MC44603P 查看數據表（PDF） - ON Semiconductor

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MC44603P

Enhanced Mixed Frequency Mode GreenLine™ PWM Controller:

ON Semiconductor 

MC44603A

In order to prevent undesired mode switching when power

is close to the threshold value, a hysteresis that is

proportional to VR P Stby is incorporated creating a second

VCS threshold level that is equal to [2.5 x (VR P Stby)/3].

When the standby comparator output is high, a second

current source (0.6 x Iref) is connected to Pin 12.

Finally, the standby mode function can be shown

graphically in Figure 42.

Pin

fS

Normal

Working

fStby

PthH

PthL

Standby

VCS

[(VR P Stby)/3]

2.5 x [(VR P Stby)/3] 1

Figure 42. Dynamic Mode Change

This curve shows that there are two power threshold

levels:

− the low one:

PthL fixed by VR P Stby

− the high one:

PthH

+

(2.5)2 x PthL x

fStby

fS

PthH

+

6.25 x PthL x

fStby

fS

Maximum Duty Cycle and Soft−Start Control

Maximum duty cycle can be limited to values less than

80% by utilizing the Dmax and soft−start control. As

depicted in Figure 43, the Pin 11 voltage is compared to the

oscillator sawtooth.

Vref

0.4 Iref

11

DZ

2.4 V CDmax

Output

Control

Output

Dmax Drive

Soft−Start

Capacitor

VOSC

Oscillator

Figure 43. Dmax and Soft−Start

Voltage

Dmax

Pin 11

VCT

(Pin 10)

Figure 44. Maximum Duty Cycle Control

Using the internal current source (0.4 Iref), the Pin 11

voltage can easily be set by connecting a resistor to this pin.

If a capacitor is connected to Pin 11, the voltage increases

from 0 to its maximum value progressively (refer to

Figure 45), thereby, implementing a soft−start. The

soft−start capacitor is discharged internally when the VCC

(Pin 1) voltage drops below 9.0 V.

Pin 11 R Connected to Pin 11

C

I = 0.4 Iref

RI

VZ

C // R

VZ

RI

t = RC

Figure 45. Different Possible Uses of Pin 11

If no external component is connected to Pin 11, an

internal zener diode clamps the Pin 11 voltage to a value VZ

that is higher than the oscillator peak value, disabling

soft−start and maximum duty cycle limitation.

Foldback

As depicted in Figures 33 and 49, the foldback input

(Pin 5) can be used to reduce the maximum VCS value,

providing foldback protection. The foldback arrangement is

a programmable peak current limitation.

If the output load is increased, the required converter peak

current becomes higher and VCS increases until it reaches its

maximum value (normally, VCS max = 1.0 V).

Then, if the output load keeps on increasing, the system is

unable to supply enough energy to maintain the output

voltages in regulation. Consequently, the decreasing output

can be applied to Pin 5, in order to limit the maximum peak

current. In this way, the well known foldback characteristic

can be obtained (refer to Figure 46).

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