7.3.1 VCC Regulator

The VCC pin is the output of the internal linear regulator for providing an 8.45V typical supply voltage to the MOSFET driver and internal circuits. The output current of the VCC pin is limited to 30mA typical. A low ESR ceramic capacitor of 0.47-μF or higher capacitance should be connected across the VCC and SRC pins to supply transient current to the MOSFET driver.

7.3.2 MOSFET Driver

The GATE pin is the output of the gate driver which referenced to the SRC pin. The gate driver is powered directly by the VCC regulator which the maximum gate driving current is limited to 30 mA (typical). To prevent hitting the VCC current limit, TI suggests using a low gate charge MOSFET when high switching frequency is needed.

7.3.3 ADJ Pin

The voltage on the ADJ pin determines the reference voltage for the input current regulation. Typically, the ADJ pin voltage is divided from the output voltage of the circuit by a voltage divider, thus the average input current is adjusted with respect to the number of LEDs used. The voltage of the ADJ pin determines the input current following the expression:

Equation 1.

7.3.3.1 Output OVP

In the TPS92560, a function of output overvoltage protection (OVP) is provided to prevent damaging the circuit due to an open circuit of the LED. The OVP function is implemented to the ADJ pin. When the voltage of the ADJ pin exceeds 0.384V typical, the OVP circuit disables the MOSFET driver and turns off the main switch to allow the output capacitor to discharge. As the voltage of the ADJ pin decreases to below 0.353 V (typical), the MOSFET driver is enabled and the TPS92560 returns to normal operation. The triggering threshold of the output voltage is determined by the value of the resistors R_ADJ1 and R_ADJ2, which can be calculated using the following equation:

Equation 2.

When defining the OVP threshold voltage, it is necessary to certain that the OVP threshold voltage does not exceed the rated voltage of the output rectifier and capacitor to avoid damaging of the circuit.

7.3.4 AC1 and AC2 Pins

The TPS92560 provides two internal active rectifiers for input voltage rectification. Each internal rectifier connects across the ACn pin to GND. These internal active rectifiers function as the low-side diode rectifiers of a generic bridge rectifier. The integrating of the active rectifiers helps in saving two external diodes of a bridge rectifier along with an improvement of power efficiency. For high power applications, for instance, 12-W output power, external diode rectifiers can be added across the ACn pin to GND to reduce heat dissipation on the TPS92560.

7.3.5 Detection of Power Source

Figure 13. Inherent Dead Time of the Output Voltage of an Electronic Transformer

Both the voltages of a generic AC source (50/60Hz) and an electronic transformer carry certain amount of dead time inherently, as shown in Figure 13. The existing of the dead time leads to a drop of the RMS input power to the driver circuit. In order to compensate the drop of the RMS input power, the ADJ pin sources current to the resistor, R_ADJ2 to increase the reference voltage for the current regulation loop and in turn increase the RMS input power accordingly when an AC voltage source or electronic transformer is detected. The output current of the ADJ pin for an AC input voltage and electronic transformer are 9.5μA and 11.5μA typical respectively. Practically the amount of the power for compensating the dead time of the input power source differs case to case depending on the characteristics of the power source, the value of the R_ADJ1 and R_ADJ2 might need a fine adjustment in accordance to the characteristics of the power source. The additional output power for compensating the dead time of the power sources (ΔP_LED) are calculated using the following Equation 3 and Equation 4.

For 50/60Hz AC power source:

Equation 3.

For electronic transformer:

Equation 4.

7.3.6 Current Regulation

In the TPS92560, the input current regulation is attained by limiting the peak and valley of the inductor current. Practically the inductor current sensing is facilitated by detecting the voltage on the resistor, R_SEN. Because the current flows through the R_SEN is a sum total of the currents of the main switch and LEDs, the voltage drop on the R_SEN reflects the current of the inductor that is identical to the input current to the LED driver circuit. Figure 14 shows the waveform of the inductor current ripple with the peak and valley values controlled.

Figure 14. Inductor Current Ripple in Steady State

The voltage of the ADJ pin is determined by the forward voltage of the LED and divided from the V_VP by a resistor divider. The equation for calculating the V_ADJ as shown in Equation 5.

