Download Datasheet

LED7708
16 channels x 85 mA LED driver with boost controller and 4-wire
serial interface
Datasheet —production data
Features
■
■
Boost controller section
– 3.6 V to 36 V input voltage range (LDO)
– Adaptive output voltage for high efficiency
– Internal +5 V LDO for gate driver supply
– Internal +3.3 V LDO for device supply
– High performance external MOSFET driver
– 250 kHz to 1 MHz switching frequency
– Programmable OV and OC protection
– Fixed frequency peak current-mode control
– External synchronization for multi-device
application
– Overtemperature alert and thermal
shutdown
LED array driver section
– 16 channels with 85 mA/ch current
capability
– ±2% channel current accuracy
– ±2% channel-to-channel current matching
– LED short-circuit and open channel fault
detection and management
– 4-wire, 30 MHz serial interface
– 16 x16-bit, 1x256-bit or 1x192-bit serial
data formats
– Grouped or independent channel PWM
control
– Selectable 12/16-bit grayscale brightness
control for local dimming
– Programmable internal dimming oscillator
– Programmable grayscale latency
– Master/slave chain configuration supported
Applications
6&1&0.XX,
PITCH
Description
The LED7708 has been specifically designed to
supply several LEDs starting from a single lowvoltage rail. It integrates a boost controller,
sixteen current generators and a 4-wire serial
interface. The boost controller regulates the
output voltage in an adaptive way, according to
the LED requirements, resulting in an improved
overall efficiency. All the current generators are
40 V-rated, allowing the LED7708 to drive several
LEDs in series on each channel. The channels
can be put in parallel for higher output current.
The brightness of the LEDs is controlled by using
the serial interface. A selectable 12-bit or 16-bit
grayscale brightness control allows independent
PWM on each channel. A programmable on-chip
dimming oscillator is provided for external circuitry
simplification. The device has dedicated pins to
lock to an external synchronization with other
devices (master or slave) for noise reduction in
multi-device applications. The LED7708
implements basic protection (OVP, OCP and
thermal shutdown) as well as LED array
protection. It can detect and manage open-LED
and shorted-LED faults and different faultmanagement options are available in order to
cover most application needs.
Table 1.
■
TV and monitor backlight units for LCD panels
■
Medium and large size LCD panel backlights
■
RGB/RGGB backlight solutions
Order codes
LED7708
LED7708TR
May 2012
This is information on a product in full production.
Device summary
Doc ID 022698 Rev 1
Package
VFQFPN-48 7x7
(exposed pad)
Packing
Tube
Tape and
reel
1/53
www.st.com
53
Contents
LED7708
Contents
1
Typical application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2
Pin function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3
Absolute maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4
Recommended operating conditions . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5
Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
6
Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7
Device description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
7.1
8
2/53
Device supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Recommended operating conditions . . . . . . . . . . . . . . . . . . . . . . . . . . 19
8.1
Device power-up and soft-start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
8.2
Device shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
8.3
Boost controller section and output voltage control . . . . . . . . . . . . . . . . . 21
8.3.1
Output voltage optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
8.3.2
Boost converter loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
8.3.3
Slope compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
8.3.4
External MOSFET control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
8.4
Current generators section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
8.5
Serial interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
8.5.1
Serial interface architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
8.5.2
Serial interface internal registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
8.5.3
Internal registers access . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
8.5.4
Brightness data formats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
8.5.5
Brightness registers (BRTxx) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
8.5.6
DEVCFG0 register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
8.5.7
DEVCFG1 register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
8.5.8
GSLAT register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
8.5.9
CHSEL register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
Doc ID 022698 Rev 1
LED7708
Contents
8.6
8.7
8.8
8.5.10
CHSTA register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
8.5.11
Internal registers initialization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Dimming options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
8.6.1
Internal dimming oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
8.6.2
Dimming waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
8.6.3
Grayscale synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
Fault management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
8.7.1
Overvoltage protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
8.7.2
Overtemperature alert and thermal protection . . . . . . . . . . . . . . . . . . . . 44
8.7.3
Overcurrent protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
8.7.4
LED short-circuit and open channel faults . . . . . . . . . . . . . . . . . . . . . . . 44
Special function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
8.8.1
Current generators gain adjust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
8.8.2
Continuous status reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
9
Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
10
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
Doc ID 022698 Rev 1
3/53
List of tables
LED7708
List of tables
Table 1.
Table 2.
Table 3.
Table 4.
Table 5.
Table 6.
Table 7.
Table 8.
Table 9.
Table 10.
Table 11.
Table 12.
Table 13.
Table 14.
Table 15.
Table 16.
Table 17.
Table 18.
Table 19.
Table 20.
Table 21.
Table 22.
Table 23.
Table 24.
Table 25.
Table 26.
4/53
Device summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Absolute maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Thermal data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Recommended operating conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Recommended operating conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Output voltage states during regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Handshake signals summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
LED current vs. external resistor setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Internal registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Internal register operations encoding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Brightness data formats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
DEVCFG0 register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
DEVCFG1 register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Available LED short detection thresholds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
CHSEL register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
CHSTA register. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Default configuration of internal registers after POR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
FOSC pin setting options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Function of pins involved in dimming control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Dimming synchronization control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Detected faulty conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
XFAULT pin behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
VFQFPN48 (7 x 7 x 1.0 mm.) package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . 49
Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
Doc ID 022698 Rev 1
LED7708
List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
Figure 18.
Figure 19.
Figure 20.
Figure 21.
Figure 22.
Basic application circuit schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Pin connection (through top view) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Simplified block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Soft-start waveforms (standalone operation) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Simplified output regulation circuitry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Effect of slope compensation on small inductor current perturbation (D>0.5) . . . . . . . . . . 26
Simplified serial interface block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Timing diagram related to a DEVCFGx write operation . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
16-bit data format, write operation to the brightness register . . . . . . . . . . . . . . . . . . . . . . . 31
256-bit data format, write operation to the brightness registers . . . . . . . . . . . . . . . . . . . . . 32
192-bit data format, write operation to the brightness registers . . . . . . . . . . . . . . . . . . . . . 32
Dimming signals and synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Normal timing mode waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
Inverted timing mode waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
Dimming waveform and GSSY output pulse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
Dimming waveform triggered by GSSY input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
Internal signals connected to the XFLT pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
Channels current gain adjust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
Continuous reading mode disabled, 1x256-bit data format . . . . . . . . . . . . . . . . . . . . . . . . 46
Continuous reading mode enabled, 1x256-bit data format . . . . . . . . . . . . . . . . . . . . . . . . . 47
VFQFPN48 (7 x 7 x 1.0 mm.) package outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
Recommended PCB footprint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
Doc ID 022698 Rev 1
5/53
Typical application circuit
1
LED7708
Typical application circuit
Figure 1.
Basic application circuit schematic
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6/53
Doc ID 022698 Rev 1
LED7708
Pin function
Pin function
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Pin connection (through top view)
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6).
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Pin description
Pin
Function
VSI
Serial interface (SI) core and digital I/O buffer supply input. Connect to LDO3 or to an
external supply (3 V-5 V). Insert a 100 nF bypass ceramic capacitor between this pin and
SGND. Connect to the host controller’s supply when possible.
2
GSSY
Grayscale synchronization output or VSYNC input. When the internal dimming generation
is set, a pulse at the end of each dimming cycle is provided for data stream
synchronization. Also used as dimming synchronization input to lock the dimming cycles
on the vertical sync pulses.
3
DIN
4
DOUT
Serial interface data out.
5
DCLK
Serial interface data clock.
6
LE
7
GSCK
Grayscale clock I/O. When the internal dimming oscillator is used, the grayscale clock is
provided at this pin. The pin becomes the grayscale clock input if the internal dimming
oscillator is disabled (see FOSC pin).
8
XFLT
Fault signal, open drain output. The pin goes low when a faulty condition occurs.
9
IOK
Channel current OK handshake I/O. Used for extended output voltage regulation in multidevice applications. Used by slave devices to request a higher output voltage to the
master. A 100 k pull-up resistor to LD03 is required.
10
XVOK
Channel voltage OK I/O (inverted). Used for extended output voltage regulation in multidevice applications. Used by slave devices to request an output voltage reduction to the
master. A 100 k pull-up resistor to LD03 is required.
1
Serial interface data input.
Serial interface latch enable.
Doc ID 022698 Rev 1
7/53
Pin function
Table 2.
LED7708
Pin description (continued)
Pin
Function
XOVF
Internal DAC overflow I/O. Used for extended output voltage regulation and fault
management in multi-device applications. It is used to inform slave devices that the
maximum output voltage has been reached. A 100 k pull-up resistor to LD03 is required.
12
XMSK
Fault detection masking I/O. This signal is used for extended output voltage regulation &
fault management in multi-device applications. It is used to avoid incorrect fault detection
by forcing slave devices to ignore LED-short detection during output voltage steering. A
100 k pull-up resistor to LD03 is required.
