Operation description - boost section

LED7706

14/31

6.3

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 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 to its final value. In addition, the switching

frequency of the boost converter is reduced to half of the nominal value to avoid the

saturation of the inductor due to current runaway; the nominal switching frequency is

restored after the SS pin voltage has crossed 0.8 V.

Figure 6.

Soft-start sequence waveforms in case of floating rows

During the soft-start phase is also performed the floating rows detection. In presence of one

or more floating rows, the error amplifier is unbalanced and the output voltage increases;

when it reaches the Floating row Detection (FRD) threshold (95 % of the OVP threshold),

the floating rows are managed according to Table 7 (see Section 8 on page 25 ). After the

SS voltage reaches a 2.4 V threshold, the start-up finishes and all the protections turn

Equation 2

AVCC

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

2.3V

Nominal switching

frequency release

4

4

.

2

t

I

C

SS

SS

SS

⋅

≅