ISL58781 Datasheet, PDF (10/15 Pages) Intersil Corporation – Laser Diode Driver with Serial Control and Write Current DAC

ISL58781
Applications Information
I_OUT
The data sheet values for oscillator current, and write
current are based on an R_SET of 620 when PMAX and
WriteDAC are both set to full scale. The user may choose
R_SET to match the output current needs of the
application.
The PMAX DAC is biased by I_RSET (= V_RSET/R_SET). See
the “Typical Performance Curves” on page 10.
The write channel output capability for a typical part is
shown in Figure 1. The amount of I_OUT will be limited by
the available headroom voltage at the I_OUTx pins.
A four input DAC (Reg 1-0A bits 3, 2, 1, 0) can be used
to control the amount of RC snubbing applied to the
outputs.
Read current may be controlled by either the Read DAC
or the IAPC input. When set by PREAD, I_READ is limited
to the data sheet value, whereas the I_APC input will allow
a significantly higher value to be obtained. The ReadDAC
and I_APC currents sum together.
Glitches could occur if two or three WEN lines are
changed simultaneously, and the propagation delay is
different for the two lines between the inner circuits of
the controller and the inner circuits of the LDD. Because
the WEN lines are encoded, the selected write current will
be correct before the change in code, and again after the
code changes. But some other output could result
momentarily if the propagation delays are not matched.
The skew detector detects the first rising edge at the
LVDS outputs.
F_OSC
Both F_OSC and R_FREQ may be chosen to accommodate
the desired range or operating point of the HFMFREQ
Typical Performance Curves
1200
1000
800
I_OUT2 vs V_HEAD vs PMAX CODE
(REG 1-21 = 88h) (VSO = 4.5V)
(R_SET = 620)
600
3FFh
300h
200h
400
100h
200
80h
40h
0 _{0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5}
V_HEADROOM (V)
FIGURE 1. I_OUT WRITE CURRENT vs V_SD vs
I_{PMAX_BIAS}
DAC. Although F_OSC is relatively linear with DAC code,
monotonicity is not guaranteed (see Figure 5). Under
extreme conditions, where VSO = 4.5V and temperature
is +125°C, the HFM frequency is only capable of 900MHz
(see Figure 6).
The oscillator may be turned on either by the OSCEN
lines or by the WEN code selected. The particular code
that selects the oscillator is under program control. The
Pcool function is only available through the program
control and WEN selection.
The WEB enables write current. Thus WEN code 000 may
also select a write current.
Power
The main power consumption is caused by the headroom
voltage across the output stage (V_SO - V_IOUT) xI_OUT. For
I_OUT1 and I_OUT2, the V_SO can be reduced below 5.0V
(but above V_SO-GOOD), as long as sufficient headroom
voltage is available to obtain the desired output current.
For the blue outputs, the built in ADC can be used to
obtain the output voltage, which is also the headroom
voltage. The HFM oscillator power consumption will
increase with increasing frequency and amplitude (see
Figure 7).
Note that in the QFN package, the die is mounted directly
on the thermal pad. This provides a very low thermal
resistance Junction to thermal pad of just a few °C/W.
The problem is in moving the heat from the thermal pad
to some other heat sink.
Figure 10 shows that when mounted well on a 4-layer
PCB with 3 ground plane layers, and an area of
10cmx10cm, the _JA is +37°C/W. The typical application
will not afford this good of a heat sink.
500
400
3FFh
300h
300
200h
200
100h
100
40h
00
1
2
3
4
5
6
V_HEADROOM (V)
FIGURE 2. I_BLUE vs P_MAX vs V_HEADROOM
(V_SO = 5.0V) (R_SET = 620)
(1-21 = FFh), (R_LOAD = 10)
001h
7
8
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FN6909.3
December 3, 2015