ld7578 Leadtrend Technology, ld7578 Datasheet - Page 9

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

Application Information
Operation Overview
As long as the green power requirement becomes a trend
and the power saving is becoming more and more
important for switching power supplies and switching
adaptors, the traditional PWM controllers are not able to
support such new requirements. Furthermore, the cost
and size limitation forces the PWM controllers to
powerfully integrate more functions, thereby reducing the
external part count.
applications to provide an easy and cost effective solution;
and its detailed features are described as below.
Internal High-Voltage Startup Circuit and
Under Voltage Lockout (UVLO)
Traditional circuits power on the PWM controller through a
startup resistor to constantly provide current from a
rectified voltage to the capacitor connected to Vcc pin.
Nevertheless, this startup resistor was usually of larger
resistance, and it therefore required more power and
longer time to start up.
To achieve the optimized topology, as shown in figure 13,
The LD7578 is built in with high voltage startup circuit to
optimize the power saving. During the startup sequence, a
Leadtrend Technology Corporation
LD7578-DS-00 October 2008
The LD7578 is ideal for these
Fig. 13
www.leadtrend.com.tw
9
high-voltage current source sinks current from C
capacitor to provide the startup current as well as to
charge the Vcc capacitor C1. During the initialization of
the startup, Vcc voltage is lower than the UVLO(off)
threshold thus the current source is on to supply a current
of 1mA. Meanwhile, the Vcc current consumed by the
LD7578 is as low as 320μA thus most of the HV current is
utilized to charge the Vcc capacitor.
configuration, the turn-on delay time will be almost the
same no matter whether operation condition is under
low-line or high-line.
When Vcc voltage reaches UVLO(on) threshold, the
LD7578 is powered on to start issuing the gate drive
signal, the high-voltage current source is then disabled,
and the Vcc supply current will be only provided from the
auxiliary winding of the transformer.
power losses on the startup circuit beyond the startup
period can be eliminated and the power saving can be
easily achieved. In general application, a 39KΩ resistor is
still recommended to be placed in high voltage path to
limit the current if there is a negative voltage applying in
any case.
An UVLO comparator is included to detect the voltage on
the V
power on the LD7578 PWM controller and to drive the
power MOSFET. As shown in Fig. 14, a Hysteresis is
provided to prevent shutdown caused by the voltage dip
during startup. The turn-on and turn-off threshold levels
are set at 16V and 10.0V, respectively.
CC
pin to ensure the supply voltage is high enough to
LD7578
By using such
Therefore, the
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