ld7577j Leadtrend Technology, ld7577j Datasheet - Page 12

• Manufacturer Part Number: ld7577j
• Description: High Voltage Green-mode Pwm Controller With Brown-out Protection
• Manufacturer: Leadtrend Technology

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 LD7577J, 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 V
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 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).
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.
Leadtrend Technology Corporation
LD7577J-DS-00b July 2009
COMP
www.leadtrend.com.tw
trips to the OLP
The
12
OVP (Over Voltage Protection) on Vcc
The V
mostly with 30V maximum. To prevent the V
condition, the LD7577J 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(
The Vcc OVP function in the LD7577J 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.
UVLO(off)
UVLO(on)
5.0V
GS
VCC
COMP
OUT
Switching
ratings of the nowadays power MOSFETs are
OLP
OLP Delay
Time
Non-Switching
OLP Counter Reset
Fig. 18
2nd UVLO(off)
OLP trip Level
ON
).
LD7577J
Switching
GS
from fault
t
t
t