MC34163, MC33163

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INTRODUCTION

The MC34163 series are monolithic power switching

regulators optimized for dc−to−dc converter applications.

The combination of features in this series enables the system

designer to directly implement step−up, step−down, and

voltage−inverting converters with a minimum number of

external components. Potential applications include cost

sensitive consumer products as well as equipment for the

automotive,

computer,

and

industrial

markets.

A

Representative Block Diagram is shown in Figure 19.

OPERATING DESCRIPTION

The MC34163 operates as a fixed on−time, variable

off−time voltage mode ripple regulator. In general, this

mode of operation is somewhat analogous to a capacitor

charge pump and does not require dominant pole loop

compensation for converter stability. The Typical Operating

Waveforms are shown in Figure 20. The output voltage

waveform shown is for a step−down converter with the

ripple and phasing exaggerated for clarity. During initial

converter startup, the feedback comparator senses that the

output voltage level is below nominal. This causes the

output switch to turn on and off at a frequency and duty cycle

controlled by the oscillator, thus pumping up the output filter

capacitor. When the output voltage level reaches nominal,

the feedback comparator sets the latch, immediately

terminating switch conduction. The feedback comparator

will inhibit the switch until the load current causes the output

voltage to fall below nominal. Under these conditions,

output switch conduction can be inhibited for a partial

oscillator cycle, a partial cycle plus a complete cycle,

multiple cycles, or a partial cycle plus multiple cycles.

Oscillator

The oscillator frequency and on−time of the output switch

are programmed by the value selected for timing capacitor

internal current source and sink, generating a negative going

pulse is generated at the oscillator output. This pulse is

connected to the NOR gate center input, preventing output

switch conduction, and to the AND gate upper input,

allowing the latch to be reset if the comparator output is low.

Thus, the output switch is always disabled during ramp−up

and can be enabled by the comparator output only at the start

of ramp−down. The oscillator peak and valley thresholds are

1.25 V and 0.55 V, respectively, with a charge current of

225

mA and a discharge current of 25 mA, yielding a

maximum on−time duty cycle of 90%. A reduction of the

maximum duty cycle may be required for specific converter

configurations. This can be accomplished with the addition

resistor increases the discharge current which reduces the

on−time of the output switch. A graph of the Output Switch

On−Off Time versus Oscillator Timing Capacitance for

NPN small−signal transistor.

Feedback and Low Voltage Indicator Comparators

Output voltage control is established by the Feedback

comparator. The inverting input is internally biased at 1.25 V

and is not pinned out. The converter output voltage is

typically divided down with two external resistors and

monitored by the high impedance noninverting input at Pin 2.

The maximum input bias current is

±0.4 mA, which can cause

an output voltage error that is equal to the product of the input

bias current and the upper divider resistance value. For

applications that require 5.0 V, the converter output can be

directly connected to the noninverting input at Pin 3. The high

impedance input, Pin 2, must be grounded to prevent noise

pickup. The internal resistor divider is set for a nominal

voltage of 5.05 V. The additional 50 mV compensates for a

1.0% voltage drop in the cable and connector from the

converter output to the load. The Feedback comparator’s

output state is controlled by the highest voltage applied to

either of the two noninverting inputs.

The Low Voltage Indicator (LVI) comparator is designed

for use as a reset controller in microprocessor−based

systems. The inverting input is internally biased at 1.125 V,

which sets the noninverting input thresholds to 90% of

nominal. The LVI comparator has 15 mV of hysteresis to

prevent erratic reset operation. The Open Collector output is

capable of sinking in excess of 6.0 mA (see Figure 11). An

threshold. Refer to Figure 21.

1

1 −

Current Limit Comparator, Latch and Thermal

Shutdown

With a voltage mode ripple converter operating under

normal conditions, output switch conduction is initiated by

the oscillator and terminated by the Voltage Feedback

comparator. Abnormal operating conditions occur when the

converter output is overloaded or when feedback voltage

sensing is lost. Under these conditions, the Current Limit

comparator will protect the Output Switch.