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
8.3.3 Shunt FET or Matrix dimming: Maximum Off-timer Calculation
Shunt FET or Matrix dimming is typically used where precise control of the LED current is required. For the LED current and light output to most accurately match the control signal, the current source supplying the LEDs must be as close to ideal as possible. With the TPS92518-Q1 hysteretic control and maximum off-time setting, the LED current can approach ideal. Two waveforms show the result during shunt FET dimming with and without optimized maximum off-time control. The result with maximum off-time control is superior and approaches an ideal current source.
| CH4 (Green) : Inductor Current (200 mA per division) | CH2 (Cyan) : VVLEDx (10 V per division) | CH1 (Blue) : Shunt FET Control Signal |
| CH4 (Green) : Inductor Current (200 mA per division) | CH2 (Cyan) : VVLEDx (10 V per division) | CH1 (Blue) : Shunt FET Control Signal |
To ensure the correct maximum off-time when shunt dimming it is necessary to calculate the off-time required when the output is in the shunted condition. The following procedure may be used:
- Estimate or measure the output voltage during the shunted condition.
- Compute the off-time required (tOFF-Shunt) when the output is shunted.
- Compute the Maximum Off-time Register Value.
Or for the Matrix approach:
