L6699D STMicroelectronics, L6699D Datasheet - Page 18

• Manufacturer Part Number: L6699D
• Description: AC/DC Switching Converters Enhanced High Volt Res CTRL 600V
• Manufacturer: STMicroelectronics
• Type: High Voltage Controllersr

Specifications of L6699D

• Product Category: AC/DC Switching Converters
• Output Voltage: 13.3 V
• Input / Supply Voltage (max): 8.85 V
• Input / Supply Voltage (min): 16 V
• Switching Frequency: 235 kHz
• Supply Current: 3 mA
• Operating Temperature Range: - 25 C to + 125 C
• Mounting Style: SMD/SMT
• Package / Case: SO-16
• Number Of Outputs: 1
• Output Current: 800 mA

Application information
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Figure 10. Comparison startup behavior: traditional controller (left), with L6699
A non-zero initial voltage on the resonant capacitor may cause the very first turn-on of the
high-side MOSFET to occur with non-zero drain-to-source voltage while the body diode of
the low-side MOSFET is conducting, therefore invoking its reverse recovery. More hard-
switching cycles may follow (see the left-hand image in
potentially hazardous: they could cause the destruction of both MOSFETs, should the
resulting dv/dt across the low-side MOSFET exceed its maximum rating (see
Capacitive-mode detection function
To prevent this hard-switching cycle(s) with body diode reverse recovery, the L6699 waits
about 50
Figure 10
zero in case of an initially charged resonant capacitor. On the other hand, it is too short for
the bootstrap capacitor to be significantly discharged.
To understand the origin of transformer flux imbalance it is worth remembering that the half
bridge is driven with 50% duty cycle, so that under steady-state conditions the voltage
across the resonant capacitor Cr has a DC component equal to Vin/2. Consequently, the
transformer's primary winding is symmetrically driven by a ± Vin/2 square wave.
At startup, however, the voltage across Cr is often quite different from Vin/2, so it takes
some time for its DC component to reach the steady-state value Vin/2. During this transient,
the transformer is not driven symmetrically and, then, there is a significant V·s imbalance in
two consecutive half-cycles. If this imbalance is large, there is a significant difference in the
up and down slopes of the tank current and, the duration of the two half-cycles being the
same, the current may not reverse in a switching half-cycle, as shown in the left-hand image
in
is conducting and this may happen for a few cycles.
To prevent this, the L6699 is provided with a proprietary circuit that modifies the normal
operation of the oscillator during the initial switching cycles, so that the initial V·s unbalance
is nearly eliminated. Its operation is such that current reversal in every switching half-cycle
and, then, soft-switching is ensured.
Figure 11
µ
). This idle time is normally long enough to let the tank current decay to essentially
s after the pre-charge time before starting switching (see the right-hand image in
. Once again, one MOSFET can be turned on while the body diode of the other
(right)
Doc ID 022835 Rev 2
for more details).
Figure 10
). These events are few but
Section 9:
L6699