MC44605P datasheet(9/20 Pages) ONSEMI

Part #	MC44605P

Description	High Safety, Latched Mode, GreenLine PWM Controller for (Multi)Synchronized Applications

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MC44605

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The Error Amp Output (Pin 13) is provided for external

loop compensation. The output voltage is offset by two

diodes drops (

[1.4 V) and divided by three before it

connects to the inverting input of the Current Sense

Comparator. This guarantees that no drive pulses appear at

the Source Output (Pin 3) when Pin 13 is at its lowest state

(VOL). This occurs when the power supply is operating and

the load is removed, or at the beginning of a soft–start

interval. The Error Amp minimum feedback resistance is

limited by the amplifier’s minimum source current (0.2 mA)

and the required output voltage (VOH) to reach the current

sense comparator’s 1.0 V clamp level:

R1(min)

+

(3

1V)

) 1.4 V

0.2 mA

+ 22 kΩ

Current Sense Comparator and PWM Latch

The MC44605 operates as a current mode controller. The

circuit uses a current sense comparator to compare the

inductor current to the threshold level established by the

Error Amplifier output (Pin 13). When the current reaches

the threshold, the current sense comparator terminates the

output switch conduction that has been initiated by the

oscillator, by resetting the PWM Latch. Thus the error signal

controls the peak inductor current on a cycle–by–cycle

basis. This configuration ensures that only one single pulse

appears at the Source Output during the appropriate

oscillator cycle.

Figure 2. Output Totem Pole

R

S

Q

UVLO

Disout

Vdemag out

Thermal

Protection

PWM

Latch

Current Sense

Comparator

Substrate

Current

Sense

14

3

7

C

RS

R

R3

Q1

Vin

VC

R2

VS

The inductor current is converted to a voltage by inserting

the ground referenced sense resistor RS in series with the

power switch Q1.

This voltage is monitored by the Current Sense Input

(Pin 7) and compared to a level derived from the Error Amp

output. The peak inductor current under normal operating

conditions is controlled by the voltage at Pin 13 where:

I

pk [

V

(pin13) *

1.4 V

3

R

S

The Current Sense Comparator threshold is internally

clamped to 1.0 V. Therefore the maximum peak switch

current is:

I

pk(max) +

1V

R

S

Undervoltage Lockout Section

As depicted in Figure 3, an undervoltage lockout has been

incorporated to guarantee that the IC is fully functional

before allowing the system working.

In effect, the VCC is connected to the non inverting input

of a comparator that has an upper threshold equal to 14,5 V

(typical Vstup–th) and a lower one equal to 7.5 V (typical

Vdisable 2). This hysteresis comparator enables or disables

the reference block that generates the voltage and current

sources required by the system.

This block particularly, produces Vref (pin 16 voltage) and

Iref that is determined by the resistor Rrefconnectedbetween

pin 16 and the ground:

I

ref +

V

ref

R

ref

where V

ref +

2.5 V (typically)

Figure 3. VCC Management

Reference Block:

Voltage and Current

Sources Generator

(Vref, Iref, ...)

Vref enable

CSTART–UP

10

1

0

Vdisable

7.5 V

START–UP

14.5 V

UVLO1

(to SOFTSTART)

Vdisable1

9.0 V

CUVLO1

Rref

Pin 16

MC44605

VCC

(Pin 1)

In addition to this, VCC is compared to a second threshold

level that is nearly equal to 9 V (Vdisable1) so that a signal

UVLO1 is generated to reset the soft start block and so, to

disable the output stage (refer to the Soft–Start §) as soon as

VCC becomes lower than Vdisable 1. In this way, the circuit

is reset and made ready for a next start–up, before the

reference block is disabled (refer to Figure 3). Thus, finally

the upper limit for the minimum normal operating voltage

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