LED7708 STMicroelectronics, LED7708 Datasheet - Page 23

• Manufacturer Part Number: LED7708
• Description: LED Lighting Drivers 16 Ch 85mA LED 3.6 to 36V 250kHz
• Manufacturer: STMicroelectronics

Specifications of LED7708

• Rohs: yes

LED7708
Table 9.
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 V
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 V
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
characteristics).
As a consequence, the output voltage can be varied by the device in the following range:
Equation 2
Equation 3
where R1 and R2 are the resistors of the divider connected to the output rail and V
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
be ensured that the minimum output voltage is below the minimum expected output voltage
XMSK
XVOK
XOVF
IOK
Pin
Used by the slaves to increase the output voltage (algorithm is overdriven)
Used by the slaves to ask for an output voltage reduction
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)
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
Handshake signals summary
V
V
BOOST MAX
Doc ID 022698 Rev 1
BOOST min
,
,
=
=
⎛
⎝
⎛
⎝
R
------------------- -
R
------------------- -
1
1
R
R
Function
+
+
1
1
R
R
2
2
⎞
⎠
⎞
⎠
Recommended operating conditions
⋅
⋅
V
V
VFB min
VFB MAX
,
,
Section 5: Electrical
(Equation 2:
RWL
RWH
), the device that
). The wire-or
VFB
). It must
is the
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