13
FSW
Switching frequency selection / synchronization input. Used to set the desired switching
frequency of the boost controller. Also used as synchronization input.
14
SYNC
Boost converter synchronization output. The buffered switching clock signal is provided at
this pin to eventually synchronize other SMPS in the host system.
15
FOSC
Dimming oscillator frequency. A resistor to ground sets the frequency of the internal
grayscale clock dimming oscillator. If set high, the internal dimming oscillator is disabled
and the GSCK pin becomes the clock input (see GSCK pin).
16
CLIM
Boost section current limit setting. A resistor between this pin and SGND sets the boost
converter current limit.
17
SLOPE
Slope compensation setting. A resistor between this pin and SGND is required to set the
proper amount of slope compensation to avoid sub-harmonic instability.
18
COMP
Transconductance amplifier output. Connect a simple RC series between this pin and
SGND to properly compensate the loop-gain of the boost converter.
19
ISETH
Output current setting (on). Connect a resistor between this pin and SGND to set the
current sunk by each channel during the on-phase of the dimming.
20
ISETL
Output current setting (off). Connect a resistor between this pin and SGND to set the
current sunk by each channel during the off-phase of the dimming (LED biasing). Connect
to LDO3 to disable the bias current.
21
SGND
Signal ground. Common return for signals and settings.
22
AGND
Analog Ground. Common return for the current generators (channels).
23
CH1
Driver output (channel) #1.
24
CH2
Driver output (channel) #2.
25
CH3
Driver output (channel) #3.
26
CH4
Driver output (channel) #4.
27
CH5
Driver output (channel) #5.
28
CH6
Driver output (channel) #6.
29
CH7
Driver output (channel) #7.
30
CH8
Driver output (channel) #8.
31
CH9
Driver output (channel) #9.
32
CH10
Driver output (channel) #10.
33
CH11
Driver output (channel) #11.
34
CH12
Driver output (channel) #12.
35
CH13
Driver output (channel) #13.
11
8/53
Doc ID 022698 Rev 1
LED7708
Pin function
Table 2.
Pin description (continued)
Pin
Function
36
CH14
Driver output (channel) #14.
37
CH15
Driver output (channel) #15.
38
CH16
Driver output (channel) #16.
39
AGND
Analog ground. Common return for the current generators (channels).
40
VMIN
Minimum output voltage setting. Multi-level input (high, low, floating or resistor to SGND).
Used in conjunction with the VFB pin to set the best output voltage range swing for a given
LED array. See related section for further details.
41
CSNS
External MOSFET current sense input. Connect to the sensing resistor in series with the
power switch (source of the power MOSFET node).
42
PGND
Power ground. Reference for the external MOSFET current sensing circuit and return for
the gate driver.
43
GATE
External MOSFET gate driver output. Connected to the gate of the power MOSFET.
44
VDR
Gate driver supply. Internally connected to the 5 V LDO regulator. Bypass with a 1 µF
ceramic capacitor to PGND as close as possible to the chip.
45
LREN
3.3 V linear regulator enable. When high or floating, the internal 3.3 V linear regulator is
used to supply the device. Also used to turn off the device for power consumption
reduction. To be left floating the internal 3.3 V LDO is overdriven by an external 3.3 V
supply rail.
46
VIN
Input of the LDO linear regulators. Bypass with a 1µF ceramic capacitor to ground as close
as possible to the chip.
47
LDO3
3.3 V device supply and internal 3.3 V linear regulator output. Bypass with a 1 µF ceramic
capacitor to ground as close as possible to the chip.
48
VFB
Output voltage feedback for the boost controller. Connect to the central tap of the output
resistor divider. See the related section for details.
Doc ID 022698 Rev 1
9/53
Absolute maximum ratings
3
LED7708
Absolute maximum ratings
Table 3.
Absolute maximum ratings
Symbol
Parameter
Min.
Max.
VIN,
CH1 to CH16
-
40
LDO3
-
VIN+0.3
3.6
COMP, SYNC,
ISETH, ISETL,
SLOPE, CLIM,
FOSC, VFB, XOVF,
VMIN
-
LDO3+0.3
3.6
VSI
-
6
DCLK, LE, DIN,
GSCK, DCLK,
DOUT, GSSY
-
VSI+0.3
6
IOK, XMASK, XVOK,
XFLT, FSW, LREN,
-
6
VDR
-
6
VIN+0.3
GATE
-
VDR+0.3
6
CSNS
-
LDO3+0.3
3.6
Continuous channel
current
CH1 to CH16
-
90
mA
Maximum current
generator power
dissipation
CH1 to CH16
-
0.5
W
ESD Rating
Human body model
JEDEC JESD22A114
-
2
kV
Maximum pin voltage
ESD
Table 4.
Pin
V
Thermal data
Symbol
Parameter
Conditions
Min.
Max.
TA = 25 °C
PD
Unit
Power
dissipation(1)
Unit
W
TA = 50 °C
TA = 85 °C
TJ,OP
10/53
Operating junction temperature
Doc ID 022698 Rev 1
-40
150
°C
LED7708
Absolute maximum ratings
Table 4.
Symbol
Thermal data (continued)
Parameter
TSTG
Storage temperature range
Rth,JA
Junction-ambient thermal
resistance
Conditions
Min.
Max.
-50
150
30
Unit
°C/W
1. Device soldered to the STEVAL-ILL035V1 demonstration board.
Doc ID 022698 Rev 1
11/53
Recommended operating conditions
LED7708
4
Recommended operating conditions
Table 5.
Recommended operating conditions
Symbol
Parameter
Conditions
Min.
Max.
3.6
36
Unit
DC Characteristics
VVIN
VIN pin input voltage range
VLDO3
LDO pin input voltage range
VIN, LDO3 and VDR
shorted together
3.0
3.6
VVDR
VDR pin input voltage range
VDR and VIN shorted
together
3.0
5.5
VCHx
Channel voltage range
VVSI
Serial interface supply voltage
3.0
5.5
ICHx
Channel continuous current
20
85
mA
Channel current when OFF
(adjustable)
2
200
µA
V
36
AC Characteristics (VSI = 3.3 V)
fGSC
Grayscale clock frequency
25
fDCLK
Serial clock frequency
30
MHz
Boost converter switching frequency
200
Tw,DCLK
DCLK pulse width
20
Tw,GSC
GSC pulse width
20
Tw,LE
LE pulse width
15
Tsu,DIN
DIN setup time
5
Th,DIN
DIN hold time
5
Tsu,LE
LE setup time
5
Th,LE
LE hold time
5
fSW
12/53
Doc ID 022698 Rev 1
1000
kHz
ns
LED7708
5
Electrical characteristics
Electrical characteristics
(VIN = 12 V; TAMB = 25 °C and VSI connected to LDO3 if not otherwise specified.)
Table 6.
Electrical characteristics
Symbol
Parameter
Conditions
Min.
Typ.
Max.
Unit
1.4
V
Supply section
LREN turn-on threshold
LREN turn-off threshold
1
µA
LREN pull-up current
VUVLO,ON
LDO3 undervoltage lockout
upper threshold
VUVLO,OFF
LDO3 undervoltage lockout
lower threshold
VVSI,ON
VSI turn-on threshold
VVSI,OFF
VSI turn-off threshold
VLDO3
2.9
2.6
3.6 V ≤VVIN ≤28 V
ILDO3=0 mA, DEN=0
3.3 V LDO load regulation
VVIN=3.6 V, DEN=0
0 mA ≤ILDO3 ≤40 mA
3.0
2.7
3.0
3.3 V LDO output voltage
V
3.1
2.6
2.7
3.2
3.3
3.4
30
100
mV
ILDO3=40 mA, VLDO3=3.2 V
180
270
5 V LDO output voltage
6 V ≤VVIN ≤28 V
IVDR=0 mA, DEN=1
4.5
5.0
5.5
V
5 V LDO load regulation
VVIN=6 V, DEN=1
0 mA ≤IVDR≤40 mA
60
200
mV
3.3 V LDO drop-out voltage
VVDR
3
5 V LDO drop-out voltage
IVDR =40 mA, VVDR=4.5 V
150
200
mV
Boost controller
tON,min
Minimum switching on-time
Default switching frequency
fSW
KFSW
IGATE,Pk
tr,GATE
tf,GATE
FSW to LDO3
Gate driver current capability
Gate driver rise time
(10 to 90%)
Gate driver fall time
(90 to10%)
170
610
670
ns
kHz
Synchronization input
frequency
Switching frequency
constant
550
100
180
RFSW = 100 kΩ
4.4
x1010
1020
5.0
x1010
CL=3.3 nF, sourcing
1.9
CL=3.3 nF, sinking
2.1
5.6
x1010
Hz • Ω
A
9
CL=3.3 nF
ns
10
Doc ID 022698 Rev 1
13/53
Electrical characteristics
Table 6.
