L6699D STMicroelectronics, L6699D Datasheet - Page 17

• 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

L6699
6.3
There are three contributors to T
●
●
●
It is important to point out that the value of T
is essentially tdet: therefore the minimum observable T
other hand, is counted starting from the negative-going edge of the gate-drive signal, so it
actually fixes a maximum limit for T
Finally, it is worth stating that the adaptive deadtime function does not significantly increase
efficiency by itself. It is a degree of freedom that must be exploited for this purpose when
designing the resonant tank. Essentially, it allows the use of a higher magnetizing
inductance in the transformer, which minimizes the magnetizing current and, then, the
conduction losses associated to it. Additionally, this may reduce the switched current I
the minimum required to achieve soft-switching, therefore reducing turn-off switching losses
in MOSFETs. Efficiency at medium and light load greatly benefits from this optimization.
Safe-start procedure
In the L6699 a new startup procedure, termed “safe-start”, has been implemented to
prevent loss of soft-switching during the initial switching cycles, which is not 100%
guaranteed by the usual soft-start procedure.
Sweeping the operating frequency from an initial high value, that should not exceed
300 kHz, down to the point where the control loop takes over, which is commonly referred to
as soft-start, has a twofold benefit. On the one hand, since the deliverable power depends
inversely on frequency, it progressively increases the converter's power capability, therefore
avoiding excessive inrush current. On the other hand, it makes the converter initially work at
frequencies higher than the upper resonance frequency of the LLC tank circuit, which
ensures inductive-mode operation (i.e. with the tank current lagging the square wave
voltage generated by the half bridge) and, therefore, soft-switching.
However, the last statement is true under a quasi-static approximation, i.e. when the
operating point of the resonant tank is slowly varying around a steady-state condition. This
approximation is not correct during the very first switching cycles of the half bridge, where
the initial conditions of the tank circuit can be away from those under steady-state.
Therefore, hard-switching is possible during the transient period needed to reach the slowly
varying steady-state condition dictated by the soft-start action. A non-zero initial voltage on
the resonant capacitor Cr and transformer flux imbalance during the previously mentioned
transient period are the possible causes of hard-switching in the initial cycles.
In high voltage half bridge controllers it is customary to start the switching activity by turning
on the low-side MOSFET for a preset time to pre-charge the bootstrap capacitor (see
Section 12: Bootstrap section
first cycle. In traditional controllers, normal switching starts right at the end of the pre-charge
time, as shown in the left-hand image in
The turn-off delay t
characteristics of the specific MOSFET and the speed its gate is driven
The transition time T
The detection time t
the gate-drive signal of the other MOSFET going high; this includes the detection time
as well as the propagation delay along the downstream logic circuitry up to the driver
output.
OFF
det
T
the half bridge midpoint takes for a rail-to-rail swing
that elapses from the end of the half bridge midpoint swing to
of the Power MOSFET, which depends on the input
) and ensure proper driving of the high-side MOSFET from the
Doc ID 022835 Rev 2
D
:
D
: T
D
Figure 10
≤ T
D_MAX
D_MIN
.
.
specified in the electrical characteristics
D
is always longer. T
Application information
D_MAX
, on the
S
17/38
to