ld7522 Leadtrend Technology, ld7522 Datasheet - Page 13

• Manufacturer Part Number: ld7522
• Description: Smart Green-mode Pwm Controller With Multiple Protections
• Manufacturer: Leadtrend Technology

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.
Over Voltage Protection (OVP)
The Vgs ratings of the nowadays power MOSFETs are most
with maximum 30V. To prevent the component damage
from the fault condition, LD7522 is implemented the
protection through the OVP pin. Figure 26 and figure 27
show 2 different configurations to programming the OVP
setting point --- zener detection and voltage divider. Figure
26 provided zero bias current under normal operation so
that it will not affect the startup timing. But the tolerance of
OVP trip point will be higher due to the distribution of the
breakdown voltage of zener diode.
On the other hand, the circuit of figure 27 will get the
benefits on the cost and that OVP accuracy but the value of
R1 and R2 must be very high to avoid affecting the startup
timing by the load effect.
As shown in figure 28, whenever the voltage on the OVP pin
is higher than the threshold voltage 2.5V, the output gate
drive circuit will be shutdown simultaneous thus to stop the
switching of the power MOSFET. Whenever the voltage
on the OVP pin gets back to lower than 2.5V, the output is
automatically returned to the normal operation on the next
UVLO(on) level.
Leadtrend Technology Corporation
LD7522-DS-03 January 2008
V(OVP)= Vz+2.5V
Vz
OVP
Fig. 26
Vbulk-cap
LD7522
VCC
GND
www.leadtrend.com.tw
13
(-)LATCH
Protection (OTP) --- Latched Mode Protection
Under some abnormal conditions, the ambient temperature
may be increased significantly and causes some damage on
the components or further inhibits the dangerous. To
prevent the power circuit damage from the system abnormal,
the OTP is required. The OTP circuit is implemented by
sensing the hot-spot of power circuit like power MOSFET or
output rectifier. It can be easily achieved by connect a
NTC on the (-)LATCH pin of LD7522. When the device
temperature or ambient temperature rises high, the
resistance of NTC will be decreased so that the voltage on
the (-)LATCH pin will be
V
(
−
)
LATCH
=
V
(
100
OVP
Pin
μ
)
A
=
2
×
5 .
R
V
NTC
⋅
and
1 (
+
Fig. 27
Fig. 28
R
R
2
1
)
Over
LD7522
Temperature