LED7708
Electrical characteristics (continued)
Symbol
Parameter
Conditions
Min.
Typ.
FSW synchronization input
low level
Max.
Unit
0.5
V
FSW synchronization input
high level
2.2
SYNC output duty cycle
FSW connected to LDO3
25
SYNC output high level
ISYNC=1 mA
VLDO3
-0.3 V
30
35
%
V
SYNC output low level
ISYNC=-1 mA
Adjustable power MOSFET
current sensing voltage
RCLIM=100 kΩ
100
RCLIM=300 kΩ
300
0.4
mV
Power consumption
IVIN1
Shutdown current (SI off,
LED drivers off, SMPS off,
LDOs off)
IVIN2
Shutdown current (SI active,
LED drivers off, SMPS off,
LDOs on)
LREN floating VSI to LDO3
DEN=0
300
IVIN3
Quiescent current (LED
drivers off, boost section not
switching)
LREN floating; VSI, FOSC
and ISETL to LDO3; DIN,
DCLK, LE, GSCK and GSSY
to GND; VCHx=2 V;
RISETH=60 kΩ
4
IVIN4
Operating current (serial
interface active, boost not
switching)
LREN floating VSI and FOSC
to LDO3, DIN and DCLK
toggling at 15 MHz, DOUT,
GSSY and GSCK floating, LE
to GND. VCHx=2 V,
RISETH=60 kΩ
7
VSI = 3.3 V
7
IVSI1
Standby current (DIN, DCLK
and LE to SGND, LED
drivers off, SMPS off, LREN
floating)
VSI = 5.0 V
9
LREN low, VSI low
30
µA
450
mA
µA
OV protection, output regulation
VFB,OVP
VFB,OVP
14/53
Overvoltage protection
reference threshold
VMIN tied to GND
1.275
1.30
1.325
VMIN to LDO3
1.105
1.13
1.155
RVMIN=220 kΩ
0.985
1.01
1.035
VMIN floating
0.765
0.8
0.815
OVP hysteresis
V
6
Doc ID 022698 Rev 1
mV
LED7708
Table 6.
Electrical characteristics
Electrical characteristics (continued)
Symbol
Parameter
Maximum internal reference
voltage for output regulation
Conditions
Min.
Typ.
Max.
VMIN to GND
1.136
1.161
1.186
VMIN to LDO3
0.967
0.992
1.017
RVMIN=220 kΩ
0.847
0.872
0.897
VMIN floating
0.630
0.655
0.680
VMIN to GND
0.836
0.861
0.886
VMIN to LDO3
0.667
0.692
0.717
RVMIN=220 kΩ
0.547
0.572
0.597
VMIN floating
0.330
0.355
0.380
±2%
±3%
Unit
V
VDAC,FB
Minimum internal reference
voltage for output regulation
Current generators
ΔICHxn
Maximum output current
accuracy with respect to
nominal value(1)
ΔICHxy
Maximum channel-tochannel output current
mismatch(1)
(all channels active)
VCHx
Minimum master channel
feedback voltage
VRW
Regulation window
amplitude
ICHx,OFF1
Off-state channels current
ICHx,OFF2
VTH,ISETL
VCHx=0.6 V,
20 mA ≤ ICHx ≤85 mA
VCHx=0.6 V, RISETH=60 kΩ
RISETH=60 kΩ, CRS=0
500
600
RISETH=15 kΩ, CRS=1
700
800
190
250
310
RISETH=60 kΩ, RWAS=1
400
500
600
VCHx=10 V, RISETL=50 kΩ
70
80
90
1
2
KISETL
Output current low-state
constant
µA
VCHx=10 V, ISETL to LDO3
VLDO3-
ICHx, OFF2
Output current high-state
constant
mV
RISETH=60 kΩ, RWAS=0
ISETL input threshold
KISETH
±2%
(ICHx=20 mA)
V
1
20 mA ≤ ICHx ≤85 mA
1164
1200
1236
3.5
4.0
4.5
Serial interface
DIN, DCLK, LE, GSCK,
GSSY low level input
threshold
1.32
VSI=LDO3=3.3 V
DIN, DCLK, LE, GSCK,
GSSY high level input
threshold
1.6
V
DOUT and GSSY output low
level
VSI=LDO3=3.3 V
ILOAD=-200 µA
DOUT and GSSY output
high level
VSI=LDO3=3.3 V
ILOAD=200 µA
Doc ID 022698 Rev 1
1.22
1.75
15/53
Electrical characteristics
Table 6.
LED7708
Electrical characteristics (continued)
Symbol
Parameter
Conditions
Min.
Typ.
Max.
Unit
DTS0=1&
DTS1=0
3.0
3.2
3.4
DTS0=0&
DTS1=1
6.2
6.4
6.6
DTS0=1&
DTS1=1
7.7
8.0
8.3
200
220
mV
4 x1011
4.14
x1011
Hz•Ω
Fault management
VCHx,FLT1
VCHx,FLT2
LED short-circuit detection
threshold
VCHx,FLT3
XFLT output low level
IXFLT, SINK=4 mA
V
Dimming oscillator
KOSC
Internal dimming oscillator
constant
FGSCK,OSC
Internal dimming oscillator
frequency
RFOSC=33.2 kΩ
3.86
x1011
RFOSC=100 kΩ
4
RFOSC=16 kΩ
25
MHz
Thermal flag and protection
TOTA
TOTP
Overtemperature alert
threshold
125
Overtemperature alert
hysteresis
20
Overtemperature protection
threshold
155
1. Channel current accuracy is calculated as:
( I CHX – ITARGET )
ΔICHX = max ------------------------------------------------ • 100
ITARGET
( ICHX – I CHY )
ΔI CHY = max -------------------------------------- • 100
ITARGET
where X, Y=1..16, X≠Y.
16/53
Doc ID 022698 Rev 1
°C
LED7708
6
Block diagram
Block diagram
Figure 3.
Simplified block diagram
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17/53
Device description
7
LED7708
Device description
The LED7708 is an LED driver that integrates a boost controller, sixteen current generators
and a 4-wire serial interface. It has been specifically designed for driving the LED backlight
in medium to large LCD panels.
The device is controlled by a 4-wire serial interface and can operate both as standalone
driver and as master driver in conjunction with other slave devices.
Basic and advanced dimming functions are supported in order to meet different application
requirements. Regardless of the dimming control technique, the LED7708 can optimize the
power dissipation by controlling the output voltage of the boost converter. Two internal linear
regulators derive the device supply (3.3 V) and the gate driver supply (5 V) from a single
power rail.
7.1
Device supply
The LED7708 integrates two low drop-out linear regulators to derive the +3.3 V (typ.) main
supply and the +5 V supply for the gate driver. The VIN pin is the input terminal for both
linear regulators. The output of the 3.3 V LDO is the LDO3 pin, the output of the 5 V LDO is
the VDR pin. The 3.3 V LDO (LDO3 pin) is active if the LREN (linear regulator enable) pin is
left floating or externally set high (see Section 5: Electrical characteristics for the thresholds
involved).
The serial interface I/O stage is supplied through the VSI pin. This pin is used to enable the
serial interface and it allows the setting of the logic level of the serial interface signals (3.3 V
or 5 V) at the same time. The serial interface can be powered by the internal 3.3 V LDO (by
simply connecting the LDO3 pin to the VSI pin) or by an external source, usually the same
supply rail as the host controller.
When an external +3.3 V source is available, it is possible to bypass the 3.3 V LDO by
connecting VIN, VDR and LDO3 together to the +3.3 V rail; in this case the gate driver is
supplied at 3.3 V too and a super-logic level MOSFET is required as the power switch.
The 3.3 V regulator is self-protected against overcurrent and thermal damage thanks to a
foldback mechanism. The continuous current capability of the 3.3 V LDO is calculated to not
exceed the junction temperature limit in worst power dissipation conditions.
A 1 µF MLCC on the LDO3 pin is required for ripple filtering and LDO output stability.
The LDO3 pin is internally connected to the undervoltage lockout (UVLO) protection; the
upper and lower UVLO thresholds are reported in Section 5: Electrical characteristics.
When the voltage at the LDO3 pin falls below the lower UVLO threshold, the device is
turned off (non-latched condition). The device turns on when the voltage at the LDO3 pin
crosses the upper UVLO threshold. Crossing the lower and upper thresholds in sequence
performs the so called “power-on reset” (POR). The POR sequence is used, as well as
toggling the internal DEN bit (see internal registers), when the device is asked to restore
normal operation after a latched fault condition. The 5 V linear regulator, whose output is
connected to the VDR pin, is turned off when the device is disabled, while the 3.3 V linear
regulator is controlled by the LREN pin only.
18/53
Doc ID 022698 Rev 1
LED7708
Recommended operating conditions
8
Recommended operating conditions
Table 7.
