LD7577JAGR View Datasheet(PDF) - Unspecified

Part Name

Description

Manufacturer

LD7577JAGR

High Voltage Green-Mode PWM Controller with Brown-Out Protection

Unspecified 

LD7577JA

What the LD7577JA uses to implement the power-saving

operation is Leadtrend Technology’s own IP. By using this

dual-oscillator control, the burst -mode frequency can be

well controlled and further to avoid the generation of

audible noise.

Over Load Protection (OLP)

To protect the circuit from being damaged under over load

condition or short condition, a smart OLP function is built

in with the LD7577JA, as shown in Figure 18 shows the

waveforms of the OLP operation. If output voltage drops,

the feedback system tends to force the voltage loop

toward the saturation (5.8V) and then pulls the voltage of

COMP pin to high. Whenever the VCOMP trips to the OLP

threshold 5.0V and continues over 30mS, OLP 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 input

average power through 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 power circuit is recovered to switch again.

By using such protection mechanism, the average input

power can be reduced to a very low level so that the

component temperature and stress can be controlled

within the safe operating area.

VCC

UVLO(on)

UVLO(off)

OLP

2nd UVLO(off)

OLP Counter Reset

t

COMP

OLP Delay

Time

5.0V

OLP trip Level

t

OUT

Switching

Non-Switching

Switching

t

Fig. 18

OVP (Over Voltage Protection) on Vcc

The VGS ratings of the nowadays power MOSFETs are

mostly with 30V maximum. To prevent the VGS from fault

condition, the LD7577JA is implemented with over-voltage

protection on Vcc. As long as the Vcc voltage is higher

than OVP threshold voltage, the output gate drive circuit

will be shut down, thus to stop the switching of the power

MOSFET until the next UVLO(ON).

The Vcc OVP function in the LD7577JA is an

auto-recovery type protection. If the OVP condition,

usually caused by the feedback loop opened, is not

released, the Vcc will trip to the OVP level again,

re-shutting down the output. The Vcc waveform in Fig.

19 shows this auto-recovery type protection, presenting a

hiccup mode.

On the other hand, the removal of the OVP condition

should render the Vcc level back to normal level, causing

the output automatically returning to the normal operation.

Leadtrend Technology Corporation

LD7577JA-DS-00b August 2010

12

www.leadtrend.com.tw

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