SLUSCZ1 May 2017 TPS92518-Q1
PRODUCTION DATA.
- 1 Features
- 2 Applications
- 3 Description
- 4 Revision History
- 5 Pin Configuration and Functions
- 6 Specifications
- 7 Parameter Measurement Information
-
8 Detailed Description
- 8.1 Overview
- 8.2 Functional Block Diagram
- 8.3
Feature Description
- 8.3.1 General Operation
- 8.3.2 Important System Considerations: Off-Timer and Maximum Peak Threshold Values
- 8.3.3 Shunt FET or Matrix dimming: Maximum Off-timer Calculation
- 8.3.4 VIN and the VCC Internal Regulators
- 8.3.5 Output Enable Control Logic
- 8.3.6 BOOT Capacitor and BOOT UVLO
- 8.3.7 Drop-out Operation
- 8.3.8 Analog and PWM Dimming
- 8.3.9 VIN and CSPx Pin Configuration
- 8.3.10 Enable and Undervoltage Lock-out Configuration
- 8.3.11 Voltage Sampling and DAC Operation
- 8.3.12 Device Functional Modes
- 8.4 Serial Interface
- 8.5
Registers
- 8.5.1 CONTROL Register (Address = 00h) [reset = 00h]
- 8.5.2 STATUS (FAULT) Register (Address = 01h) [reset = 10h]
- 8.5.3 THERM_WARN_LMT Register (Address = 02h) [reset = 80h]
- 8.5.4 LED1_PKTH_DAC Register (Address = 03h) [reset = 80h]
- 8.5.5 LED2_PKTH_DAC Register (Address = 04h) [reset = 80h]
- 8.5.6 LED1_TOFF_DAC Register (Address = 05h) [reset = 80h]
- 8.5.7 LED2_TOFF_DAC Register (Address = 06h) [reset = 80h]
- 8.5.8 LED1_MAXOFF_DAC Register (Address = 07h) [reset = 80h]
- 8.5.9 LED2_MAXOFF_DAC Register (Address = 08h) [reset = 80h]
- 8.5.10 VTHERM Register (Address = 09h) [reset = 0h]
- 8.5.11 LED1_MOST_RECENT Register (Address = 0Ah) [reset = 0h]
- 8.5.12 LED1_LAST_ON Register (Address = 0Bh) [reset = 0h]
- 8.5.13 LED1_LAST_OFF Register (Address = 0Ch) [reset = 0h]
- 8.5.14 LED2_MOST_RECENT Register (Address = 0Dh) [reset = 0h]
- 8.5.15 LED2_LAST_ON Register (Address = 0Eh) [reset = 0h]
- 8.5.16 LED2_LAST_OFF Register (Address = 0Fh) [reset = 0h]
- 8.5.17 Reset Register (Address = 10h) [reset = 0h]
- 8.6 Programming
- 9 Application and Implementation
- 10Power Supply Recommendations
- 11Layout
- 12Device and Documentation Support
- 13Mechanical, Packaging, and Orderable Information
8.3.2.3 Off-Time Thresholds - LEDx_TOFF_DAC and LEDx_MAXOFF_DAC
- Do not write '0' to the LEDx_TOFF_DAC or LEDx_MAXOFF_DAC registers. Writing a '0' to the LEDx_TOFF_DAC or LEDx_MAXOFF_DAC registers is the equivalent to: do not turn the FET off. Damage occurs to the device when a value of 0 is written to this register.
- Do not write an off-time value that causes the TPS92518-Q1 to operate beyond the maximum frequency possible for the conversion voltage ranges. For example: for a given application switching frequency and VIN to VLED conversion, there is a required duty cycle. If the duty cycle requirement is shorter than the TPS92518-Q1 minimum on-time ( tLEB ) current can not regulate. If the Off-time is made sufficiently short such that the inductor current is not able to reset given the V-s reached during the leading edge blanking time, the inductor current continues to increase until the inductor is saturated and the system may be damaged. The maximum frequency can be estimated, starting with Equation 9
Equation 9. 
Then the maximum frequency can be derived using the minimum on-time (tLEB leading edge blanking)
Equation 10. 
Using an example condition: VIN= 65 V, VLED= 6 V, tLEB= 250 ns, ΔIL-PP= 250 mA, we can find the maximum switching frequency and an Off-time minimum value.
Equation 11. 
Equation 12. 
The system controller must be programmed with a minimum LEDx_TOFF_DAC value of 55 allowing for some margin above the computed value of 53. (a smaller value = higher switching frequency) Note this is the correct value for this example. Each application is different. All register values, minimums and maximums must be considered for each application separately.