Recommended operating conditions
Symbol
Parameter
Conditions
Min.
Max.
3.6
36
Unit
DC Characteristics
VVIN
VIN pin input voltage range
VLDO3
LDO pin input voltage range
VIN, LDO3 and VDR
shorted together
3.0
3.6
VVDR
VDR pin input voltage range
VDR and VIN shorted
together
3.0
5.5
VCHx
Channel voltage range
VVSI
Serial interface supply voltage
3.0
5.5
ICHx
Channel continuous current
20
85
mA
Channel current when OFF
(adjustable)
2
200
µA
V
36
AC Characteristics (VSI = 3.3 V)
fGSC
Grayscale clock frequency
25
fDCLK
Serial clock frequency
30
MHz
Boost converter switching frequency
200
Tw,DCLK
DCLK pulse width
20
Tw,GSC
GSC pulse width
20
Tw,LE
LE pulse width
15
Tsu,DIN
DIN setup time
5
Th,DIN
DIN hold time
5
Tsu,LE
LE setup time
5
Th,LE
LE hold time
5
fSW
Doc ID 022698 Rev 1
1000
kHz
ns
19/53
Recommended operating conditions
8.1
LED7708
Device power-up and soft-start
The VSI pin (supply input for the serial interface) has a high level threshold lower than the
upper UVLO threshold at the LDO3 pin, ensuring that the serial interface is enabled prior to
the device being turned on. Anyway, after the POR, the DEN (device enable) bit of the
internal DEVCFG0 control register is low (default value) and the device is off. The soft-start
sequence takes place as soon as this bit is asserted.
If the LED7708 is set to operate in standalone, the soft-start procedure takes place in two
steps (Figure 4):
1.
The switching frequency is reduced to one fourth of the nominal value and the boost
current limit is set to 20%. This phase finishes approximately 1 ms after the output
voltage reaches the maximum value (not the OVP level). During this phase the
channels are disabled.
2.
Both the switching frequency and the boost current limit are released to their respective
nominal values and then the channels are enabled.
Figure 4.
Soft-start waveforms (standalone operation)
A slightly different soft-start procedure takes place if the LED7708 is set to operate as
master in conjunction with other slave devices:
20/53
1.
The switching frequency is reduced to one eighth of the nominal value and the boost
current limit is set to 20%. This phase finishes approximately 1 ms after the output
voltage reaches the maximum value (not the OVP level). During this phase the
channels are active.
2.
The switching frequency is increased to half the nominal value. This phase is held for
approximately 2 ms.
3.
The switching frequency is further increased to the nominal value while the current limit
changes to 40% (~5 ms duration).
4.
The current limit is released to its nominal values.
Doc ID 022698 Rev 1
LED7708
8.2
Recommended operating conditions
Device shutdown
The device is turned off by clearing the DEN bit of the DEVCFG0 register. The content of
most of the internal registers is not cleared acting on the DEN bit.
The LREN pin can be also used to turn the device on and off whenever the internal 3.3 V
LDO is used to supply the LED7708 (POR). In this case the content of the internal registers
is lost.
8.3
Boost controller section and output voltage control
The boost section of the LED7708 consists of:
8.3.1
●
Fixed-frequency current-mode step-up controller
●
Output voltage optimization loop
●
External MOSFET gate driver
●
Related protection circuitry.
Output voltage optimization
The output voltage of the boost section is the supply rail for the LED strings and is regulated
in an adaptive way to improve the overall efficiency. A dedicated digital-to-analog converter
(DAC) is used to change the reference voltage of the loop and the output voltage is adjusted
in order to keep the power dissipation of the LED driving section to the minimum level.
The optimization is achieved by keeping the voltage drop across the channels inside a fixed
window (regulation window) and its amplitude can be selected between two values (300 mV
and 600 mV) by acting on the internal registers. The lower threshold of the regulation
window (VRWL) is a function of the set LED current (Equation 1) and the upper threshold
(VRWH) is therefore determined.
Equation 1
V RWL = K W ⋅ R ISETH
Figure 5 shows the simplified regulation circuit.
Doc ID 022698 Rev 1
21/53
Recommended operating conditions
Figure 5.
LED7708
Simplified output regulation circuitry
6"//34
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SLOPE
3
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During normal operation, the regulation algorithm is the following:
●
if at least one active channel has a voltage drop below the lower threshold, the internal
DAC counts up and the output voltage is increased
●
if all the active channels have a voltage drop higher than the upper threshold, the
internal DAC counts down and the output voltage is reduced.
Therefore, the possible operating configurations are shown in Table 8.
Table 8.
Output voltage states during regulation
Output voltage
Excessive
Optimal
Condition
All the channels have their voltage above the upper threshold of the
regulation window.
All the channels have their voltage inside the regulation window.
Regulated
Most of the channels have their voltage inside the regulation window; the
remaining channels have a voltage drop above the upper threshold.
Insufficient
At least one channel has its voltage drop below the lower threshold of the
regulation window.
The algorithm can also manage faulty conditions that may occur on the LED array (open
channels and LED short-circuit) in order to prevent excessive power dissipation.
The LED7708 has been designed to interface with other slave devices and can provide them
with the correct LED supply voltage by extending the above mentioned output voltage
optimization. For this purpose, some handshake signals have been reserved:
22/53
Doc ID 022698 Rev 1
LED7708
Recommended operating conditions
Table 9.
Handshake signals summary
Pin
IOK
Function
Used by the slaves to increase the output voltage (algorithm is overdriven)
XVOK
Used by the slaves to ask for an output voltage reduction
XOVF
Used by the master to inform that the maximum output voltage has been reached (each
slave uses this information to tag as “faulty” its open channels)
XMSK
Used by the master to inform the slaves that the output voltage is out of regulation and
the LED fault detection must be masked to prevent false detection
The two signals IOK and XVOK are connected (wired-or) to all the slave devices sharing the
same output voltage rail: if a channel has a low-voltage drop (below VRWL), the device that
owns that channel forces low the IOK signal. In a similar way, the XVOK is released if the
voltage across all the channels of a slave is higher than required (above VRWH). The wire-or
connection of this signal ensures that the output voltage reduction takes place only if all the
slaves (and the master itself) need this operation. Refer to the special functions section for a
detailed explanation of the daisy chain connection between the master and slave devices.
The boost converter is always active regardless of how many channels are active (i.e. even
if all channels are off) in order to improve the load transient dynamic response.
The output voltage regulation can be enabled/disabled (the OVRE bit of the DEVCFG0
register), allowing the user to perform the optimization continuously or on demand.
In real applications, the spread of the forward voltage of the LEDs may cause the channels
to have different voltage drops. In any case, the leading channel (i.e. the channel requiring
the highest voltage to drive its LED string) is kept inside the regulation window.
The reference voltage, provided by the internal DAC, spans between the minimum and
maximum values, programmable through the VMIN pin (see Section 5: Electrical
characteristics).
As a consequence, the output voltage can be varied by the device in the following range:
Equation 2
R1 + R2
VBOOST ,min = ⎛ --------------------⎞ ⋅ VVFB ,min
⎝ R1 ⎠
Equation 3
R1 + R2
V BOOST ,MAX = ⎛⎝ --------------------⎞⎠ ⋅ VVFB ,MAX
R1
where R1 and R2 are the resistors of the divider connected to the output rail and VVFB is the
reference voltage at the VFB pin.
The LED7708 regulates the output voltage in order to optimize the overall efficiency and, to
do this, it needs to read the voltage drop across the active current generators. Unfortunately,
in some cases (e.g. during the soft-start phase or when all the channels are set to 0%
brightness), the current generators are off and there is no useful information coming from
them. Therefore the output voltage is regulated to the minimum value (Equation 2: ). It must
be ensured that the minimum output voltage is below the minimum expected output voltage
Doc ID 022698 Rev 1
23/53
Recommended operating conditions
LED7708
in order to guarantee enough steering room for the optimization. Because of the significant
dependence of the LED forward voltage with temperature, this must be taken into account
when calculating the minimum expected output voltage.
The best way to proceed when designing the external components is the following:
1.
Determine the output voltage range required by the LED strings, considering VF spread
and temperature variation:
N RO WS
m LE DS
ΣV
VLED = max (
i= 1
F, j
)
j= 1
2.
Calculate the middle-point of the output voltage range VBOOST,middle.
3.
Calculate the output divider through Equation 4 and 5.
4.
Calculate the resulting four output voltage ranges (VMIN setting options).
5.
Select the output voltage range closest to the one calculated at point 1.
Equation 4
( V BOOST ,middle – V VFB )
R 2 = --------------------------------------------------------------- ⋅ R 1
VVFB
Equation 5
V BOOST ,MAX
---------------------------------- ≤100μA
R1 + R2
8.3.2
Boost converter loop
The voltage drop of each current generator is sent to the minimum voltage selector: the
lowest one is used as feedback and sent to the window comparator, whose outputs are used
to increase or decrease the loop-counter. The digital outputs of the counter are connected to
the DAC and the target reference is generated. As previously mentioned, this reference
voltage is compared to a partition of the output voltage: the difference is translated into an
error current at the output of the trans-conductance amplifier (see Figure 4, COMP pin).
