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LMH6525SPNOPB bảng dữ liệu(PDF) 11 Page - National Semiconductor (TI) |
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LMH6525SPNOPB bảng dữ liệu(HTML) 11 Page - National Semiconductor (TI) |
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11 / 15 page  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 Ω 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 Ω 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 11 |
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