8

FN9066.4

July 8, 2005

Drive Signals Timing Diagrams

Timing Diagram Descriptions

The two upper drivers (UPPER1 and UPPER2) are driven at

a fixed 50% duty cycle and the two lower drivers (LOWER1

and LOWER2) are PWM-controlled on the trailing edge,

while the leading edge employs resonant delay (T2 and T4).

In current mode control, the sensed switch (FET) current

and Leading Edge Blanking (LEB) circuits and then compared

to a control signal (EAO). Spikes, due to parasitic elements in

the bridge circuit, would falsely trigger the comparator

generating the PWM signal. To prevent false triggering, the

leading edge of the sensed current signal is blanked out by

T1, which can be programmed at the R_LEB pin with a

resistor. Internal switches gate the analog input to the PWM

comparator, implementing the blanking function that

eliminates response degrading delays which would be caused

if filtering of the current feedback was incorporated. The dead

time (T3 and T5) is the delay to turn on the upper FET

(UPPER1/UPPER2) after its corresponding lower FET

(LOWER1/LOWER2) is turned off when the bridge is

operating at maximum duty cycle in normal conditions, or is

responding to load transients or input line dipping conditions.

Therefore, the upper and lower FETs that are located at the

same side of the bridge can never be turned on together, which

eliminates shoot-through currents. SYNC1 and SYNC2 are the

gate control signals for the output synchronous rectifiers. They

are biased by VDD and are capable of driving capacitive loads

up to 20pF at 1MHz clock frequency (500kHz switching

frequency). External drivers with high current capabilities are

required to drive the synchronous rectifiers, cascading with

both synchronous signals (SYNC1 and SYNC2).

CLOCK

UPPER1

UPPER2

SYNC1

SYNC2

LOWER1

RAMP ADJUST

OUTPUT TO

PWM

LOGIC

T1

T2

T3

T4

T5

T1 = Leading edge blanking

T2 = T4 = Resonant delay

T3 = T5 = dead time

In the above figure, the values for T1 through T5 are exaggerated for demonstration purposes.

LOWER2

NOTES:

EAO

EAO

EAO

ISL6551