At this point the compensation network, externally connected to the COMP pin, provides the
programmed trip level for the inductor current. An additional slope compensation ramp is
mixed, as usual, to avoid possible sub-harmonic instability when the boost converter
operates in continuous conduction mode (CCM). The SLOPE pin allows the proper setting
of the amount of slope compensation connecting a simple resistor RSLOPE between the
SLOPE pin and ground. The compensation ramp of a master device starts at 35% (typ.) of
each switching period.
The boost converter switching frequency can be set in the 200 kHz-1 MHz range by
connecting the FSW pin to ground through a resistor, calculated according to Equation 6:
Equation 6
RFSW =
24/53
KF
fSW
Doc ID 022698 Rev 1
LED7708
Recommended operating conditions
In addition, when the FSW pin is tied to LDO3, the LED7708 uses a default 610 kHz fixed
switching frequency, allowing the saving of a resistor in minimum component-count
applications.
The FSW pin can also be used as synchronization input, allowing the LED7708 to operate
both as master or slave device. If a clock signal having a frequency within the capture range
is applied to this pin, the device locks synchronized. The minimum pulse width which allows
the synchronizing pulses to be detected is 270 ns (typ.). The SYNC pin is a synchronization
output and provides a 35% (typ.) duty cycle clock when the LED7708 is set as master or a
replica of the FSW pin when it is set as slave. It is used to connect multiple devices in a
daisy chain configuration or to synchronize other switching converters running in the system
with the LED7708. When an external synchronization clock is applied to the FSW pin, the
internal oscillator is overdriven and the external power MOSFET is ignited on the rising edge
of the synchronization signal while the slope compensation ramp starts on the falling edge
of the same signal. For this reason, the duty cycle of the external synchronization clock
should be 30% to 40% to prevent sub-harmonic instability when the boost converter is
working in continuous-conduction mode (CCM).
8.3.3
Slope compensation
The constant frequency, peak current-mode topology has the advantage of very easy loop
compensation with output ceramic caps (reduced cost and size of the application) and fast
transient response. In addition, the intrinsic peak current measurement simplifies the current
limit protection, avoiding undesired saturation of the inductor. On the other side, this
topology has a drawback: there is inherent open loop instability when operating with a duty
cycle greater than 0.5. This phenomenon is known as “sub-harmonic instability” and can be
avoided by adding an external ramp to the one coming from the sensed current. This
compensating technique, based on the additional ramp, is called “slope compensation”. As
seen in Figure 6, when the switching duty cycle is higher than 0.5, the small perturbation ΔIL
dies away in subsequent cycles thanks to the slope compensation and the system reverts to
a stable situation.
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Recommended operating conditions
Figure 6.
LED7708
Effect of slope compensation on small inductor current perturbation
(D>0.5)
)NDUCTORCURRENT##-
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PERTURBATION
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The SLOPE pin allows to properly set the amount of slope compensation connecting a
simple resistor RSLOPE between the SLOPE pin and the output. The compensation ramp
starts at 35% (typ.) of each switching period and its slope is given by the following equation:
Equation 7
KS
SE = -------------------R SLOPE
where KS = 4.0 s-1 and SE is the slope ramp in [A/s].
To avoid sub-harmonic instability, the compensating slope should be at least half the slope
of the inductor current during the off-phase when the duty cycle is greater than 50%. The
value of RSLOPE can be calculated according to Equation 8.
Equation 8
2 ⋅ K s ⋅ L ⋅ ( V OUT – V IN – 0.7V )
R SLOPE ≤---------------------------------------------------------------------------------------( V OUT – VIN )
8.3.4
External MOSFET control
The LED7708 employs an external MOSFET as the power switch for the boost converter.
The internal gate driver is capable of sourcing and sinking up to 1 A and it is supplied
through the VDR pin, internally connected to the +5 V LDO. A 1 µF decoupling capacitor,
connected between the VDR pin and PGND, is highly recommended. When an external +5
V supply is available, the whole device can be supplied by connecting both the VDR and VIN
pins to this rail.
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LED7708
Recommended operating conditions
Detailed information about the MOSFET gate driver can be found in Section 5: Electrical
characteristics.
The current flowing through the power MOSFET is sensed during the on-phase by
measuring the voltage drop across an external sensing resistor (connected between the
source of the MOSFET and PGND). The overcurrent condition is detected by comparing the
sensed voltage with an internal threshold. This threshold can be set in the 100 mV - 300 mV
range according to the resistor connected between the CLIM pin and ground. The sensing
resistors are often available in few values: once a suitable one has been selected, the
resistor at the CLIM pin is calculated according to the following equation:
Equation 9
( R SNS ⋅ I MOS ,OCP + K C2 )
R CLIM = -------------------------------------------------------------------KC1
where KC1 = 1.63 x 10-5 V and KC2 = 3.1 x 10-2 A.
The maximum allowed peak current depends on the external MOSFET capability.
The actual maximum peak current is given by the above equations as long as duty cycle is
below 35% and decreases as duty cycle increases beyond 35%, depending on the slope
compensation setting. If an overcurrent (OC) condition occurs, the boost controller turns-off
the external MOSFET and skips subsequent switching cycles until the current is above the
programmed level.
In the case of a light load condition, the COMP pin voltage may fall below 0.8 V (typ.). In this
case the boost controller stops switching until the error amplifier forces the COMP voltage to
rise. The above voltage at the COMP pin corresponds to about 25% of full current limit.
In a boost converter, the RMS current through the internal MOSFET depends on both the
input and output voltages, according to Equation 10 (DCM) and 11 (CCM).
Equation 10
VIN ⋅ D D
I MOS ,rms = ---------------------- ---F SW ⋅ L 3
Equation 11
2
⎞
⎛ D
⎞
1 ⎛
VOUT
⎜
⎟ (D(1 − D ))3 ⎟
IMOS ,rms = IOUT ⎜⎜
+
⎟⎟
⎜ (1 − D )2 12 ⎜⎝ IOUT ⋅ fSW ⋅ L ⎟⎠
⎠
⎝
The current limitation works by clamping the COMP pin voltage proportionally to RBILIM. The
peak inductor current is limited to the above threshold decreased by the slope
compensation contribution.
8.4
Current generators section
The LED-driving section consists of 16 current generators connected to the internal control
logic. Each channel can sustain up to 40 V and sink up to 85 mA. The brightness of the
LEDs connected to the LED7708 is controlled by switching the current between a maximum
value (on-phase) and a minimum one (off-phase) at a given dimming frequency (PWM
control).
Doc ID 022698 Rev 1
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Recommended operating conditions
LED7708
The two current levels are set by connecting a resistor on both the ISETH and ISETL pins:
Equation 12
KH
ICHX ,ON = ------------------R ISETH
where KH= 1200 V.
Equation 13
KL
ICHX ,OFF = -----------------R ISETL
where KL= 4.0 V.
The lower level is generally used to keep the voltage across the LED string under control
during the off-phase of the dimming period; if a zero-current low level is required, the ISETL
pin must be connected to the device supply rail (LDO3).
The current generators of the LED7708 have been designed to be grouped (i.e. connected
in parallel) if required by the application. For stability reasons, the parallelization of the
current generators should be done when the programmed current is above 40 mA per
channel.
The current accuracy among different devices is guaranteed to be lower than ±2% through
the full output current range (20 mA-85 mA). Two current ranges are selectable in order to
optimize the overall current accuracy.
Table 10 provides some typical output currents for standard E96 values.
Table 10.
LED current vs. external resistor setting
Nominal LED current
RISETH [kΩ]
Actual LED current
ICHX,ON [mA]
E96 series (1%)
ICHX,ON [mA]
85
14.3
83.9
75
16.2
74.1
60
20.0
60.0
50
24.3
49.4
40
30.1
39.9
30
40.2
29.9
20
60.4
19.9
15
80.6
14.9
Unused channels should be disabled through the CHSEL register and can be physically left
floating or shorted to ground.
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LED7708
Recommended operating conditions
8.5
Serial interface
8.5.1
Serial interface architecture
The LED7708 is equipped with a 4-wire serial interface (SI) to allow full control of its
functionalities by the host controller. The LED7708 uses four pins (DIN, DOUT, DCLK and
LE) to connect itself to the serial bus. The serial interface supports a data clock frequency
up to 30 MHz.
Figure 7.
Simplified serial interface block diagram
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8.5.2
Serial interface internal registers
The internal registers are organized in control registers and brightness registers. Access to
the desired register is performed by encoding the destination address via a serial key on the
bus (a serial key consists of a certain number of clock pulses during the high-state of the LE
signal). All the internal registers are 16 bits wide.
