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Texas Instruments Using Voltage Supervisor for Input Over-Voltage Protection in LED Drivers Application notes
Application Report
SLVA594 – May 2013
Using Voltage Supervisor for Input Over-Voltage
Protection in LED Drivers
Atul Singh .......................................................................................................................................
ABSTRACT
Lighting electronics meant for AC-DC applications tend to get subjected to input line variations leading to
sustained over-voltage on the driving circuitry. Metal oxide varistors (MOV’s) meant for suppressing the
transients fail at such sustained over-voltage leading to system failure and hence the need for
incorporation of separate circuitry for the Input Over-Voltage Protection. The TLV809 family of supervisory
circuits that provide circuit initialization and timing supervision, primarily for DSPs and processor-based
systems can be used for the Over-Voltage Protection in LED drivers and other AC-DC power supply
applications.
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Contents
Introduction ..................................................................................................................
Functional Block and Timing Diagram ...................................................................................
Input Over-Voltage Protection in LED Drivers ..........................................................................
Conventional Approach ....................................................................................................
Proposed Approach .........................................................................................................
References ...................................................................................................................
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List of Figures
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Typical Application .......................................................................................................... 2
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Functional Block Diagram
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Timing Diagram .............................................................................................................
Input Over-Voltage Protection With Conventional Approach ..........................................................
Input Over-Voltage Protection With Voltage Supervisor ...............................................................
Voltage Transient Measurement ..........................................................................................
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Introduction
The TLV809K33 is a family of ultra-low current supply voltage supervisors (SVS) that monitor a single
voltage rail. Because this device is a fixed-voltage monitor, it is typically configured as shown in Figure 1.
During power-on, RESET is asserted when the supply voltage (VDD) becomes higher than 1.1 V.
Thereafter, the supervisory circuit monitors VDD and keeps RESET active as long as VDD remains below
the threshold voltage VIT. An internal timer delays the return of the output to the inactive state (high) to
ensure proper system reset. The delay time (td (typ) = 200 ms) starts after VDD has risen above the
threshold voltage, VIT. When the supply voltage drops below the VIT threshold voltage, the output becomes
active (low) again. No external components are required. All the devices in this family have a fixed sensethreshold voltage (VIT) set by an internal voltage divider.
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Functional Block and Timing Diagram
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3.3-V LDO
3.3 V
5V
OUT
IN
GND
VDD
DSP/FPGA/ASIC
VDD
TLV809K33
RESET
RESET
GND
GND
Figure 1. Typical Application
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Functional Block and Timing Diagram
The TLV809 family of voltage supervisors has an internal Schmitt trigger that provides hysteresis of 30 mV
(min) to 60 mV (max) to avoid false trigger due to VDD voltage fluctuations. The VDD voltage fluctuations
become more pronounced and more susceptible to false trigger when it is derived from the input AC.
Therefore, an internal hysteresis makes the device suitable for the shutdown in case of Input Over-Voltage
in the AC-DC LED drivers. Figure 2 shows the functional bock diagram of TLV809.
Figure 2. Functional Block Diagram
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Using Voltage Supervisor for Input Over- Voltage Protection in LED Drivers
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Input Over-Voltage Protection in LED Drivers
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Figure 3 shows that state change of the RESET incorporates an internal delay of 200 ms and the
undefined behavior of the Reset output for VDD < 1.1 V.
Figure 3. Timing Diagram
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Input Over-Voltage Protection in LED Drivers
TPS92210-based LED drivers can be prevented against Input Over-Voltage by an external shutdown/retry
by pulling VCG pin to ground. Voltage on the VCG is internally clamped; the clamp level varies with the
operating conditions. In normal use, VCG is current fed with the voltage internally clamped. The gate of
the HV Mosfet is held at a constant DC voltage using the VCG pin. It is shunt regulated to 14 V during
normal operation and regulation is increased to 16 V during fault, UVLO and start-up conditions. When
pulled low (below 9 V), the VCG pin pulls down the gate of the external HV Mosfet disabling the PWM
action.
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Conventional Approach
4
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Conventional Approach
Figure 4. Input Over-Voltage Protection With Conventional Approach
The VCG pin can be pulled low by turning ON an NPN transistor with Zener diode at the base. Once the
rectified AC line voltage exceeds the set point, the Zener diode conducts and turns ON the transistor that
pulls down the VCG pin.
Disadvantages with the approach:
• Need of a higher capacitance C10 (220 µF) to make the Zener diode to conduct and turn ON Q1.
• The set point for the input over the voltage cut off can not be precisely fixed as it depends on the
breakdown voltage of the Zener diode. The Zener diodes have a tolerance of 5% (min) to 10% (max),
which translates to shifting the AC line cutoff voltage set point.
• The AC line voltage hysteresis attainable with the above approach is more than 30 V, which is on a
higher side as C10 (220 µF) in parallel to R16 (6.2k) takes more time to discharge.
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Using Voltage Supervisor for Input Over- Voltage Protection in LED Drivers
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Proposed Approach
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5
Proposed Approach
Figure 5. Input Over-Voltage Protection With Voltage Supervisor
The disadvantages with the conventional approach mentioned in Section 4 can be easily overcome using
the voltage supervisor TLV809K33. The use of the TLV809K33 allows the use of smaller capacitor C10
(10 µF) as it requires only the voltage reference for the RESET output state change from low to high. The
TLV809XX has built-in rejection of fast transients on the VDD pin. Transient rejection depends on both the
duration and amplitude of the transient. Transient amplitude is measured from the bottom of the transient
to the negative threshold voltage (VIT–) of the device, as shown in Figure 6.
Figure 6. Voltage Transient Measurement
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References
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The TLV809XX uses a unique sampling scheme to maintain an extremely low average quiescent current
of 150 nA. The TLV809XX typically consumes only about 100 nA of dc current. However, this current rises
to approximately 15 μA for around 200 μs while the TLV809XX samples the input voltage. If the source
impedance back to the supply voltage is high, then the additional current during sampling may trigger a
false reset as a result of the apparent voltage drop at VDD. For high VDD source or trace impedance
applications, it is recommended to add a small 0.1-μF bypass capacitor near the TLV809XX VDD pin. This
bypass capacitor effectively keeps the average current at 150 nA and reduces the effects of a highimpedance voltage source The voltage supervisor pulls the RESET output high once the VDD voltage
crosses the specified threshold (2.93 V for TLV809K33) after a delay of 200 ms, which ensures precise
AC cutoff set point. The inbuilt hysteresis of 40 mV avoids false trigger and provides AC line Hysteresis of
around 10 V, which is good enough for lighting applications.
The transistor Q1 (BC547) in Figure 5 can be replaced by Signal Mosfet 2N7002 and used for pulling
down the VCG pin. The use of signal Mosfet reduces the dependency on the input gate capacitance (Ciss)
of the external HV Mosfet (M1).
The input over voltage shutdown used for the TPS92210-based LED driver can also be implemented for
other AC-DC power supply applications requiring cutoff at higher AC line voltages.
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References
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TLV809J25, TLV809L30, TLV809K33, TLV809I50 3-Pin Supply Voltage Supervisors Data Sheet
(SLVSA03)
TPS92210 Natural PFC LED Lighting Driver Controller Data Sheet (SLUS989)
Using Voltage Supervisor for Input Over- Voltage Protection in LED Drivers
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