MC33341_06 datasheet(6/19 Pages) ONSEMI

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MC33341

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INTRODUCTION

Power supplies and battery chargers require precise

control of output voltage and current in order to prevent

catastrophic damage to the system load. Many present day

power sources contain a wide assortment of building blocks

and glue devices to perform the required sensing for proper

regulation. Typical feedback loop circuits may consist of a

voltage and current amplifier, level shifting circuitry,

summing circuitry and a reference. The MC33341 contains

all of these basic functions in a manner that is easily

adaptable to many of the various power source−load

configurations.

OPERATING DESCRIPTION

The MC33341 is an analog regulation control circuit that

is specifically designed to simultaneously close the voltage

and current feedback loops in power supply and battery

charger applications. This device can control the feedback

loop in either constant−voltage or constant−current mode

with automatic crossover. A concise description of the

integrated circuit blocks is given below. Refer to the block

diagram in Figure 14.

Transconductance Amplifier

A quad input transconductance amplifier is used to control

the feedback loop. This amplifier has separate voltage and

current channels, each with a sense and a threshold input.

Within a given channel, if the sense input level exceeds that

of the threshold input, the amplifier output is driven high.

The channel with the largest difference between the sense

and threshold inputs will set the output source current of the

amplifier and thus dominate control of the feedback loop.

The amplifier output appears at Pin 8 and is a source−only

type that is capable of 15 mA.

A high impedance node within the transconductance

amplifier is made available at Pin 3 for loop compensation.

This pin can sink and source up to 10

mA of current. System

stability is achieved by connecting a capacitor from Pin 3 to

ground. The Compensation Pin signal is out of phase with

respect to the Drive Output. By actively clamping Pin 3 low,

the Drive Output is forced into a high state. This, in effect,

will shutdown the power supply or battery charger, by

forcing the output voltage and current regulation threshold

down towards zero.

Voltage Sensing

The voltage that appears across the load is monitored by

the noninverting Vsen input of the transconductance

amplifier. This voltage is resistively scaled down and

connected to Pin 5. The threshold at which voltage

regulation occurs is set by the level present at the inverting

Vth input of the transconductance amplifier. This level is

controlled by Pin 6. In source high−side and load high−side

current sensing modes, Pin 6 must be connected to the low

potential side of current sense resistor RS. Under these

conditions, the voltage regulation threshold is internally

fixed at 1.2 V. In source return low−side and load low−side

current sensing modes, Pin 6 is available, and can be used to

lower the regulation threshold of Pin 5. This threshold can

be externally adjusted over a range of 0 V to 1.2 V with

respect to the IC ground at Pin 4.

Current Sensing

Current sensing is accomplished by monitoring the

voltage that appears across sense resistor RS, level shifting

it with respect to Pin 4 if required, and applying it to the

noninverting Isen input of the transconductance amplifier. In

order to allow for maximum circuit flexibility, there are

three methods of current sensing, each with different

internal paths.

In source high−side (Figures 14 and 15) and load

high−side (Figures 18 and 19) current sensing, the

Differential Amplifier is active with a gain of 1.0. Pin 1

connects to the high potential side of current sense resistor

RS while Pin 6 connects to the low side. Logic circuitry is

provided to disable the Differential Amplifier output

whenever low−side current sensing is required. This circuit

clamps the Differential Amplifier output high which

disconnects it from the Isen input of the Transconductance

Amplifier. This happens if Pin 1 is less than 1.2 V or if Pin 1

is less than Pin 6.

With source return low−side current sensing (Figures 16

and 17), the Inverting Amplifier is active with a gain of −1.0.

Pin 1 connects to the low potential side of current sense

resistor RS while Pin 4 connects to the high side. Note that

a negative voltage appears across RS with respect to Pin 4.

In load low−side current sensing (Figures 20 and 21) a

Noninverting input path is active with a gain of 1.0. Pin 1

connects to the high potential side of current sense resistor

RS while Pin 4 connects to the low side. The Noninverting

input path lies from Pin 1, through the Inverting Amplifier

input and feedback resistors R, to the cathode of the output

diode. With load low−side current sensing, Pin 1 will be

more positive than Pin 4, forcing the Inverting Amplifier

output low. This causes the diode to be reverse biased, thus

preventing the output stage of the amplifier from loading the

input signal that is flowing through the feedback resistors.

The regulation threshold in all of the current sensing

modes is internally fixed at 200 mV with Pin 2 connected to

VCC. Pin 2 can be used to externally adjust the threshold

over a range of 0 to 200 mV with respect to the IC ground

at Pin 4.

Reference

An internal band gap reference is used to set the 1.2 V

voltage threshold and 200 mV current threshold. The

reference is initially trimmed to a

±1.0% tolerance at

TA = 25

°C and is guaranteed to be within ±2.0% over an

ambient operating temperature range of −25

° to 85°C.

Applications

Each of the application circuits illustrate the flexibility of

this device. The circuits shown in Figures 14 through 21

contain an optoisolator connected from the Drive Output at

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