mc33466 ON Semiconductor, mc33466 Datasheet - Page 8

• Manufacturer Part Number: mc33466
• Description: Fixed Frequency Pwm Micropower Dc-to-dc Converter
• Manufacturer: ON Semiconductor

DEFINITIONS
Quiescent Bias Current − Current which is used to operate
the switching regulator chip and is not delivered to the load.
Leakage Current − Current drawn through a transistor
junction, under a specified collector voltage, when the
transistor is off.
FUNCTIONAL DESCRIPTION
Introduction
regulators optimized for dc−to−dc converter applications
where power drain must be minimized. The combination of
features in this series allows the system designer to directly
implement step−up, step−down or flyback converters with
a small number of external components. Potential
applications include low power consumer products and
battery powered portable products. Typical application
circuits are shown in Figures 22 through 26.
Operating Description
frequency voltage mode regulator. Operation is intended to
be in the discontinuous mode, where the inductor current
ramps up to a peak value which is greater than or equal to
twice the value of the dc input current during the on−time of
the transistor switch. During the off−time of the transistor
switch, the inductor current ramps down to zero and remains
at zero until another switching cycle begins.
voltage for powering internal circuitry, an external startup
circuit is needed in step−down converter and flyback
designs to provide initial power to the integrated circuit to
begin switching. The startup circuit needed can be three
discrete components, as shown in Figure 24, or a
micropower undervoltage sensor, as shown in Figure 25.
Oscillator
50 kHz for the JT1 suffix and 100 kHz for the LT1 suffix.
The timing capacitor (C
oscillator is designed for a maximum duty cycle of 80% at
the Lx or EXT output. During the charge of C
generates an internal blanking pulse that holds the PWM
control off, disabling the output transistor drive. The
oscillator peak and valley thresholds are 0.5 V and ground,
respectively.
Pulse Width Modulator
the oscillator ramp voltage applied to the inverting input,
while the error amplifier output is applied to the
noninverting input. Output switch conduction is initiated
when the timing capacitor is charged to its peak voltage
value. When the timing capacitor ramp discharges to a
voltage below the error amplifier output, the comparator
resets a latch terminating output transistor drive for the
duration of the oscillator ramp period.
The MC33466 series are monolithic power switching
The MC33466 series converters operate as a fixed
Because the output voltage pin is also used as the supply
The oscillator frequency, is internally programmed to
The Pulse Width Modulator consists of a comparator with
T
) discharge to charge ratio of the
T
, the oscillator
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8
Error Amplifier and Reference
of voltage gain at dc. Internal compensation components
provide poles at 0.25 Hz, 30 kHz and 33 kHz. Two zeros are
provided at 1.0 kHz and at 2.5 kHz. The output voltage value
is set by the internal voltage divider and a 0.7 V reference
which is trimmed to an accuracy of ±2.5%. Because the loop
compensation components are located within the IC,
discontinuous mode operation is recommended for most
applications.
Driver and Output Switch
efficiency, two output driver options are provided. The
MC33466H−XXJT1 converters have an internal drive
transistor which is capable of sinking currents greater than
60 mA into the Lx pin. An internal V
if the Lx pin voltage exceeds 1.0 V during t
the drive transistor. The MC33466H−XXLT1 provides
output drive for an external transistor.
Applications
simplicity and flexibility of the converter architecture.
Three main converter topologies are demonstrated in
Figures 22 through 26.
V
V
in
An Error Amplifier is provided which has a nominal 80 dB
To aid in system design flexibility and conversion
The following converter applications show the
in
MMBT2222ALT1
22 μF
28 μH
120 μH
22 μF
Figure 22. MC33466H−50JT1 Typical
Figure 23. MC33466H−50LT1 Typical
Step−Up Application
Step−Up Application
MC33466H−50JT1
3
0.01 μF
300
Lx
MBRD520LT1
MBRD520LT1
Gnd
1
MC33466H−50LT1
3
V
O
EXT
Lx
2
limiter circuit senses
Gnd
1
on
V
O
and turns off
2
22 μF
5.0 V
V
V
100
μF
O
O