TPS92692 Datasheet, PDF (21/55 Pages) Texas Instruments – High Accuracy LED Controller With Spread Spectrum Frequency Modulation

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TPS92692, TPS92692-Q1
SLVSDD9 – MARCH 2017
Feature Description (continued)
8.3.11 Series P-Channel FET Dimming Gate Driver Output
The PDRV output is a function of the internal PWM signal and is capable of sinking and sourcing up to 50 mA of
peak current to control a high-side series connected P-channel dimming FET. The PDRV switches between V_CSP
and (V_CSP– 7 V) based on the status of PWM signal to completely turn-off and turn-on the external P-channel
dimming FET. The series dimming FET is required to achieve high contrast ratio as it ensures fast rise and fall
times of the LED current in response to the PWM input. Without any dimming FET, the rise and fall times are
limited by the inductor slew rate and the closed-loop bandwidth of the system. Leave the PDRV pin unconnected
if not used.
8.3.12 Soft-Start
The soft-start feature helps the regulator gradually reach the steady-state operating point, thus reducing startup
stresses and surges. The devices clamp the COMP pin to the SS pin, separated by a diode, until LED current
nears the regulation threshold. The internal 10-µA soft-start current source gradually increases the voltage on an
external soft-start capacitor C_SS connected to the SS pin. This results in a gradual rise of the COMP voltage from
GND.
The internal 10-µA current source turns on when VCC exceeds the UVLO threshold. At the beginning of the soft-
start sequence, the SS pull-down switch is active and is released when the voltage V_SS drops below 50 mV. The
SS pin can also be pulled down by an external switch to stop switching. When the SS pin is externally driven to
enable switching, the slew-rate on the COMP pin is controlled by the compensation capacitor. In this case, the
startup duration and LED current transient is controlled by tunning the compensation network. It is essential to
ensure that the softstart duration is longer than the time required to charge the output capacitor when selecting
the soft-start capacitor, C_SS and the compensation capacitor, C_COMP.
8.3.13 Current Monitor Output
The IMON pin voltage represents the LED current measured by the rail-to-rail current sense amplifier across the
external current shunt resistor. The linear relationship between the IMON voltage and LED current includes the
amplifier gain-factor of 14 (see Feature Description section). The IMON output can be connected to an external
microcontroller or comparator to facilitate LED open, short, or cable harness fault detection and mitigation. The
IMON voltage is internally clamped to 3.7 V.
8.3.14 Output Overvoltage Protection
The TPS92692 and TPS92692-Q1 devices include a dedicated OV pin which can be used for either input or
output overvoltage protection. This pin features a precision 1.228 V (typ) threshold with 20-µA (typ) of hysteresis
current. The overvoltage threshold limit is set by a resistor divider network from the input or output terminal to
GND. When the OV pin voltage exceeds the reference threshold, the GATE pin is immediately pulled low, the
PDRV output is disabled, and the SS and COMP capacitors are discharged. The GATE and PDRV outputs are
enabled and a new startup sequence is initiated after the voltage drops below the hysteresis threshold set by the
20-µA source current and the external resistor divider.
8.3.15 Output Short-circuit Protection
The device monitors the output of the current sense amplifier and the output voltage via OV pin to determine
LED Short-circuit fault. The device signals an output overcurrent fault when the voltage across the current sense
amplifier, (V_(CSP-CSN)), exceeds the regulation set point based on the IADJ pin voltage, V_IADJ. The overcurrent fault
threshold is calculated as follows:
V _{(CSP CSN),OCP}
1.5 u ^VIADJ
14
(7)
The device also indicates a short-circuit condition when the voltage across the OV pin and GND falls below 100
mV. In this case, the output voltage, V_O, is below the undervoltage fault threshold determined based on the
resistor divider connected to the OV pin.
V_{O(UV )}
0.1u ^{ROV1 ROV2}
R _OV1
(8)
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