ld7578 Leadtrend Technology, ld7578 Datasheet - Page 12

• Manufacturer Part Number: ld7578
• Description: Green-mode Pwm Controller With Hv Start-up Circuit And Otp Protection
• Manufacturer: Leadtrend Technology

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.
OVP (Over Voltage Protection) on Vcc
The V
mostly with 30V maximum. To prevent the V
condition, the LD7578 is implemented with over-voltage
protection on Vcc. After the Vcc voltage is higher than
OVP threshold voltage, the output gate drive circuit will be
shut down, thus stopping the switching of the power
MOSFET. As there is no power to maintain Vcc voltage,
it drops over time, and will turn on high voltage startup
circuit when it trips down to UVLO(
the power MOSFET will resume after Vcc voltage rises
back to UVLO(
The Vcc OVP function in the LD7578 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
Leadtrend Technology Corporation
LD7578-DS-00 October 2008
GS
ratings of the nowadays power MOSFETs are
ON
).
Fig. 17
OFF
). The switch of
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GS
from fault
12
Vcc waveform in Fig. 18 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.
(-)Latch
Protection
Protection
The heat generated under some fault conditions may
cause damage on components and the consequences
may be hazardous to the employed system or users. Thus,
over temperature protection (OTP) is required. The OTP
circuit can be implemented by using a NTC to sense a
hot-spot in power circuit like power MOSFET or output
rectifier. It can be easily achieved by connecting a NTC
to the (-)LATCH pin of LD7578.
temperature or ambient temperature rises high, the
resistance of the NTC decreases. So, the voltage on the
(-)LATCH pin could be read as below.
V
UVLO(off)
OVP Level
UVLO(on)
(
−
)
Latch
VCC
OUT
=
Switching
100
Pin
μ
A
(OTP)
⋅
R
OVP Tripped
NTC
and
Non-Switching
Fig. 18
---
Over
Latched
LD7578
Temperature
As the device
Switching
Mode
t
t