L6699DTR STMicroelectronics, L6699DTR Datasheet - Page 21

• Manufacturer Part Number: L6699DTR
• Description: AC/DC Switching Converters Enhanced High Volt Res CTRL 600V
• Manufacturer: STMicroelectronics

Specifications of L6699DTR

• Product Category: AC/DC Switching Converters

L6699
7
Operation at no load or very light load
When the resonant half bridge is lightly loaded or totally unloaded, its switching frequency
reaches its maximum value. To keep the output voltage under control and avoid losing soft-
switching in these conditions, there must be some current flowing through the transformer's
magnetizing inductance. This current can be kept relatively low because of the adaptive
deadtime function; however, it produces power losses that prevent the converter's no load
consumption from achieving very low values anyhow.
To overcome this issue, the L6699 enables the user to make the converter operate
intermittently (burst-mode operation), with a series of a few switching cycles spaced out by
long idle periods where both MOSFETs are in the OFF-state, so that the average switching
frequency can be substantially reduced. As a result, the average value of the residual
magnetizing current and the associated losses is considerably cut down, therefore
facilitating the converter to comply with energy saving specifications.
The L6699 can be operated in burst-mode by using pin 5 (STBY): if the voltage applied to
this pin falls below 1.26 V, the IC enters an idle state where both gate-drive outputs are low,
the oscillator is stopped, the soft-start capacitor C
reference at the
discharge. The IC resumes normal operation as the voltage on the pin exceeds 1.26 V by 30
mV.
In the L6699, the half bridge stops and restarts during burst-mode are more accurately
controlled with respect to its predecessors: the last cycle before stopping and the first cycle
after restarting are such that the DC voltage across the resonant capacitor Cr always stays
close to its steady-state value Vin/2. In this way, the anomalous current peaks due to V·s
unbalance in the transformer are minimized.
To implement burst-mode operation, the voltage applied to the STBY pin needs to be related
to the feedback loop.
input voltage range (e.g. when there is a PFC front-end).
Essentially, RF
mode operation. Once f
Figure 14. Narrow input voltage range
max
RF
defines the switching frequency f
min
Figure 14
pin stays alive to minimize IC consumption and V
max
is fixed, RF
Doc ID 022835 Rev 2
shows the simplest implementation, suitable to a narrow
max
is found from the relationship:
Figure 15. Wide input voltage range
SS
Operation at no load or very light load
max
keeps its charge and only the 2V
above which the L6699 enters burst-
CC
capacitor
21/38