out of the pin by a setting
resistor or potentiometer. The frequency and amplitude in-
crease by decreasing setting resistor value. There are two
charts in the Typical Performance Characteristics section
that relates the setting resistor value to the resulting fre-
quency or amplitude. Normally the settings for the frequency
and amplitude are done by connecting the pin via a resistor
to ground. If needed to program this settings it is possible to
connect these R
Fx and RAx pins via a current limiting resistor
to the output of an op amp or DAC. When using such a
circuitry the output can be held at a negative voltage, which
means even if the channel pins R
Fx and RAx are not se-
lected, current is drawn from the pin. This is only true when
the negative voltage has such a value that the internal
transistors connected to the pin will conduct. This will influ-
ence the settings of the active pins R
Fx and RAx. Due to this
effect it is recommended, when using a negative voltage
lower as -0.5V, to disable this voltage simultaneously with
the channel.
OUTPUT
The outputs can source currents in excess of 600 mA. The
output pins have been designed to have minimal series
inductance in order to minimize current overshoot on fast
pulses. The outputs have a saturation voltage of about 1V.
The table below shows the typical output saturation Voltages
into a 5
Ω load at various supply voltages.
TABLE 1. Output Saturation
Supply Voltage
(V)
Maximum
Output (mA) 5
Saturation
Voltage (V)
4.5V
700
0.8
5.0V
777
0.89
5.5V
846
1.02
As can be seen, even with a 4.5V supply voltage the part can
deliver 700 mA while the saturation voltage is at 0.8V. This
means the output voltage of the part can be at maximum
700e-3*5 = 3.5V. With a saturated output voltage (see Figure
1) of 0.8V the voltage on the supply pin of the part is 4.3V.
The used supply voltage is 4.5V so there is a supply voltage
loss of 0.2V over the supply line resistance, but nevertheless
the part can drive laser diodes with a forward voltage up to
3.5V with currents over 500 mA. When operating at 5.5V the
part can deliver currents over 800 mA. In this case the output
at the anode of the laser diode is 846e-3*5 = 4.23V, com-
bined with the saturated output voltage of 1.02V the supply
www.national.com
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