L6566BH Datasheet, PDF (32/51 Pages) STMicroelectronics – Multimode controller for SMPS

Application information
L6566BH
Figure 20. Operation after latched disable activation: timing diagram
DIS
(pin 8)
4.5V
Vcc
(pin 5)
VccON
VccON -0.5
VccOFF
HV generator is turned on
Vccrestart
GD
(pin 4)
Vin
VHVstart
AC_OK
(pin 16)
Vth
Restart is quicker
t
Disable latch is reset here
HV generator turn-on is disabled here
Input source is removed here
t
t
t
t
AM11496v1
5.10
Soft-start and delayed latched shutdown upon overcurrent
At device startup, a capacitor (C_SS) connected between the SS pin (14) and ground is
charged by an internal current generator, I_SS1, from zero up to about 2 V where it is
clamped. During this ramp, the overcurrent setpoint progressively rises from zero to the
value imposed by the voltage on the VFF pin (15, see Section 5.6: PWM comparator, PWM
latch and voltage feedforward blocks); MOSFET conduction time increases gradually,
therefore controlling the startup inrush current. The time needed for the overcurrent setpoint
to reach its steady-state value, referred to as soft-start time, is approximately:
Equation 12
T_SS
=
Css
I_SS1
V_csx
(V_VFF
)
=
Css
I_SS1
⎜⎜⎝⎛¹
−
V_VFF
3
⎟⎟⎠⎞
During the ramp (i.e. until V_SS = 2 V) all the functions that monitor the voltage on the COMP
pin are disabled.
The soft-start pin is also invoked whenever the control voltage (COMP) saturates high,
which reveals an open loop condition for the feedback system. This condition very often
occurs at startup, but may be also caused by either a control loop failure or a converter
overload/short-circuit. A control loop failure results in an output overvoltage that is handled
by the OVP function of the L6566BH (see next section). In the case of QR operation, a
short-circuit causes the converter to run at a very low frequency, then with very low power
capability. This makes the self-supply system that powers the device unable to keep it
operating, so that the converter works intermittently, which is very safe. In the case of
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Doc ID 16610 Rev 2