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LD7550

Green-Mode PWM Controller

Unspecified 

LD7550

reduce the switching cycles under light-load or no-load

condition either by skip some switching pulses or reduce the

switching frequency.

What LD7550 used to implement the power-saving

operation is Leadtrend Technology’s own IP2. In such

approach, as shown in the block diagram, 2 oscillators are

implemented in LD7550. 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. Under the

normal operation, this oscillator is dominated the switching

frequency.

COMP

CS

OSC

EN=1

Green-Mode Osc is not activated

under normal operation.

EN

Green-Mode

Osc

2R

R

LEB

Set

S

V+

Reset

V-

RQ

PWM

Comparator

+∑+

Ramp from

Oscillator

OUT

Fig. 10

(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.

By using this dual-oscillator control, the green-mode

frequency can be well controlled and further to avoid the

generation of audible noise.

from OSC

"Set" Signal

(to OSC & Nor gate)

Green-Mode

Oscillator

V+

V-

VCO

Level-detector

& Counter

Vgreen

(V+ -Vgreen) >=0, VCO disabled

(V+ -Vgreen) <0, VCO activated

V+

Normal Mode

Green Mode

Vgreen

Ramp of

OSC

Green-Mode

Oscillator activated

Ramp of

VCO

Fig. 11

2 Note: Patent pending

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

8

Leadtrend Technology Corporation

LD7550-DS-00 August 2004

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