ld7578j Leadtrend Technology, ld7578j Datasheet - Page 9

• Manufacturer Part Number: ld7578j
• 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 becoming more and more important
for the switching power supplies and switching adaptors,
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 counts. The
LD7578J targets on such applications, providing an easy
and cost effective solution; its detail 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 consumed more power and required longer time to
start up.
To achieve an optimized topology, as shown in Fig. 13, The
LD7578J is built in with high voltage startup circuit to
optimize the power saving. During the startup sequence, a
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, as the Vcc current consume
d by the LD7578J is as low as 320μA, most of the HV
current is utilized to charge the Vcc capacitor. By using
such 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
LD7578J is powered on to start issuing the gate drive signal,
the high-voltage current source is then disabled, and the
Leadtrend Technology Corporation
LD7578J-DS-00 March 2009
www.leadtrend.com.tw
BULK
9
Vcc supply current will be only provided from the auxiliary
winding of the transformer. Therefore, the 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 40KΩ 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 LD7578J PWM controller and to drive the
power MOSFET as well. As shown in Fig. 14, a Hysteresis
is provided to prevent the shutdown caused by the voltage
dip during startup.
levels are set at 16V and 10.0V, respectively.
CC
pin to ensure the supply voltage is high enough to
The turn-on and turn-off threshold
Fig. 13
LD7578J