TCA5600 Просмотр технического описания (PDF) - Motorola => Freescale
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TCA5600 Datasheet PDF : 12 Pages
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TCA5600 TCF5600
APPLICATIONS INFORMATION
(See Figure 18)
Voltage Reference (Vref)
The voltage reference Vref is based upon a highly stable
bandgap voltage reference and is accessible on Pin 5 for
additional tasks. This circuit part has its own supply
connection on Pin 3 and is, therefore, able to operate in
standby mode. The RC network R3, C6 improves the ripple
rejection on both regulators.
DC/DC Converter
The dc/dc converter performs according to the flyback
principle and does not need a time base circuit. The
maximum coil current is well defined by means of the current
sensing resistor R1 under all working conditions (startup
phase, circuit overload, wide supply voltage range and
extreme load current change). Figure 13 shows the Simplified
Converter Schematic.
Figure 13. Simplified Converter Schematic
VCC2
Control Feedback
L1
300µH
10
Figure 14. Voltage and Current Waveform
Coil
Voltage
VCC2
ÉÉÉÉÉÉÉÉÉÉÉÉÉ on the Coil (not to scale)
t
Coil
Current
ILpeak
t
t1
t2
The coil charging time t1 is found using Equation (4):
t1 =
(1 +
1
1
α
)⋅f
(4)
V12(H)
V12(L)
+
Comp
VF
Prog.
Regulator
VCE(sat) 9
12
R1
0.68Ω
C2 +
100 µF
C7
0.22µF
8
Vout2
R4
7
85K
C3
R5
47nF
10k
[f : minimum oscillation frequency which should be chosen
above the audio frequency band (e.g. 20 kHz)]
Knowing the dc output current Iout2 of the programmable
regulator, the peak coil current IL(peak) can now be calculated:
IL(peak) = 2 ⋅ (Iout2) (1 + α)
(5)
The coil inductance L1 of the nonsaturated coil is given by
Equation (6):
A simplified method on “how to calculate the coil inductance”
is given below. The operation point at minimum supply
voltage (VCC2) and max. output current (Iout2) for a fixed
output voltage (Vout2) determines the coil data. Figure 14
shows the typical voltage and current waveforms on the coil
L1 (coil losses neglected).
Equations (1) and (2) yield the respective coil voltage
VL – and VL + (see Figure 14):
VL+ = Vout2 + ∆V(Pin 9 – Pin 8) + VF – VCC2 (1)
VL– = VCC2 – VCE(sat) – V12(H)
(2)
[∆V(Pin 9 – Pin 8): input/output voltage drop of the regulator,
2.5 V typical]
[VF, VCE(sat), V12(H): see Electrical Characteristics Table]
The time ratio α for the charging time to dumping time is
defined by Equation (3):
L1 = IL(pt1eak)(VL–)
(6)
The formula (6a) yields the current sensing resistor R1 for a
defined peak coil current IL(peak):
R1
=
V12(H)
IL(peak)
(6a)
In order to limit the by–pass current through capacitor C7
during the energy dumping phase the value C2>>C7 should
be implemented.
For all other operation conditions, the feedback signal from
the programmable voltage regulator controls the activity of
the converter.
α=
t1
t2
=
VL+
VL–
(3)
MOTOROLA ANALOG IC DEVICE DATA
9
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