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MC33466

Fixed Frequency PWM Micropower DC-to-DC Converter

ON Semiconductor 

MC33466

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

The MC33466 series are monolithic power switching

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

The MC33466 series converters operate as a fixed

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.

Because the output voltage pin is also used as the supply

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

The oscillator frequency, is internally programmed to

50 kHz for the JT1 suffix and 100 kHz for the LT1 suffix.

The timing capacitor (CT) discharge to charge ratio of the

oscillator is designed for a maximum duty cycle of 80% at

the Lx or EXT output. During the charge of CT, the oscillator

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 Pulse Width Modulator consists of a comparator with

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.

Error Amplifier and Reference

An Error Amplifier is provided which has a nominal 80 dB

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

To aid in system design flexibility and conversion

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 VLx limiter circuit senses

if the Lx pin voltage exceeds 1.0 V during ton and turns off

the drive transistor. The MC33466H–XXLT1 provides

output drive for an external transistor.

Applications

The following converter applications show the

simplicity and flexibility of the converter architecture.

Three main converter topologies are demonstrated in

Figures 22 through 26.

MBRD520LT1

120 µH

Vin

22 µF

MC33466H–50JT1

3

Lx

2

VO

VO

22 µF

Gnd

1

Figure 22. MC33466H–50JT1 Typical

Step–Up Application

28 µH

Vin

22 µF

MMBT2222ALT1

MBRD520LT1

300

0.01 µF

MC33466H–50LT1

2

VO

3

EXT

Gnd

1

VO

5.0 V

100

µF

Figure 23. MC33466H–50LT1 Typical

Step–Up Application

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