l6599atn STMicroelectronics, l6599atn Datasheet - Page 25

• Manufacturer Part Number: l6599atn
• Description: Improved High-voltage Resonant Controller
• Manufacturer: STMicroelectronics

L6599AT
6.7
the function is not used the pin must be connected to a voltage greater than 1.24 V but lower
than 6 V (worst-case value of the 7 V threshold).
Bootstrap section
The supply of the floating high-side section is obtained by means of bootstrap circuitry. This
solution normally requires a high voltage fast recovery diode (D
the bootstrap capacitor C
external diode. It is achieved by means of a high voltage DMOS, working in the third
quadrant and driven synchronously with the low side driver (LVG), with a diode in series to
the source, as shown in
Figure 14. Bootstrap supply: a) standard circuit;
The diode prevents any current flowing from the VBOOT pin back to V
quickly turned off when the internal capacitor of the pump is not fully discharged. To drive
the synchronous DMOS a voltage higher than the supply voltage V
voltage is obtained by means of an internal charge pump (
The bootstrap structure introduces a voltage drop while recharging CBOOT (i.e. when the
low side driver is on), which increases with the operating frequency and with the size of the
external power MOS. It is the sum of the drop across the R
across the series diode. At low frequency this drop is very small and can be neglected but,
as the operating frequency increases, it must be taken into account. In fact, the drop
reduces the amplitude of the driving signal and can significantly increase the R
external high-side MOSFET and then its conductive loss.
This concern applies to converters designed with a high resonance frequency (indicatively,
> 150 kHz), so that they run at high frequency also at full load. Otherwise, the converter runs
at high frequency at light load, where the current flowing in the MOSFETs of the half-bridge
leg is low, so that, generally, an R
point anyway and the following equation is useful to compute the drop on the bootstrap
driver:
b) internal bootstrap synchronous diode
Figure 14
BOOT
. In the L6599AT, a patented integrated structure replaces this
Doc ID 15534 Rev 1
(DS)ON
b.
rise is not an issue. However, it is wise to check this
Figure 14
(DS)ON
BOOT
and the forward drop
Application information
cc
b).
,
is necessary. This
Figure 14
cc
if the supply is
(DS)ON
a) to charge
of the
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