Equation 5.

In steady state, the voltage drop on the R_ADJ1 is identical to the forward voltage of the LED (V_LED) and the voltage across the R_ADJ2 is identical to the voltage across the R_SEN. The LED current, I_LED is then calculated following the equations:

In steady state:

Equation 6.

Equation 7.

Equation 8.

Since

Equation 9.

Thus,

Equation 10.

Put the expressions (2) to (4) into (5):

Equation 11.

Due to the high input impedance of the ADJ pin, the current flows into the ADJ pin can be neglected and thus I_RADJ1 equals I_RADJ2. The LED current is then calculated following the expressions below:

Equation 12.

Practically, the conversion efficiency of a boost circuit is almost a constant around 85%. Being assumed that the efficiency term in the I_LED expression is a constant, the LED current depends solely on the magnitude of the input voltage, V_IN. Without changing a component, the output power of the typical application circuits of the TPS92560 is adjustable by using different number of LEDs.

The output power is calculated by following the expression:

Equation 13.

7.3.7 Switching Frequency (Boost Configuration)

In the following sections, the equations and calculations are limited to the boost configuration only (that is, the LED forward voltage higher than the input voltage), unless otherwise specified. The application information for the SEPIC and other circuit topologies are available in separate application notes and reference designs. In the boost configuration, including the propagation delay of the control circuit, the ON and OFF times of the main switch are calculated using Equation 14 and Equation 15.

Equation 14.

Equation 15.

In the previous equations, the V_D is the forward voltage of D₃, R_L is the DC resistance of L₁, R_DS(ON) is the ON resistance of Q₁ and R_AC-FET is the turn ON resistance of the internal active rectifier with respect to the typical application circuit diagram.

Practically the resistance of the R_L, R_DS(on) and R_AC-FET is in the order if several tenth of mΩ, by assuming a 0.5-V diode forward voltage and the sum total of the R_L, R_DS(ON) and R_AC-FET is close to 1 Ω, the on and off times of Q₁ can be approximated using the Equation 16 and Equation 17.

Equation 16.

Equation 17.

With the switching on and OF times determined, the switching frequency can be calculated using Equation 18.

Equation 18.

Because of the using of hysteretic control scheme, the switching frequency of the TPS92560 in steady state is dependent on the input voltage, output voltage and inductance of the inductor. Generally a 1-MHz to 1.5-MHz switching frequency is suggested for applications using an electronic transformer as the power source.

7.3.8 Inductor Selection (Boost Configuration)

Because of the using of the hysteretic control scheme, the switching frequency of the TPS92560 in a boost configuration can be adjusted in accordance to the value of the inductor being used. Derived from the equations (12) and (13), the value of the inductor can be determined base on the desired switching frequence by using Equation 19.

Equation 19.

When selecting the inductor, it is essential to ensure the peak inductor current does not exceed the the factory suggested saturation current of the inductor. The values of the peak and valley inductor current are calculated using the following equations:

Peak inductor current:

Equation 20.

Valley inductor current:

Equation 21.

Assume the total resistance of the R_L, R_DS(on) and R_AC-FET is 1 Ω and the diode drop, V_D equal to 1 V, the peak and valley currents of the inductor can be approximated using Equation 22 and Equation 23.

Equation 22.

Equation 23.

In order not to saturate the inductor, an inductor with a factory guranteed saturation current (I_SAT) 20% higher than the I_L(peak) is suggested. Thus the I_SAT of the inductor should fulfill the following requirement:

Equation 24.

7.3.9 Input Surge Voltage Protection

When use with an electronic transformer, the surge voltage across the input terminals can be sufficiently high to damage the TPS92560 depending on the characteristics of the electronic transformer. To against potential damaging due to the input surge voltage, a 36-V Zener diode can be connected across the input bridge rectifier as shown in Figure 15.

Figure 15. Input Surge Voltage Protection Using an External Zener Diode

7.4.1 Thermal Shutdown

The TPS92560 includes a thermal shutdown circuitry that ceases the operation of the device to avoid permanent damage. The threshold for thermal shutdown is 165°C with a 30°C hysteresis typical. During thermal shutdown the VCC regulator is disabled and the MOSFET is turned off.