MC34063A — 1.5A Universal DC-DC Converter

Overview

The MC34063A (and industrial-grade MC33063A) is a monolithic control circuit containing the primary functions required for DC-to-DC converters. Originally designed by Motorola and now produced by ON Semiconductor, Texas Instruments, and STMicroelectronics, it contains an internal temperature-compensated reference, comparator, controlled duty cycle oscillator with active peak current limit circuit, driver, and a 1.5A high-current output switch.

Unlike specialized single-topology regulators, the MC34063A is a universal switcher that can be configured into three fundamental topologies:

Step-Down (Buck) converter (e.g., 12V to 5V)
Step-Up (Boost) converter (e.g., 5V to 12V or 170V for Nixie tubes)
Voltage Inverting converter (e.g., +5V to -5V dual supply rail)

Quick reference


Input Voltage (`VCC`)	3.0 V to 40.0 V DC
Output Voltage Range	Adjustable from 1.25 V to 40.0 V DC (or negative voltages)
Max Peak Switch Current (`Ipk`)	1.5 A (expandable using external NPN/PNP transistor)
Oscillator Frequency	Up to 100 kHz ( $C_T$ capacitor programmed)
Internal Reference Voltage	1.25 V ( $\pm 2\%$ )
Low Standby Current	2.5 mA typical
Package	8-pin DIP / SOIC-8

Pinout (DIP-8 Package)

text

             ┌───┴───┐
     Switch  │ 1   8 │ Driver Collector
  Collector  │       │
             │ 2   7 │ IPK Sense
      Switch │       │
    Emitter  │ 3   6 │ VCC
    Timing   │       │
  Capacitor  │ 4   5 │ Comparator Inverting Input
             └───────┘

Pin	Name	Description
1	`Switch Collector`	Collector of internal output Darlington NPN switch
2	`Switch Emitter`	Emitter of internal output Darlington NPN switch
3	`Timing Capacitor`	Connect timing capacitor $C_T$ to GND to set switching frequency
4	`GND`	Power and signal ground reference
5	`Comparator Inverting Input`	Feedback pin ( $V_{TH} = 1.25\text{V}$ ) connected to output resistor divider
6	`VCC`	Positive power supply voltage pin (+3.0V to +40V DC)
7	`IPK Sense`	Peak current sense pin. Connected to $VCC$ through current sense resistor $R_{SC}$
8	`Driver Collector`	Collector of internal driver transistor

Specifications

Parameter	Symbol	Min	Typ	Max	Unit	Conditions
Power Supply Voltage	$V_{CC}$	3.0	—	40.0	V	DC input
Output Switch Current	$I_{SW}$	—	1.0	1.5	A	Continuous switch current
Saturation Voltage	$V_{CE(sat)}$	—	1.0	1.3	V	Darlington switch at $I_{SW}=1.0\text{A}$
Internal Reference Voltage	$V_{th}$	1.225	1.250	1.275	V	$T_A = 25^\circ\text{C}$
Current Limit Sense Voltage	$V_{ISENSE}$	250	300	350	mV	Pin 7 to Pin 6
Operating Frequency	$f_{OSC}$	24	33	42	kHz	$C_T = 1.0\text{ nF}$

Basic Configurations

1. Step-Down (Buck) Converter Circuit

text

            R_sc (Current Sense)
  VCC ─────███████───┬───────────────┬─────── [Pin 6: VCC]
                     │               │
                 [Pin 7: IPK]  [Pin 1: Sw Coll]
                               [Pin 8: Drv Coll]
                                     │
   GND ───┤<───┬───────────────[Pin 2: Sw Emit]
         Diode │                     │
            [Inductor L]       [Timing Cap C_T]
               │                     │
  VOUT ────────┴───[C_OUT]─── GND   GND

Key Design Formulas

$\text{Current Sense Resistor:} \quad R_{SC} = \frac{0.3\text{V}}{I_{PK}}$

$\text{Feedback Output Voltage:} \quad V_{OUT} = 1.25\text{V} \times \left(1 + \frac{R_2}{R_1}\right)$

Common mistakes

Omission of the current sense resistor $R_{SC}$ : Shorts Pin 7 to Pin 6 without a low-ohm sense resistor (e.g., $0.22\ \Omega \dots 0.5\ \Omega$ ). Without $R_{SC}$ , overcurrent protection is disabled, causing immediate Destruction of the internal NPN switch when overloaded.
High voltage drop on internal Darlington switch: The internal bipolar Darlington switch has a relatively high $V_{CE(sat)}$ of about $1.0\text{V} \dots 1.3\text{V}$ . At high currents ( $>0.75\text{A}$ ), the IC will get warm. Use an external MOSFET or external PNP transistor for efficiency above 80%.
Incorrect $C_T$ capacitor calculation: If $C_T$ is chosen too large, the switching frequency drops into the audible range ( $<20\text{ kHz}$ ), causing audible inductor whine.

Notes

MC34063A vs MC33063A: MC34063A is specified for $0^\circ\text{C}$ to $+70^\circ\text{C}$ ; MC33063A is industrial grade specified for $-40^\circ\text{C}$ to $+85^\circ\text{C}$ .