Datasheet

LED7707 Operation description
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Figure 7. External sync waveforms
5.1.3 Soft-start
The soft-start function is required to perform a correct start-up of the system, controlling the
inrush current required to charge the output capacitor and to avoid output voltage overshoot.
The soft-start duration is set connecting an external capacitor between the SS pin and
ground. This capacitor is charged with a 5 μA (typ.) constant current, forcing the voltage on
the SS pin to ramp up. When this voltage increases from zero to nearly 1.2 V, the current
limit of the power MOSFET is proportionally released from zero to its final value. However,
because of the limited minimum on-time of the switching section, the inductor might saturate
due to current runaway. To solve this problem the switching frequency is reduced to one half
of the nominal value at the beginning of the soft-start phase. The nominal switching
frequency is restored after the SS pin voltage has crossed 0.8 V.
Figure 8. Soft-start sequence waveforms in case of floating rows
During the soft-start phase the floating rows detection is also performed. In presence of one
or more floating rows, the voltage across the involved current generator drops to zero. This
voltage becomes the inverting input of the error amplifier through the minimum voltage drop
selector (see
Figure 5). As a consequence the error amplifier is unbalanced and the loop
DIM
EN
SGND
LED7707
220k
100n
BAS69
AM00584v1
AM00585v1
AVCC
t ss
SS pin voltage
t
Protections turn active
1.2V
0.8V
Current limit
EN pin voltage
100%
Output voltage
OVP
95% of
OVP
Floating ROWs detection
2.4V
Nominal switching
frequency release
AVCC
t ss
SS pin voltage
t
Protections turn active
1.2V
0.8V
Current limit
EN pin voltage
100%
Output voltage
OVP
95% of
OVP
Floating ROWs detection
2.4V
Nominal switching
frequency release