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MC44602

HIGH PERFORMANCE CURRENT MODE CONTROLLER

ON Semiconductor 

MC44602

A narrow spike on the leading edge of the current

waveform can usually be observed and may cause the power

supply to exhibit an instability when the output is lightly

loaded. This spike is due to the power transformer

interwinding capacitance and the output rectifier recovery

time. The addition of an RC filter on the Current Sense Input

with a time constant that approximates the spike duration will

usually eliminate the instability; refer to Figure 30.

Undervoltage Lockout

Two undervoltage lockout comparators have been

incorporated to guarantee that the IC is fully functional before

the output stage is enabled. The positive power supply

terminal (VCC) and the reference output (Vref) are each

monitored by separate comparators. Each has built–in

hysteresis to prevent erratic output behavior as their

respective thresholds are crossed. The VCC comparator

upper and lower thresholds are 14.1 V/10.2 V. The Vref

comparator upper and lower thresholds are 3.6 V/3.3 V. The

large hysteresis and low startup current of the MC44602

make it ideally suited for off–line converter applications

(Figures 33, 34) where efficient bootstrap startup techniques

are required.

A 20 V zener is connected as a shunt regulator from VCC to

ground. Its purpose is to protect the IC from excessive

voltage that can occur during system startup. The upper limit

for the minimum operating voltage of the MC44602 is 11V.

Outputs

The MC44602 contains a high current split totem pole

output that was specifically designed for direct drive of

Bipolar Power Transistors. By splitting the totem pole into

separate source and sink outputs, the power supply designer

has the ability to independently adjust the turn–on and

turn–off base drive to the external power transistor for optimal

switching. The Source and Sink outputs are capable of up to

1.0 A and 1.5 A respectively and feature 50 ns switching

times with a 1.0 nF load. Additional internal circuitry has been

added to keep the Source Output “Off” and the Sink Output

“On” whenever an undervoltage lockout is active. This

feature eliminates the need for an external pull–down resistor

and guarantees that the power transistor will be held in the

“Off” state.

Separate output stage power and ground pins are

provided to give the designer added flexibility in tailoring the

base drive circuitry for a specific application. The Source

Output high–state is controlled by applying a positive voltage

to VC (Pin 14) and is independent of VCC. A zener clamp is

typically connected to this input when driving power

MOSFETs in systems where VCC is greater than 20V. The

Sink Output low–state is controlled by applying a negative

voltage to the Sink Ground (Pins 4, 5, 12, 13). The Sink

Ground can be biased as much as 5.0 V negative with

respect to Ground (Pin 7). Proper implementation of the VC

and Sink Ground pins will significantly reduce the level of

switching transient noise imposed on the control circuitry.

This becomes particularly useful when reducing the Ipk(max)

clamp level.

Reference

The 5.0 V bandgap reference has a tolerance of ±6.0%

over a junction temperature range of –25°C to 85°C. Its

primary purpose is to supply charging current to the oscillator

timing capacitor. The reference has short circuit protection

and is capable of providing in excess of 20 mA for powering

additional control system circuitry.

Figure 30. Bipolar Transistor Drive

and Current Spike Suppression

Vin

TQ

S

RQ

R

PWM

Latch

Current Sense

Comparator

IB

+

0

–

Base Charge

VC

Removal

14

CB

Source

11

RB1 RB2

Q1

Sink

10

LB

Sink Gnd

Substrate 4, 5, 12, 13

Current Sense

R

6

C RS

Thermal Protection and Package

Internal Thermal Shutdown circuitry is provided to protect

the integrated circuit in the event that the maximum junction

temperature is exceeded. When activated, typically at 160°C,

the PWM Latch is held in the “reset” state, forcing the Source

Output “Off” and the Sink Output “On”. This feature is

provided to prevent catastrophic failures from accidental

device overheating. It is not intended to be used as a

substitute for proper heatsinking.

The MC44602 is contained in a heatsinkable 16–lead

plastic dual–in–line package in which the die is mounted on a

special heat tab copper alloy lead frame. This tab consists of

the four center Sink Ground pins that are specifically

designed to improve the thermal conduction from the die to

the circuit board. Figure 14 shows a simple and effective

method of utilizing the printed circuit medium as a heat

dissipater by soldering these pins to an adequate area of

copper foil. This permits the use of standard layout and

mounting practices while having the ability to halve the

junction to air thermal resistance. This example is for a

symmetrical layout on a single–sided board with two ounce

per square foot of copper.

MOTOROLA ANALOG IC DEVICE DATA

11

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