MC34063ABuck / boost / inverting controller
One switching controller. Three ways to change a DC rail.
MC34063A is an eight-pin switching controller with an internal NPN power switch, reference, comparator, oscillator and current-limit circuit. External connections configure it as a buck, boost or inverting converter. Its 1.5 A headline rating describes peak switch current. Available output current depends on topology, voltage ratio, inductor ripple and cooling; a boost output cannot simply deliver 1.5 A at every voltage. This reference follows TI’s commercial 0–70°C MC34063A in PDIP-8 and SOIC-8. The illustrated buck circuit follows the manufacturer’s example, with its resistor-selected output and distinct current-sense, timing and feedback nodes.
Power switchOscillatorGroundFeedbackSupplyCurrent senseSwitch driver
MC34063APPDIP-8Top view
MC34063ADRSOIC-8Top view
Click a lead or use the arrow keys. Top view
PIN 1 · Power switch
Switch collectorSC
Function
Collector of the internal high-current NPN switch.
How to connect
In the buck example, connect to the current-sense resistor output and driver collector pin 8.
Electrical note
The 1.5 A rating is a peak switch limit, not a guaranteed load current.
PIN 2 · Power switch
Switch emitterSE
Function
Emitter of the internal output switch.
How to connect
In the buck circuit, connect to L1 and the catch-diode cathode.
Electrical note
This is a switching node, not the filtered DC output.
PIN 3 · Oscillator
Timing capacitorCT
Function
An external capacitor sets the oscillator timing.
How to connect
Connect the 470 pF example capacitor from CT to GND.
Electrical note
The 33 kHz typical specification is measured with CT = 1 nF; do not assign it to every timing capacitor.
PIN 4 · Ground
Common returnGND
Function
Return for the reference, oscillator and comparator.
How to connect
Join the input/output capacitor returns and diode anode with short power paths.
Electrical note
Keep the feedback return away from large switching currents.
PIN 5 · Feedback
Comparator inverting inputINV
Function
Compares the output-divider voltage with the internal 1.25 V reference.
How to connect
Connect to the junction of R2 from output and R1 to ground.
Electrical note
Output setting is approximately 1.25 × (1 + R2/R1).
PIN 6 · Supply
Control supplyVCC
Function
Supplies the control circuitry.
How to connect
Connect to VIN before the current-sense resistor and bypass locally.
Electrical note
Recommended supply range 3–40 V; the example uses 25 V.
PIN 7 · Current sense
Peak current senseIPK
Function
Senses the drop between the supply and the current-sense node.
How to connect
Connect after RSC, at the switch/driver collector feed.
Electrical note
Approximately 300 mV across RSC invokes current limiting; 250–350 mV at 25°C.
PIN 8 · Switch driver
Driver collectorDC
Function
Collector supply of the internal driver transistor.
How to connect
Join to pin 1 after RSC in this Darlington buck configuration.
Electrical note
Alternative driver configurations need the manufacturer’s base-drive design checks.
At a glance
Buck / boost / inverting controller
The external inductor and diode carry the energy. CT controls timing, RSC limits peak current and the output divider sets the regulated voltage.
3–40 VSupply range
1.5 A · not load currentPeak switch limit
0–70 °C · MC34063AAmbient range
1.25 VNominal reference
Operating profile
MC34063AP
Operating conditions
Key parameters and their test conditions
- Supply
- 3–40 VConditions / notes: Recommended VCC range
- Ambient temperature
- 0 to +70°CConditions / notes: MC34063A; MC33063A is a separately specified −40 to +85°C option
- Topologies
- Buck / boost / invertingConditions / notes: External wiring and calculations differ
- Maximum oscillator capability
- 100 kHzConditions / notes: External CT and the operating conditions determine actual timing
- Peak switch current
- 1.5 AConditions / notes: Not a guaranteed continuous load current
- Packages shown
- PDIP-8 / SOIC-8Conditions / notes: TI P / D orderables
Package details
Follows the selected physical package
- Manufacturer
- Texas Instruments
- Part number
- MC34063AP
- Package
- PDIP-8
- Connections
- 8 pins
- Mounting
- Through-hole
- Lead pitch
- 2.54 mm
- Package code
- dip
- Thermal reference
- Board and package dependent; see manufacturer data
Connections by function
Select a pin to open its function, electrical notes and wiring guidance.
Power switch2 pins
Oscillator1 pins
Ground1 pins
Feedback1 pins
Supply1 pins
Current sense1 pins
Switch driver1 pins
Power switch2 pins
Oscillator1 pins
Ground1 pins
Feedback1 pins
Supply1 pins
Current sense1 pins
Switch driver1 pins
Start an energy pulse
The timing capacitor controls the oscillator. When feedback requests energy, the driver turns the output switch on.
Limit the current
The drop across RSC increases with switch-path current. The sense circuit changes the timing when its threshold is reached.
Hold the output near its setting
Pin 5 compares the divided output against 1.25 V. The inductor and catch diode continue supplying current between switching pulses.
The example divider gives 1.25 × (1 + 3.8/1.2) = 5.208 V ideally. RSC = 0.33 Ω gives a nominal 0.909 A current-limit estimate, not a guaranteed output-current rating.
Pin reference
Select a pin to explore its function and connection.
8 pins
| Pin | Signal | Type | Function |
|---|---|---|---|
| SC | Switch collector | Collector of the internal high-current NPN switch. | |
| SE | Switch emitter | Emitter of the internal output switch. | |
| CT | Timing capacitor | An external capacitor sets the oscillator timing. | |
| GND | Common return | Return for the reference, oscillator and comparator. | |
| INV | Comparator inverting input | Compares the output-divider voltage with the internal 1.25 V reference. | |
| VCC | Control supply | Supplies the control circuitry. | |
| IPK | Peak current sense | Senses the drop between the supply and the current-sense node. | |
| DC | Driver collector | Collector supply of the internal driver transistor. |
No pins match. Try a signal name or choose All pins.
Operating conditions
Selected commercial MC34063A
Manufacturer specifications 6 parameters
| 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 |
No specifications match. Try a parameter name or a unit.
Keep voltage overshoot and thermal stress within the exact device ratings.
Oscillator and current limit
VCC = 5 V; TA = 25°C unless noted
Manufacturer specifications 6 parameters
| 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 |
No specifications match. Try a parameter name or a unit.
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 specifications 6 parameters
| 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 |
No specifications match. Try a parameter name or a unit.
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 specifications 7 parameters
| 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 |
No specifications match. Try a parameter name or a unit.
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 specifications 6 parameters
| 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 |
No specifications match. Try a parameter name or a unit.
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 specifications 6 parameters
| 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 |
No specifications match. Try a parameter name or a unit.
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 specifications 6 parameters
| 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 |
No specifications match. Try a parameter name or a unit.
A headline current rating cannot replace dissipation and temperature calculations.
Step-down rails
Create a lower-voltage supply using the emitter switch node, inductor and catch diode.
Manufacturer application guideStep-up supplies
Rewire the power stage for boost operation and calculate switch current from the higher voltage ratio.
Manufacturer application guideNegative rails
Use the inverting topology with appropriate grounding, component stress and feedback design.
Manufacturer application guideTake it into ProTech
Find MC34063A in the Components Library to place its schematic symbol and connect its pins.
Reference page · circuit simulation model not yet availableOpen ProTech