ld7552 Leadtrend Technology, ld7552 Datasheet - Page 8

• Manufacturer Part Number: ld7552
• Description: Green-mode Pwm Controller
• Manufacturer: Leadtrend Technology

basic operation theory of all these approaches intended to
reduce the switching cycles under light-load or no-load
condition either by skip some switching pulses or reduce the
switching frequency.
What LD7552 used to implement the power-saving
operation is Leadtrend Technology’s own IP
approach, as shown in the block diagram, 2 oscillators are
implemented in LD7552. The first oscillator is to take care
the normal switching frequency, which can be set by the RT
pin through an external resistor. Under this operation mode,
as shown in Fig. 10, the 2nd oscillation (green-mode
oscillator) is not activated. Therefore, the rising-time and the
falling-time of the internal ramp will be constant to achieve
good stability over all temperature range.
normal operation, this oscillator is dominated the switching
frequency.
As shown in Fig. 11, the green-mode oscillator detects the
Comp pin signal to determine if it is within the green-mode
operation. When the detected signal V+ is lower than the
green-mode threshold Vgreen, the green-mode oscillator is
on. The green-mode oscillator, implemented by a VCO
(voltage controlled oscillator), is a variable frequency
oscillator. The rising time of the VCO is proportional to
(Vgreen-V+), thus the lower voltage on V+ will generate
longer rising time on VCO as well as lower frequency on
VCO.
Leadtrend Technology Corporation
LD7552-DS-00 February, 2005
COMP
CS
Green-Mode
Osc
OSC
LEB
2R
R
EN=1
Fig. 10
+
Green-Mode Osc is not activated
under normal operation.
∑
V+
V-
+
PWM
Comparator
Set
Reset
Ramp from
Oscillator
S
R
EN
Q
2
.
Under the
In such
OUT
8
By using this dual-oscillator control, the green-mode
frequency can be well controlled and further to avoid the
generation of audible noise.
2
Oscillator activated
Ramp of
Note: Patent pending
Green-Mode
OSC
Green-Mode
V-
Oscillator
Ramp of
VCO
V+
Vgreen
from OSC
VCO
V+
Normal Mode
Fig. 11
(V+ -Vgreen) >=0, VCO disabled
(V+ -Vgreen) <0, VCO activated
Level-detector
"Set" Signal
(to OSC & Nor gate)
& Counter
Green Mode
LD7552
Vgreen