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MC33261 Datasheet PDF : 12 Pages
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MC34261 MC33261
Timer
A watchdog timer function was added to the IC to eliminate
the need for an external oscillator when used in stand alone
applications. The Timer provides a means to automatically
start or restart the preconverter if the Drive Output has been
off for more than 400 µs after the inductor current reaches
zero.
Undervoltage Lockout
An Undervoltage Lockout comparator guarantees that the
IC is fully functional before enabling the output stage. The
positive power supply terminal (VCC) is monitored by the
UVLO comparator with the upper threshold set at 10 V and
the lower threshold at 8.0 V (Figure 14). In the standby mode,
with VCC at 7.0 V, the required supply current is less than
0.5 mA (Figure 13). This hysteresis and low startup current
allow the implementation of efficient bootstrap startup
techniques, making these devices ideally suited for wide
input range off line preconverter applications. An internal 36
V clamp has been added from VCC to ground to protect the IC
and capacitor C5 from an overvoltage condition. This feature
is desirable if external circuitry is used to delay the startup of
the preconverter.
Output
The MC34261/MC33261 contain a single totem pole
output stage specifically designed for direct drive of power
MOSFETs. The Drive Output is capable of up to ±500 mA
peak current with a typical rise and fall time of 50 ns with a
1.0 nF load. Additional internal circuitry has been added to
keep the Drive Output in a sinking mode whenever the
Undervoltage Lockout is active. This characteristic
eliminates the need for an external gate pull–down resistor.
The totem pole output has been optimized to minimize cross
conduction current during high speed operation. The addition
of two 10 Ω resistors, one in series with the source output
transistor and one in series with the sink output transistor,
reduces the cross conduction current, as shown in Figure 12.
A 16 V clamp has been incorporated into the output stage to
limit the high state VOH. This prevents rupture of the
MOSFET gate when VCC exceeds 20 V.
Notes
Calculate the maximum required output power.
Calculated at the minimum required ac line for
regulation. Let the efficiency n = 0.95.
Let the switching cycle t = 20 µs.
Table 1. Design Equations
Calculation
Required Converter Output Power
Peak Inductor Current
Inductance
In theory the on–time ton is constant. In practice ton
tends to increase at the ac line zero crossings due to
the charge on capacitor C6.
The off–time toff is greatest at peak ac line and
approaches zero at the ac line zero crossings.
Theta (θ) represents the angle of
the ac line voltage.
The minimum switching frequency occurs at peak ac
line and increases as toff decreases.
Switch On–Time
Switch Off–Time
Switching Frequency
Set the current sense threshold VCS to 1.0 V for
universal input (85 Vac to 265 Vac) operation and
to 0.5 V for fixed input (92 Vac to 138 Vac, or
184 to 276 Vac) operation.
Set the multiplier input voltage VM to 3.0 V at high
line. Empirically adjust VM for the lowest distortion
over the ac line range while guaranteeing startup at
minimum line.
The IIB R1 error term can be minimized with a divider
current in excess of 100 µA.
Peak Switch Current
Multiplier Input Voltage
Converter Output Voltage
The bandwidth is typically set to 20 Hz for minimum
output ripple over the ac line haversine.
Error Amplifier Bandwidth
The following converter characteristics must be chosen:
VO – Desired output voltage
IO – Desired output current
Vac – AC RMS line voltage
Vac(LL) – AC RMS low line voltage
Formula
PO = VO IO
IL(pk) =
2 2 PO
ηVac(LL)
2t ǒ VO – VacǓ Vac2
2
L=
VO Vac(LL) IL(pk)
ton =
2 PO L
η Vac2
toff =
ton
VO
–1
2 Vac Sin θ
f= 1
ton + toff
R9 =
VCS
IL(pk)
VM = Vac 2
ǒ R7 + 1Ǔ
R3
VO
=
Vref
ǒ
R2
R1
+ 1Ǔ
– IIB R2
BW =
1
R1 R2
2 π R1 + R2 C1
MOTOROLA ANALOG IC DEVICE DATA
7
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