ld7575b Leadtrend Technology, ld7575b Datasheet - Page 8

• Manufacturer Part Number: ld7575b
• Description: Green-mode Pwm Controller With High-voltage Start-up Circuit
• 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 limitation force
the PWM controllers need to be powerful to integrate more
functions to reduce the external part counts. The LD7575B
is targeted on such 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 provide the startup current. However, the
startup resistor consumes significant power which is more
and more critical whenever the power saving requirement is
coming tight.
very high resistance value. However, higher resistor value
will cause longer startup time.
To achieve an optimized topology, as shown in figure 13,
LD7575B implements a high-voltage startup circuit for such
requirement. During the startup, a high-voltage current
source sinks current from the bulk capacitor to provide the
startup current as well as charge the Vcc capacitor C1.
During the startup transient, the Vcc is lower than the UVLO
Leadtrend Technology Corporation
LD7575B-DS-00 May 2008
Vin
Cbulk
Theoretically, this startup resistor can be
Comp
LD7575B
HV
Fig. 13
R1
GND
VCC
OUT
CS
C1
D1
Rs
8
threshold thus the current source is on to supply a current
with 1mA. Meanwhile, the Vcc supply current is 500μA
thus half of the HV current is utilized to charge the Vcc
capacitor. Thus start-up time can be calculated as:
almost same no matter under low-line or high-line
conditions.
Whenever the Vcc voltage is higher than UVLO(on) to
power on the LD7575B and further to deliver the gate drive
signal, the high-voltage current source is off and the supply
current is provided from the auxiliary winding of the
transformer. Therefore, the power losses on the startup
circuit can be eliminated and the power saving can be easily
achieved.
An UVLO comparator is included to detect the voltage on
the Vcc pin to ensure the supply voltage enough to power
on the LD7575B PWM controller and in addition to drive the
power MOSFET.
provided to prevent the shutdown from the voltage dip
during startup. The turn-on and turn-off threshold level are
set at 16V and 10.0V, respectively.
By using such configuration, the turn-on delay time will be
C is capacitance value of C1 in uF
T
Start
As shown in Fig. 14, a hysteresis is
−
up
=
0.032C
LD7575B
Second