HCPL-4562 Datasheet, PDF (17/17 Pages) Agilent(Hewlett-Packard) – High Bandwidth, Analog/Video Optocouplers

Conversion from HCPL-4562 to
HCNW4562
In order to obtain similar circuit performance when
converting from the HCPL-4562 to the HCNW4562,
it is recommended to increase the Quiescent Input
Current, I_FQ, from 6 mA to 10 mA. If the application
circuit in Figure 4 is used, then potentiometer R4
should be adjusted appropriately.
Design Considerations of the
Application Circuit
The application circuit in Figure 4 incorporates
several features that help maximize the bandwidth
performance of the HCPL-4562/HCNW4562. Most
important of these features is peaked response of
the detector circuit that helps extend the frequency
range over which the voltage gain is relatively
constant. The number of gain stages, the overall
circuit topology, and the choice of DC bias points
are all consequences of the desire to maximize
bandwidth performance.
To use the circuit, first select R₁ to set V_E for the
desired LED quiescent current by:
I_FQ
=
–V–^E
R₄
≅
–––G––^V–V–^E––R–¹â€“⁰––
(∂I_PB/∂I_F) R₇R₉
(1)
For a constant value V_INp-p, the circuit topology
(adjusting the gain with R₄) preserves linearity by
keeping the modulation factor (MF) dependent only
on V_E.
i_Fpp--pp ≅ V_IN/R₄
(2)
–i–^Fp–^p-–^pp ≅ –i–^PB–^p–^p-–^p = –V–^IN–^p–^--pp–
(3)
I_FQ
I_PBQ
V_E
Modulation
Factor (MF):
–ⁱ–^F–^((pp–^--p–^{p) )}=
V_{––IN–pp–--–pp}
(4)
2 I_FQ 2 V_E
For a given G_V, V_E, and V_CC, DC output voltage will
vary only with h_FEX.
V_O
=
V_CC
–
V_BE4
–
–R–⁹–
R₁₀
[V_BEX
–
(I_PBQ
–
I_BXQ)
R₇]
(5)
Where:
I_PBQ
≅
–G––^V–V–^E–R–¹â€“⁰
R₇R₉
(6)
and,
I_BXQ
≅
–V–^C–^C–^–––2––V–^B–^E
R₆ h_FEX
(7)
Figure 15 shows the dependency of the DC output
voltage on h_FEX.
For 9 V < V_CC < 12 V, select the value of R₁₁ such
that
I_CQQ44 ≅
–V–^O– ≤
R₁₁
4.25 V
––––––
470 Ω
≤
9.0
mA
(8)
The voltage gain of the second stage (Q₃) is
approximately equal to:
–R–⁹–
R₁₀
*
–––––––––––––1––––––––––––
1
+
s
R₉
C_CQ3 +
––––1–––––
2π R′₁₁ f_T44
(9)
Increasing R′₁₁ (R′₁₁ includes the parallel
combination of R₁₁ and the load impedance) or
reducing R₉ (keeping R₉/R₁₀ ratio constant) will
improve the bandwidth.
If it is necessary to drive a low impedance load,
bandwidth may also be preserved by adding an
additional emitter following the buffer stage (Q₅ in
Figure 16), in which case R₁₁ can be increased to
set I_CQ4 ≅ 2 mA.
Finally, adjust R₄ to achieve the desired voltage
gain.
G_V
≅
–V–^O–^U–^T
V_IN
≅
–^∂–I–^PB–
∂I_F
–R––⁷–R–⁹–
R₄R₁₀
(10)
where typically –^∂–I–^P–^B = 0.0032
∂I_F
Definition:
G_V = Voltage Gain
I_FQ = Quiescent LED forward current
i_Fp-p = Peak-to-peak small signal LED forward
current
V_INp-p = Peak-to-peak small signal input voltage
i_PBp-p = Peak-to-peak small signal
base photo current
I_PBQ = Quiescent base photo current
V_BEX = Base-Emitter voltage of HCPL-4562/
HCNW4562 transistor
I_BXQ = Quiescent base current of HCPL-4562/
HCNW4562 transistor
h_FEX = Current Gain (I_C/I_B) of HCPL-4562/
HCNW4562 transistor
V_E = Voltage across emitter degeneration
resistor R₄
f_T4 = Unity gain frequency of Q₅
C_{CQ 3} = Effective capacitance from collector of Q₃
to ground
1-401