LD7575PN データシートの表示（PDF） - Unspecified

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LD7575PN

Green-Mode PWM Controller

Unspecified 

LD7575

Oscillator and Switching Frequency

Connecting a resistor from RT pin to GND according to the

equation can program the normal switching frequency:

fSW

=

65.0

RT(KΩ )

× 100(KHz)

The suggested operating frequency range of LD7575 is

within 50KHz to 130KHz.

Internal Slope Compensation

A fundamental issue of current mode control is the stability

problem when its duty-cycle is operated more than 50%. To

stabilize the control loop, the slope compensation is needed

in the traditional UC384X design by injecting the ramp signal

from the RT/CT pin through a coupling capacitor. In LD7575,

the internal slope compensation circuit has been

implemented to simplify the external circuit design.

threshold 5.0V and keeps longer than 30mS (when

switching frequency is 65KHz), the protection is activated

and then turns off the gate output to stop the switching of

power circuit. The 30mS delay time is to prevent the false

trigger from the power-on and turn-off transient.

A divide-2 counter is implemented to reduce the average

power under OLP behavior. Whenever OLP is activated,

the output is latched off and the divide-2 counter starts to

count the number of UVLO(off). The latch is released if

the 2nd UVLO(off) point is counted then the output is

recovery to switching again.

By using such protection mechanism, the average input

power can be reduced to very low level so that the

component temperature and stress can be controlled within

the safe operating area.

On/Off Control

The LD7575 can be controlled to turn off by pulling COMP

pin to lower than 1.2V. The gate output pin of LD7575 will

be disabled immediately under such condition. The off mode

can be released when the pull-low signal is removed.

Dual-Oscillator Green-Mode Operation

There are many difference topologies has been

implemented in different chips for the green-mode or power

saving requirements such as “burst-mode control”,

“skipping-cycle Mode”, “variable off-time control “…etc. The

basic operation theory of all these approaches intended to

reduce the switching cycles under light-load or no-load

condition either by skipping some switching pulses or

reduce the switching frequency.

Over Load Protection (OLP)

To protect the circuit from the damage during over load

condition or short condition, a smart OLP function is

implemented in the LD7575. Figure 17 shows the

waveforms of the OLP operation. Under such fault

condition, the feedback system will force the voltage loop

toward the saturation and thus pull the voltage on COMP pin

(VCOMP) to high. Whenever the VCOMP trips the OLP

Fig. 17

OVP (Over Voltage Protection) on Vcc

The Vgs ratings of the nowadays power MOSFETs are most

with maximum 30V. To prevent the Vgs from the fault

condition, LD7575 is implemented an OVP function on Vcc.

Whenever the Vcc voltage is higher than the OVP threshold

voltage, the output gate drive circuit will be shutdown

simultaneous thus to stop the switching of the power

MOSFET until the next UVLO(on).

The Vcc OVP function in LD7575 is an auto-recovery type

protection. If the OVP condition, usually caused by the

feedback loop opened, is not released, the Vcc will tripped

Leadtrend Technology Corporation

LD7575-DS-04b November 2007

10

www.leadtrend.com.tw

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