L6699D STMicroelectronics, L6699D Datasheet - Page 19

• 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
Application information
Figure 11. Comparison of initial cycles after startup: traditional controller (left), with
L6699 (right)
It goes without saying that when either MOSFET is turned on for the very first time, this
occurs with a non-zero drain-to-source voltage. Therefore, strictly speaking, hard-switching
is still there. However, this type of one-shot hard-switching, where the body diode of the
other MOSFET is not reverse recovered, is of little concern. In fact, the related capacitive
power loss is thermally insignificant and, with a proper gate-drive circuit, spurious turn-on of
the other MOSFET through Cgd injection is easily prevented.
The timing diagrams of
Figure 11
compare the startup behavior of a resonant converter
driven by a traditional resonant controller with that of a converter driven by the L6699.
During the initial phase, the ramps of the oscillator are synchronized to the zero-crossings of
the tank current, so that a trapezoidal waveform appears across CF. As a result, the duty
cycle of the half bridge is initially considerably less than 50% and the tank current changes
its sign every half-cycle. The device goes to normal operation after approximately 50 µs
Table 6
from the first switching cycle. If the timing capacitor CF is selected according to
, this
transition is nearly seamless and just a small perturbation of the tank current can be
observed.
Table 6
Using capacitor values significantly different from those provided in
might cause
large perturbations during the transition. This might bring the half bridge close to losing soft-
switching with a consequent activation of the capacitive-mode detection function.
With the L6699 the soft-start function is easily realized with the addition of an R-C series
circuit from pin 4 (
) to ground (see
Figure 13
).
RF
min
Initially, the capacitor C
is totally discharged, so that the series resistor R
is effectively
SS
SS
parallel with RF
and the resulting initial frequency is determined by R
and RF
only,
min
SS
min
as the optocoupler's phototransistor is cut off (as long as the output voltage is not too far
away from the regulated value):
Equation 4
1
f
=
start
(
)
· 3
CF
·
RF
//
R
min
SS
The C
capacitor is progressively charged until its voltage reaches the reference voltage (2
SS
V) and, consequently, the current through R
goes to zero. This conventionally takes 5
SS
times the constants R
·C
, however, the soft-start phase really ends when the output
SS
SS
voltage has got close to the regulated value and the feedback loop has taken over, so that
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