LED7707 STMicroelectronics, LED7707 Datasheet - Page 21

• 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
5.2.2
5.3
PWM dimming
The brightness control of the LEDs is performed by a pulse-width modulation of the rows
current. When a PWM signal is applied to the DIM pin, the current generators are turned on
and off mirroring the DIM pin behavior. Actually, the minimum dimming duty-cycle depends
on the dimming frequency.
The real limit to the PWM dimming is the minimum on-time that can be managed for the
current generators; this minimum on-time is approximately 10 μs.
Thus, the minimum dimming duty-cycle depends on the dimming frequency according to the
following formula:
Equation 10
For example, at a dimming frequency of 1 kHz, 1% of dimming duty-cycle can be managed.
During the off-phase of the PWM signal the boost converter is paused and the current
generators are turned off. The output voltage can be considered almost constant because of
the relatively slow discharge of the output capacitor. During the start-up sequence (see
Section 5.1.3 on page 13
regardless of the applied dimming signal.
Figure 15. PWM dimming waveforms
Fault management
The main loop keeps the row having the lowest voltage drop regulated to about 700 mV.
This value slightly depends on the voltage across the remaining active rows. After the soft-
start sequence, all protections turn active and the voltage across the active current
generators is monitored to detect shorted LEDs.
) the dimming duty-cycle is forced to 100% to detect floating rows
D
DIM
,
min
=
10
μ
s
⋅
f
DIM
Operation description
10µs minimum on-time
10µs minimum on-time
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