LM3697_15 Datasheet, PDF (15/43 Pages) Texas Instruments – High-Efficiency Three-String White LED Driver

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LM3697
SNOSCS2C – NOVEMBER 2013 – REVISED OCTOBER 2015
Device Functional Modes (continued)
• Apply power to the LM3697
• Enable Short Fault (Register 0xB4, bit [1] = 1)
• Assign HVLED1 to Bank A (Register 0x10, Bits [2:0] = (1, 1, 0)
• Set the startup ramp times to the fastest setting (Register 0x11 = 0x00)
• Set Bank A full-scale current to 20.2 mA (Register 0x17 = 0x13)
• Enable Feedback on the HVLED Current Sinks (Register 0x19, bits[2:0] = (1,1,1))
• Set Control A Brightness MSB to max (Register 0x21 = 0xFF)
• Enable Bank A (Register 0x24 Bit[0] = 1)
• Wait 4 ms
• Read back bits[0] of register 0xB2. 1 = HVLED1 short.
• Disable all banks (Register 0x24 = 0x00)
• Repeat the procedure for the HVLED2 and HVLED3 strings
7.4.2.3 Overvoltage Protection (Inductive Boost)
The overvoltage protection threshold (OVP) on the LM3697 has 4 different configurable options (16 V, 24 V, 32
V, and 40 V). The OVP protects the device and associated circuitry from high voltages in the event the high-
voltage LED string becomes open. During normal operation, the LM3697 device’s inductive boost converter
boosts the output up so as to maintain V_HR at the active, high-voltage (COUT connected) current sink inputs.
When a high-voltage LED string becomes open, the feedback mechanism is broken, and the boost converter
over-boosts the output. When the output voltage reaches the OVP threshold the boost converter stops switching,
thus allowing the output node to discharge. When the output discharges to V_OVP minus 1 V the boost converter
begins switching again. The OVP sense is at the OVP pin, so this pin must be connected directly to the inductive
boost output capacitor’s positive terminal.
For high-voltage current sinks that have the HVLED Current Sink Feedback Enable setting such that the high-
voltage current sinks anodes are not connected to COUT (feedback is disabled), the overvoltage sense
mechanism is not in place to protect the input to the high-voltage current sink. In this situation the application
must ensure that the voltage at HVLED1, HVLED2 or HVLED3 doesn’t exceed 40 V.
The default setting for OVP is set at 16 V. For applications that require higher than 16 V at the boost output, the
OVP threshold must be programmed to a higher level after power up.
7.4.2.4 Current Limit (Inductive Boost)
The NMOS switch current limit for the LM3697 device’s inductive boost is set at 1 A (typical). When the current
through the LM3697’s NFET switch hits this overcurrent protection threshold (OCP), the device turns the NFET
off, and the inductor’s energy is discharged into the output capacitor. Switching is then resumed at the next
cycle. The current limit protection circuitry can operate continuously each switching cycle. The result is that
during high-output power conditions the device can continuously run in current limit. Under these conditions the
LM3697’s inductive boost converter stops regulating the headroom voltage across the high-voltage current sinks.
This results in a drop in the LED current.
7.4.3 I²C-Compatible Interface
7.4.3.1 Start And Stop Conditions
The LM3697 is controlled via an I²C-compatible interface. START and STOP conditions classify the beginning
and the end of the I²C session. A START condition is defined as SDA transitioning from HIGH to LOW while SCL
is HIGH. A STOP condition is defined as SDA transitioning from LOW to HIGH while SCL is HIGH. The I²C
master always generates START and STOP conditions. The I²C bus is considered busy after a START condition
and free after a STOP condition. During data transmission the I²C master can generate repeated START
conditions. A START and a repeated START condition are equivalent function-wise. The data on SDA must be
stable during the HIGH period of the clock signal (SCL). In other words, the state of SDA can only be changed
when SCL is LOW.
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