Table 11.
Internal registers
Control registers
Brightness registers
DEVCFG0
DEVCFG1
GSLAT
CHSTA
CHSEL
BRT01 to BRT16
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Recommended operating conditions
8.5.3
LED7708
Internal registers access
The LED7708 is capable of supporting different data formats to satisfy different application
needs. The control registers are accessed one at a time by using 16-bit read/write
operations, while the brightness registers can be loaded in sequence (16x16-bit) or in a
single shot (1x256-bit). An additional data format (1x192-bit) allows data flow optimization
when only 12 bits are used for brightness control. After the startup sequence, the 16x16-bit
data format is set by default. Figure 8 details the signal timing for a write operation to the
DEVCFGx registers. Table 12 summarizes the available operations:
Table 12.
Internal register operations encoding
DCLK rising edges with LE=1
Operation
1
Data latch (BRTxx)
2-3
Global latch (BRTxx)
4-5
Write CHSEL
6-7
Read CHSTA
8-9
Write DEVCFGx(*)
10-11
Read DEVCFGx(1)
12-13
Write GSLAT
1. The content of the shift register is loaded into DEVCFG0 or DEVCFG1 according to the DEST bit (the most
significant bit, common to both registers). The data bits are shifted in starting from the most significant one
(MSB first).
Figure 8.
$).
Timing diagram related to a DEVCFGx write operation
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LED7708
8.5.4
Recommended operating conditions
Brightness data formats
The format for the brightness data is chosen by acting on the BDFS (brightness data format
selector) and the BRLS (brightness register length selector) bits of the DEVCFG1 register.
Table 13.
Brightness data formats
BDFS
BRLS
Brightness data format
0
0
16x16, 12-bit
(the 4 most significant bits are not used)
0
1
16x16, 16-bit
1
0
192x1, 12-bit
1
1
256x1, 16-bit
16x16-bit data format (BDFS=0, BRLS=X)
The brightness registers BRT01:BRT16 are sequentially loaded at the end of 16-bit write
operations. The LE is managed to encode two serial keys: one for the so called “data latch”
operation and another to encode the final “global latch” operation. Therefore, a complete
brightness data transfer to the LED7708 consists of 15 “data latch” operations followed by a
single “global latch” operation.
If the 4096 steps (12-bit) brightness control is used (BRLS low), the four most significant bits
of each brightness register are unused. The brightness registers are written from BRT16 to
BRT01 with auto-decrement after each write operation (the LE signal of the last write
operation to BRT01 must encode a “global latch”).
Figure 9.
',1
16-bit data format, write operation to the brightness register
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Recommended operating conditions
LED7708
1x256-bit data format (BDFS=1, BRLS=1)
This option allows a complete brightness data transfer in a single 256-bit write operation.
The brightness registers BRT01:BRT16 are simultaneously loaded at the falling edge of the
LE signal. Only 65536 steps (16-bit) brightness control is supported by this data format.
Figure 10. 256-bit data format, write operation to the brightness registers
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1x192-bit data format (BDFS=1, BRLS=0)
The brightness registers BRT01:BRT16 are simultaneously loaded at the end of a single
192-bit single write operation. This option is similar to the 1x256-bit format but optimizes the
serial bus communication when 12 bits only are used for the brightness control.
Figure 11. 192-bit data format, write operation to the brightness registers
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LED7708
Recommended operating conditions
Normally, the serial data coming from the DIN pin is clocked out at the DOUT pin. Optionally,
the LED7708 can continuously transmit the status of the control registers when the
continuous status reading (CSRD) bit of the DEVCFG0 register is asserted. In this case the
device replaces (with a 16-clock-cycle delay) the received brightness data stored in the
FIFO with the CHSTA, CHSEL, LATENCY, DEVCFG0 and DEVCFG1 content. This option is
useful to get the device status without interrupting the brightness data flow.
The continuous status reading is available only with 1x256-bit and 1x192-bit data formats.
See Section 8.8.2 for details.
8.5.5
Brightness registers (BRTxx)
The BRT16:01 registers are 16-bit wide, write-only registers where the brightness levels,
received through the serial interface, are stored. These registers may be 12 or 16 bits wide,
depending on the BRLS and BDFS bits of the DEVCFG1, see Table 15. The LED7708
supports two brightness control options: group dimming and grayscale dimming.
The first option uses BRT01 only to set the desired brightness to all the channels, while the
second one allows independent brightness level on each channel (all BRTxx registers are
used). Group dimming and grayscale dimming modes are selected through the GSME bit of
the DEVCFG0 register.
The PWM signals at the outputs are generated by comparing the content of the BRT16:01
registers to a 16-bit or 12-bit counter, according to BRLS and BDFS bit status. The clock
source for the counter can be either external or internal, depending on the FOSC pin status.
See Section 8.6.1: Internal dimming oscillator for details.
All the brightness registers are not directly connected to the digital comparators of the
grayscale circuitry: a 256-bit buffer is used to store the value. The content of the buffer is
transferred to the brightness registers at the end of the LE pulse (falling edge) or at the end
of the current dimming cycle according to the selected option (See the DASS bit of the
DEVCFG1 register for details).
8.5.6
DEVCFG0 register
Table 14.
Bit
DEN
DEVCFG0 register
Position
0
Attribute
R/W
Default
Description
0
Device enable
0= device off
1= device on
EDMI
1
Read only
1
Error detection mode indicator
0= valid data on CHSTA
1= error detection in progress or output voltage
not optimized
OVFW
2
Read only
0
Regulation DAC overflow
0= regulation DAC in range
1= regulation DAC overflow
0
Overtemperature alert flag
0= die temp <120 °C
1= die temp >130 °C
OTAF
3
Read only
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Recommended operating conditions
Table 14.
Bit
DEVCFG0 register (continued)
Position
Attribute
Default
Description
OCAD
4
R/W
0
Open channel auto disconnection
0= open channels not disconnected
1= open channels auto-disconnection
SCAD
5
R/W
0
Shorted channel auto disconnection
0= shorted channels not disconnected
1= shorted channels auto-disconnection
CFP
6
Read only
0
Critical fault protection
0= no protection occurred
1= OV or OT protection occurred
RWAS
7
R/W
0
Regulation window amplitude selection
0= 300 mV regulation window
1= 600 mV regulation window
DTS0
8
R/W
0
DTS1
9
R/W
0
GSME
10
R/W
0
Grayscale mode enable
0= group dimming mode
1= grayscale dimming mode
CRS†
11
R/W
0
Current range selector†
0= lower current range (20 mA-45 mA)
1= upper current range (40 mA-85 mA)
LED short detection threshold selection
(See Table 17).
OVRS
12
Read only
0
Output voltage regulation status
0= output voltage not optimized
1= output voltage optimized
OVRE
13
R/W
0
Output voltage regulation enable
0= output voltage regulation disabled
1= output voltage regulation enabled
SDMS†
14
R/W
0
Slave device mode selection†
0= no slave devices
1= slave devices connected
DEST
15
R/W
X
Destination register address
0= FIFO loaded into DEVCFG0
1= FIFO loaded into DEVCFG1
Warning:
34/53
LED7708
(†) In order to prevent damage to the device or to the external
circuitry, these bits are locked when the DEN bit of DEVCFG0
is asserted.
Doc ID 022698 Rev 1
LED7708
8.5.7
Recommended operating conditions
DEVCFG1 register
Table 15.
DEVCFG1 register
Bit
Position
Attribute
Default
ADJ0
0
R/W
0
ADJ1
1
R/W
0
ADJ2
2
R/W
0
ADJ3
3
R/W
0
ADJ4
4
R/W
0
ADJ5
5
R/W
0
Description
Current gain adjust
Current gain range selector
0= range 1 selected
1= range 2 selected
See Section 8.8.1: Current generators gain
adjust.
CGRS
6
R/W
1
LAT16
7
R/W
0
LAT17
8
R/W
0
BDFS
9
R/W
0
Brightness data format selector
0= 16x16-bit format
1= 1x256-bit or 1x192-bit format
BRLS
10
R/W
0
Brightness registers length selector
0= 12-bit resolution
1= 16-bit resolution
0
Dimming timing inversion selector
0= dimming cycle off-phase first
1= dimming cycle on-phase first
0
Data synchronization selector
0= brightness data immediately updated at
the falling edge of LE
1= brightness data updated at the end of the
current dimming cycle
DTIN
DASS
11
12
R/W
R/W
Bits #16 and #17 of dimming latency
(See GSLAT register)
DSYS†
13
R/W
0
Dimming synchronization selector†
0= GSSY pin is pulse output
1= GSSY pin is trigger input
CSRD
14
R/W
0
Continuous status reading
0= continuous status reading disabled
1= continuous status reading enabled
DEST
15
R/W
X
Destination register address
0= FIFO loaded into DEVCFG0
1= FIFO loaded into DEVCFG1
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Recommended operating conditions
Warning:
Table 16.
