ld7578h Leadtrend Technology, ld7578h Datasheet - Page 10

• Manufacturer Part Number: ld7578h
• Description: High Voltage Green-mode Pwm Controller With Over Temperature Protection
• Manufacturer: Leadtrend Technology

Application Information
Operation Overview
As long as the green power requirement becomes a trend
and the power saving is getting more and more important
for the switching power supplies and switching adaptors,
the traditional PWM controllers are not able to support
such new requirements. Furthermore, the cost and size
limitations force PWM controllers to be more powerful by
integrating more functions and, thus, reducing the
external part count.
application to provide an easy and cost effective solution.
Its detail features are described as below:
Internal High-Voltage Startup Circuit and
Under Voltage Lockout (UVLO)
Traditional circuit powers up the PWM controller through a
startup resistor to constantly provide startup current.
However, this startup resistor was usually of larger
resistance, and it therefore required more power and
longer time to start up. To achieve an optimized topology,
as shown in figure 13, the LD7578H was built in with high
voltage startup circuit to optimize the power saving.
During the startup sequence, a high-voltage current
source sinks current from Cbulk capacitor to provide the
Leadtrend Technology Corporation
LD7578H-DS-00 February 2010
The LD7578H designed for such
Fig. 13
www.leadtrend.com.tw
10
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 LD7578H is as low as 300μA thus most of
the HV current is utilized to charge the Vcc capacitor. By
using such configuration, the turn-on delay time will be
almost same no matter whether operation condition is
under low-line or high-line.
When
LD7578H is powered on to start issuing the gate drive
signal, the high-voltage current source is then disabled,
and the 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.
An UVLO comparator is included to detect the voltage on
the V
power on the LD7578H PWM controller and in addition to
drive the power MOSFET.
hysteresis is provided to prevent the 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
Vcc
voltage
reaches
As shown in Fig. 14, a
LD7578H
UVLO(on)
Therefore, the
threshold,