MC34063A Reference Tables, Pins & Specifications

Pin reference

Signal names, electrical functions and connection guidance

PDIP-8

Physical pin numbers8 pins

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MC34063A pin functions and connections
Signal	Type	Function & connection
SC	Switch collector	Collector of the internal high-current NPN switch. Connection guidance In the buck example, connect to the current-sense resistor output and driver collector pin 8. The 1.5 A rating is a peak switch limit, not a guaranteed load current.
SE	Switch emitter	Emitter of the internal output switch. Connection guidance In the buck circuit, connect to L1 and the catch-diode cathode. This is a switching node, not the filtered DC output.
CT	Timing capacitor	An external capacitor sets the oscillator timing. Connection guidance Connect the 470 pF example capacitor from CT to GND. The 33 kHz typical specification is measured with CT = 1 nF; do not assign it to every timing capacitor.
GND	Common return	Return for the reference, oscillator and comparator. Connection guidance Join the input/output capacitor returns and diode anode with short power paths. Keep the feedback return away from large switching currents.
INV	Comparator inverting input	Compares the output-divider voltage with the internal 1.25 V reference. Connection guidance Connect to the junction of R2 from output and R1 to ground. Output setting is approximately 1.25 × (1 + R2/R1).
VCC	Control supply	Supplies the control circuitry. Connection guidance Connect to VIN before the current-sense resistor and bypass locally. Recommended supply range 3–40 V; the example uses 25 V.
IPK	Peak current sense	Senses the drop between the supply and the current-sense node. Connection guidance Connect after RSC, at the switch/driver collector feed. Approximately 300 mV across RSC invokes current limiting; 250–350 mV at 25°C.
DC	Driver collector	Collector supply of the internal driver transistor. Connection guidance Join to pin 1 after RSC in this Darlington buck configuration. Alternative driver configurations need the manufacturer’s base-drive design checks.

Operating conditions

Selected commercial MC34063A

Manufacturer-referenced values6 parameters

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Parameter	Value	Conditions / notes
Supply	3–40 V	Recommended VCC range
Ambient temperature	0 to +70°C	MC34063A; MC33063A is a separately specified −40 to +85°C option
Topologies	Buck / boost / inverting	External wiring and calculations differ
Maximum oscillator capability	100 kHz	External CT and the operating conditions determine actual timing
Peak switch current	1.5 A	Not a guaranteed continuous load current
Packages shown	PDIP-8 / SOIC-8	TI P / D orderables

Keep voltage overshoot and thermal stress within the exact device ratings.

Oscillator and current limit

VCC = 5 V; TA = 25°C unless noted

Manufacturer-referenced values6 parameters

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Parameter	Value	Conditions / notes
Frequency with CT = 1 nF	24 / 33 / 42 kHz	Minimum / typical / maximum; pin 5 at 0 V
Capacitor charge current	24 / 35 / 42 µA	VCC = 5–40 V
Capacitor discharge current	140 / 220 / 260 µA	VCC = 5–40 V
Discharge / charge ratio	5.2 / 6.5 / 7.5	Pin 7 tied to VCC
Current-limit sense voltage	250 / 300 / 350 mV	Characterized at equal charge and discharge currents
Nominal sense resistor	RSC ≈ 0.3 / IPEAK	Use threshold tolerance and waveform margins

The CT = 1 nF frequency is a test point, not the frequency of the illustrated 470 pF circuit.

Reference, switch and supply

TI electrical-characteristic conditions

Manufacturer-referenced values6 parameters

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Parameter	Value	Conditions / notes
Reference at 25°C	1.225 / 1.250 / 1.275 V	Minimum / typical / maximum
Reference over ambient range	1.21–1.29 V	Commercial operating range
Reference line change	1.4 mV typical; 5 mV maximum	VCC = 5–40 V
Darlington switch saturation	1 V typical; 1.3 V maximum	ISW = 1 A; pins 1 and 8 joined
Non-Darlington saturation	0.45 V typical; 0.7 V maximum	ISW = 1 A, 82 Ω driver feed, forced beta about 20
Supply current	4 mA maximum	VCC = 5–40 V; output-switch-off test conditions

Driver topology changes switching loss and turn-off behavior. Use the complete manufacturer conditions.

Buck example components

TI Figure 11 · 25 V input, approximately 5 V output

Manufacturer-referenced values7 parameters

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Parameter	Value	Conditions / notes
RSC	0.33 Ω	Between VIN and the joined SC1 / IPK7 / DC8 feed
CT	470 pF	Pin 3 to ground
L1	220 µH	Pin 2 to filtered output; select suitable saturation and RMS ratings
Catch diode	1N5819	Cathode at pin 2; anode at GND
CIN / COUT	100 µF / 470 µF	Choose voltage/ripple ratings and suitable ESR; illustrated 50 V / 16 V
R2 / R1	3.8 kΩ / 1.2 kΩ	R2 from output to INV5; R1 from INV5 to GND
Reference load	500 mA	Manufacturer application example, not the part’s universal current rating

The optional extra LC output filter is omitted from the illustration. The nominal resistor ratio gives 5.208 V before tolerance and losses.

Read the example correctly

Calculated illustrations

Manufacturer-referenced values6 parameters

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Parameter	Value	Conditions / notes
Divider output	5.208 V	1.25 × (1 + 3.8/1.2)
Nominal current limit	0.909 A	0.300 V / 0.33 Ω
Threshold-only limit interval	0.758–1.061 A	0.250–0.350 V / 0.33 Ω; excludes resistor tolerance and dynamic effects
Ideal duty at 25 V	20.8%	5.208 / 25; excludes switch/diode drops
Output power at 0.5 A	Approximately 2.60 W	5.208 V × 0.5 A
Simple resistor load	About 10.4 Ω	For an illustrative 0.5 A load; rate it above the dissipated power

These calculations explain the circuit. They are not live measurements or a switching simulation.

Layout and connection checks

Protect quiet nodes from switching currents

Manufacturer-referenced values6 parameters

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Parameter	Value	Conditions / notes
Current-sense path	VIN6 → RSC → IPK7 / SC1 / DC8	Use short, low-inductance paths and sense the resistor terminals
Switch node	SE2 → L1 and diode cathode	Keep copper compact
Catch path	GND → diode → SE2 → L1	Short loop with the output capacitor
Feedback divider	Output → R2 → INV5 → R1 → GND	Route away from switch-node copper
Timing capacitor	CT3 → GND4	Place close to the IC
Physical package mapping	Eight pins in both selected packages	Pin 1 indication remains visible

Changing from buck to boost or inverting operation requires a different power-stage connection.

Stress and thermal limits

Absolute maxima are not normal targets

Manufacturer-referenced values6 parameters

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Parameter	Value	Conditions / notes
Supply / switch voltage	40 V	Absolute limits require checking each referenced voltage
Driver collector current	100 mA	Absolute maximum
Switch current	1.5 A	Absolute maximum
Junction temperature	150°C	Absolute maximum; thermal design must stay below it
Storage temperature	−65 to +150°C	Unpowered
Comparator input	−0.3 to +40 V	Absolute input range, not a desired feedback voltage

A headline current rating cannot replace dissipation and temperature calculations.