8.5.8
LED7708
In order to prevent damage to the device or to the external
circuitry, these bits are locked when the DEN bit of DEVCFG0
is asserted.
Available LED short detection thresholds
DTS1
DTS0
LED short-circuit detection threshold
0
0
LED short detection disabled
0
1
LED short detection threshold fixed to 3.4 V
1
0
LED short detection threshold fixed to 6.8 V
1
1
LED short detection threshold fixed to 8.4 V
GSLAT register
The latency is used to insert a programmable delay at the beginning of each dimming period
to implement a certain amount of scanning. The scanning technique is basically a way to
improve the dynamic performance of an LCD panel by powering the backlight only when the
pixels are stable.
The GSLAT register is a write-only register and stores the 16 less significant bits of the
dimming period latency. The additional two most significant bits are located in the
DEVCFG1 register (LAT16 and LAT17 bits). Thanks to the 18-bit wide latency, the LED7708
is capable of supporting down to 20% of scanning waveform (see Section 8.6: Dimming
options for details).
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LED7708
8.5.9
Recommended operating conditions
CHSEL register
The CHSEL register is a readable/writable configuration register. Every bit of this register
allows the user to enable/disable the channels. Independent control of the channels (e.g.
scrolling backlight or phase shift) can be achieved by acting on this register.
Table 17.
Bit
CHSEL register
Position
Attribute
Meaning
R/W
1= the corresponding channel is active.
0= the corresponding channel is disabled.
0
1
2
3
4
5
6
CHSEL0 to
CHSEL15
7
8
9
10
11
12
13
14
15
8.5.10
CHSTA register
The CHSTA register is a read only status register. Each bit of this register shows the status
of the corresponding channel. After the error detection sequence takes place, the channels
which have an excessive voltage drop (LED short-circuit) or that are unable to regulate the
nominal current (open channels) are tagged as faulty and the related CHSTA bits are
asserted.
The CHSEL register is not affected by the error detection sequence. The channels disabled
by clearing the corresponding bits of the CHSEL register are not involved in the error
detection and the corresponding bits of the CHSTA register are forced low. The CHSTA
register is automatically overwritten during subsequent error detection occurrences.
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Recommended operating conditions
Table 18.
LED7708
CHSTA register
Bit
Position
Attribute
Meaning
Read only
0= The channel is working properly or is not
used (the corresponding CHSEL bit is cleared)
1= The channel is open or an LED short-circuit
has been detected during the error detection
sequence
0
1
2
3
4
5
6
7
CHSTA0 to CHSTA15
8
9
10
11
12
13
14
15
If at least one bit of the CHSTA register is asserted after the error detection sequence, the
XFLT pin is tied low. Both the CHSTA register and the XFLT pin are reset by clearing the
DEN bit of DEVCFG0 or by performing a POR.
8.5.11
Internal registers initialization
The internal registers show the following configuration after a POR.
Table 19.
38/53
Default configuration of internal registers after POR
Register
Value (binary)
DEVCFG0
0000 0000 0000 0010
DEVCFG1
0000 0000 0100 0000
GSLAT
0000 0000 0000 0000
BRTxx
0000 0000 0000 0000
CHSEL
0000 0000 0000 0000
CHSTA
0000 0000 0000 0000
Doc ID 022698 Rev 1
LED7708
Recommended operating conditions
The content of the internal registers, except the read-only bits of DEVCFG0 and DEVCFG1
(i.e. CFP, OVRS, OVFW and EDMI), is not affected when the DEN bit is cleared.
Warning:
8.6
In order to prevent damage to the device or to the external
circuitry, some bits of DEVCFG0 and DEVCFG1 registers
(CRS, SDMS and DSYS) are locked when DEN is high.
Dimming options
The brightness of the LEDs connected to the output channels can be basically controlled in
two ways, depending on the status of the GSME bit of the DEVCFG1 register:
●
Group dimming mode (GSME=0): common dimming performed by using the brightness
value received through the serial interface and stored in the BRT01 register: all the
channels have the same PWM duty cycle;
●
Grayscale dimming mode (GSME=1): independent PWM dimming on each channel
performed by loading the BRT01:BRT16 brightness registers.
In practice, a third way to control the brightness of the LEDs should rely on an intensive use
of the CHSEL register by the host controller. The direct control of the CHSEL register,
combined with either the group dimming or the grayscale dimming, allows the user to
implement particular dimming algorithms.
Once the dimming control mode has been chosen through the GSME bit, the source of the
clock for the PWM generation (grayscale clock) must be selected. The LED7708 can use
either an external clock source or its internal high-frequency oscillator.
8.6.1
Internal dimming oscillator
The LED7708 has a programmable oscillator dedicated to the dimming circuitry.
The output clock is provided at the GSCK pin for the slave devices. If the FOSC pin is forced
high, the oscillator is turned off and the GSCK pin becomes the grayscale clock input.
Table 20.
FOSC pin setting options
FOSC pin setting
Oscillator
GSCK pin function
High (to LDO3)
OFF
Grayscale clock input
Resistor to GND
Active (4 MHz-25 MHz)
Grayscale clock output
The oscillator frequency is simply set by connecting a resistor between the FOSC pin and
ground:
Equation 14
fGSCK =
K OSC
RFOSC
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Recommended operating conditions
LED7708
Equation 15
fDIM =
Table 21.
fOSC
2N
Function of pins involved in dimming control
Pin
Function
GSCK
Input or output, depending on FOSC pin status;
FOSC high: grayscale clock input, internal oscillator disabled.
FOSC to resistor: grayscale clock output. The clock, internally generated by the dimming
oscillator, is provided at this pin.
FOSC
Internal oscillator frequency setting. See above.
GSSY
Input or output pin, depending on DSYS bit value. Used for dimming synchronization.
See Table 21.
Table 22.
Dimming synchronization control
DSYS
Dimming sync selector (locked if DEN is asserted)
0
The GSSY pin is set as output and provides a pulse at the end of each dimming cycle.
The device continuously performs a latency+dimming sequence.
1
The GSSY pin is set as input. The ongoing dimming sequence is reset as soon as a
rising edge on the GSSY input is detected; a new latency+dimming cycle is triggered by
the falling edge on this pin.
Figure 12. Dimming signals and synchronization
3YNCHRONIZATIONOFDIMMINGCYCLESIS
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!-V
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LED7708
8.6.2
Recommended operating conditions
Dimming waveforms
Normal dimming mode
In normal timing mode the basic cycle is represented by a latency period followed by a
dimming period starting with the on-phase (current generators active). This option is
selected by clearing the DTIN (dimming timing inversion) bit of the DEVCGF1 register.
Figure 13. Normal timing mode waveforms
$IMMINGPERIOD
,ATENCY
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5PTOGRAYSCALECLOCKCYCLES
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!-V
Inverted dimming mode
In inverted dimming mode, the basic cycle is represented by a latency period followed by a
dimming period starting with the off-phase (current generators turned off). This option,
normally used when a common turn-off is required, is selected by asserting the DTIN
(dimming timing inversion) bit of the DEVCGF1 register.
Figure 14. Inverted timing mode waveforms
'LPPLQJSHULRG
/DWHQF\
7RI I
8SWRJUD\VFDOHFORFNF\FOHV
7RQ
)L[HGRUJUD\VFDOHFORFNF\FOHV
!-V
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Recommended operating conditions
8.6.3
LED7708
Grayscale synchronization
Grayscale sync (GSSY) used as synchronization output
When the DSYS bit of the DEVCFG1 register is set low, the GSSY pin of the LED7708 is set
as output. A 4xTGSC high level pulse is provided at the GSSY pin at the end of each
dimming cycle (latency followed by a dimming period).
Figure 15. Dimming waveform and GSSY output pulse
X4'3#
!'339PULSEISGENERATEDATTHEENDOF
EACH,ATENCY$IMMINGPERIODSEQUENCE
'339
$IMMINGPERIOD
,ATENCY
4OF F
4ON
#(X
CURRENT
5PTO GRAYSCALECLOCKCYCLES
&IXED OR GRAYSCALECLOCKCYCLES
'3#+
!-V
Grayscale sync (GSSY) used as synchronization input
When the DSYS bit of the DEVCFG1 register is set high, the GSSY pin of the LED7708 is
set as input. This option can be used to lock the dimming cycles (latency followed by a
dimming period) to the vertical sync signal (VSYNC) coming from the video processor. The
minimum pulse duration is 2xTGSC. As soon as the GSSY signal goes high, the ongoing
dimming sequence is stopped and the grayscale counter is reset. A new dimming sequence
is triggered when the GSSY pin goes low.
