LED7707 STMicroelectronics, LED7707 Datasheet - Page 13

• Manufacturer Part Number: LED7707
• Description: LED Drivers 6-ROW 85mA LED DRIV BOOST REG LCD PANEL
• Manufacturer: STMicroelectronics

Specifications of LED7707

• High Level Output Current: 85 mA
• Operating Supply Voltage: 4.5 V to 36 V
• Maximum Supply Current: 1 mA
• Maximum Power Dissipation: 2.3 W
• Maximum Operating Temperature: + 150 C
• Mounting Style: SMD/SMT
• Package / Case: VFQFPN-24
• Minimum Operating Temperature: - 40 C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

LED7707
5.1.3
Figure 7.
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.
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
External sync waveforms
Soft-start sequence waveforms in case of floating rows
Figure 5
AVCC
AVCC
100%
100%
OVP
OVP
2.4V
2.4V
1.2V
1.2V
0.8V
0.8V
). As a consequence the error amplifier is unbalanced and the loop
95% of
95% of
OVP
OVP
220k
BAS69
100n
t
t
ss
ss
EN
DIM
Floating ROWs detection
Floating ROWs detection
Current limit
Current limit
LED7707
Output voltage
Output voltage
SGND
Protections turn active
Protections turn active
EN pin voltage
EN pin voltage
SS pin voltage
SS pin voltage
Nominal switching
Nominal switching
frequency release
frequency release
Operation description
t
t
AM00584v1
AM00585v1
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