Figure 16. Dimming waveform triggered by GSSY input
4HEFALLINGEDGEOFTHE'339SIGNAL
TRIGGERSANEWDIMMINGSEQUENCE
4HERISINGEDGEOFTHE'339SIGNAL
STOPSTHEDIMMINGSEQUENCEAND
RESETSTHEGRAYSCALECOUNTER
'339
$IMMINGPERIOD
,ATENCY
4ON
4OFF
#(X
CURRENT
5PTO GRAYSCALECLOCKCYCLES
&IXED OR GRAYSCALECLOCKCYCLES
'3#+
!-V
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LED7708
8.7
Recommended operating conditions
Fault management
The LED7708 includes some protection to prevent damage to itself and to the backlight. In
order to satisfy these requirements, the following protection has been implemented:
Table 23.
Detected faulty conditions
Faulty Condition
Involved Section
Boost output overvoltage (OV)
Boost controller
External MOSFET overcurrent (OC)
Open channel
Current generators
Excessive channel voltage (LED short-circuit)
Overtemperature protection
Whole device
The XFLT pin is an open drain output (active low) and is affected by the following faulty
conditions:
Table 24.
XFAULT pin behavior
Detected fault
Involved flag
XFLT status
Overvoltage protection
OVPF
Low
Overtemperature alert
OTAF
Low
Open channel
CHSTA16:CHSTA01
Low
LED short-circuit
CHSTA16:CHSTA01
Low
Figure 17. Internal signals connected to the XFLT pin
&)3
'76
'76
;)/7
/('VKRUWFLUFXLW
2SHQFKDQQHO
27$)
!-V
8.7.1
Overvoltage protection
The LED7708 protects itself and the external components from excessive output voltage by
monitoring the voltage at the VFB pin. The overvoltage threshold is given by the following
equation, where R1 and R2 are the resistors of the output divider:
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Recommended operating conditions
LED7708
Equation 16
R1 + R2
V OUT ,OVP = -------------------- ⋅ V FB ,OV
R2
Once the voltage at the VFB pin has crossed the related threshold, the LED7708 shuts
down, the CFP (critical fault protection) flag of the DEVCFG0 register is asserted, the XFLT
pin is set low and the condition is latched.
Normal operation is restored by toggling the DEN bit of the DEVCFG0 register or by
performing a POR.
8.7.2
Overtemperature alert and thermal protection
The LED7708 implements a two-level thermal protection. When the junction temperature
rises above 130 °C (typ.), the overtemperature alert flag (OTAF) is asserted and the XFLT
pin goes low. The OTAF flag is cleared as soon as the chip temperature drops below 125 °C
(typ.). This bit is used by the host controller to enable output voltage regulation if temporarily
disabled.
If the temperature of the chip crosses 150 °C, the device shuts down. Automatic restart
occurs as soon as the temperature drops to 130 °C.
8.7.3
Overcurrent protection
Overcurrent occurring in the power MOSFET is managed by skipping subsequent switching
cycles until the faulty condition is present. The XFLT pin is not affected. See also
Section 8.3.3 for details.
8.7.4
LED short-circuit and open channel faults
The faults that can occur to the current generators section are basically the following:
●
one or more open (floating) channels
●
excessive voltage drop (shorted LEDs).
The LED7708 manages these faulty conditions following a particular algorithm.
When a channel opens, its voltage drop falls to zero: the output voltage regulation circuitry
tags it as “probably faulty” and increases the output voltage. If the “open” condition is
temporary, the voltage drop across the generator is self-recovered and the output voltage is
corrected. In case of a real open channel, the regulation DAC reaches the maximum value
(DAC overflow) and the OVFW bit of the DEVCFG0 register is asserted.
The LED7708 can automatically disconnect the open channels if required. Therefore, if the
open channels auto-disconnection (OCAD) bit of the DEVCFG0 register is asserted, the
faulty channel is suddenly turned off, excluded from the regulation loop and the output
voltage is quickly adjusted to the optimal level. During the open channel detection (i.e. when
the regulation DAC is rolling up), the LED short-circuit detection circuitry is masked to avoid
false detection due to the high output voltage.
If the OCAD bit is cleared, the output voltage is kept to the maximum value and the fault
management is demanded to the host controller (XFLT pin low).
In case of LED short-circuit, the voltage across the related channel increases. The LED7708
compares the drop across each channel with a fixed threshold in order to detect this
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Doc ID 022698 Rev 1
LED7708
Recommended operating conditions
condition. Four options can be chosen by acting on the DTS1:DTS0 bits of the DEVCFG0
register (see Table 16).
To prevent false triggering, the LED short-circuit detection is masked if the output voltage is
out of regulation (i.e. the OVRS bit of the DEVCFG0 is low). Each channel detected as faulty
(open or shorted) sets the corresponding bit of the CHSTA register.
The fault management algorithm can operate only if the channels are active and at a
minimum (10 µs typ.) PWM on-time is required at the outputs in order to detect the faulty
condition. After the detection of a “probably open channel”, the device begins to ramp-up the
output voltage, but this operation cannot be performed if the involved channel is turned off
(no feedback information available). In this case, the algorithm should wait until the channel
is turned on again, leading to a delayed output voltage steering because of the dimming
function. This situation can induce excessive power dissipation on the other channels (highvoltage drop for a long period). To prevent this, the LED7708 automatically forces the
“probably open channel” to remain on (regardless of its programmed status) until the
maximum output voltage has been reached (confirmation of open channel fault) or the
output current has been recovered. Once the algorithm has recognized the open condition,
the faulty channel is excluded from the regulation loop and the output voltage is quickly
decreased to the optimal value in order to avoid excessive power dissipation (only if the
OCAD bit is asserted). Therefore, the open channel/LED-short fault detection algorithm can
operate in a transparent way without the need to stop the dimming function.
8.8
Special function
8.8.1
Current generators gain adjust
The gain of the current generators can be varied by changing the ADJ5:ADJ0 bits of the
DEVCFG1 register (64 steps). Two selectable ranges are provided by asserting/clearing the
current gain range selector (CGRS) bit of the DEVCFG1 register. After POR, the current
gain is unitary and can be changed according to the following formulas:
Equation 17
0.5
G CGRS = 1 = 1 + GD ⋅ -------64
Equation 18
0.25
G CGRS = 0 = 0.75 + G D ⋅ ----------64
Equation 19
5
4
2
2
G D = ADJ5x2 + ADJ4x2 + ADJ3x2 + ADJ2x2 + ADJ1x2 + ADJ0
Doc ID 022698 Rev 1
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Recommended operating conditions
LED7708
Figure 18. Channels current gain adjust
* 6WHSV
&*56 * 6WHSV
&*56 * !-V
8.8.2
Continuous status reading
The continuous status-reading is a feature that allows the content of the internal registers to
be obtained without stopping the brightness data flow. The content of the DEVCFG0,
DEVCFG1, CHSEL, CHSTA and GSLAT is automatically loaded into the FIFO after the
brightness data has been moved to the buffer. This feature is supported in 1x1256-bit and
1x192-bit data formats only.
Figure 19. Continuous reading mode disabled, 1x256-bit data format
BITBRIGHTNESSDATA
BIT"RIGHTNESS$ATA3TREAM
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.
.
.
.
.
.
.
.
,%
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0REVIOUS$ATA3TREAM
.
!-V
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Doc ID 022698 Rev 1
LED7708
Recommended operating conditions
Figure 20. Continuous reading mode enabled, 1x256-bit data format
BITBRIGHTNESSDATA
BIT"RIGHTNESS$ATA3TREAM
$).
.
.
DUMMY
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.
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'3,!4
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$%6#&'
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0REVIOUS2EGISTERS#ONFIGURATION
Doc ID 022698 Rev 1
DUMMY
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!-V
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Package mechanical data
9
LED7708
Package mechanical data
In order to meet environmental requirements, ST offers these devices in different grades of
ECOPACK® packages, depending on their level of environmental compliance. ECOPACK
specifications, grade definitions and product status are available at: www.st.com.
ECOPACK is an ST trademark.
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Doc ID 022698 Rev 1
LED7708
Package mechanical data
Table 25.
VFQFPN48 (7 x 7 x 1.0 mm.) package mechanical data
(mm.)
Dim.
Min.
Typ.
Max.
0.80
0.90
1.00
A1
0.02
0.05
A2
0.65
1.00
A3
0.25
A
b
0.18
0.23
0.30
D
6.85
7.00
7.15
D2
4.95
5.10
5.25
E
6.85
7.00
7.15
E2
4.95
5.10
5.25
e
0.45
0.50
0.55
L
0.30
0.40
0.50
ddd
0.08
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Package mechanical data
LED7708
Figure 21. VFQFPN48 (7 x 7 x 1.0 mm.) package outline
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Doc ID 022698 Rev 1
LED7708
Package mechanical data
Figure 22. Recommended PCB footprint
0.28
0.50
1.20
5.00
8.60
AM11137v1
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Revision history
10
LED7708
Revision history
Table 26.
52/53
Document revision history
Date
Revision
07-May-2012
1
Changes
Initial release.
Doc ID 022698 Rev 1
LED7708
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