8.3.1 Switching Frequency

The boost converter of the LM3492HC device employs a projected-on-time (POT) control method to determine the on-time period of the MOSFET with respect to the input and output voltages and an external resistor R_RT. During the on-time period, the boost inductor charges up, and the output capacitor discharges to provide power to the output. A cycle-by-cycle current limit (which is 3.9 A typically and programmable by an external resistor) protects the MOSFET. After the on-time period, the MOSFET turns off and boost inductor discharges. The next on-time period starts when the voltage of the FB pin drops below a threshold which is determined by dynamic headroom control (DHC) and operates from 1.05 V to 2 V. DHC affects the threshold when either the DIM1 pin is high or the DIM2 pin is high.

During POT control operation, the boost converter maintains switching at a nearly constant frequency. During most operating conditions, the switching frequency depends on mainly the value of R_RT (Figure 19) but may see some variation with changes in input or output voltage. Also, POT control operation requires no compensation circuit and offers fast transient response of the output voltage. Applications that require very wide input voltage or very wide output voltage ranges may see some variation in the switching frequency as shown in Figure 20 and Figure 21. More switching frequency graphs can be found in the Typical Characteristics section.

ILED = 150 mA	VOUT = 30 V	VVIN = 12 V

Figure 19. Switching Frequency vs RT Resistance

	ILED = 150 mA	RRT = 274 kΩ	VVIN = 12 V

Figure 21. Switching Frequency vs Output Voltage

ILED = 150 mA	VOUT = 30 V	RRT = 274 kΩ

Figure 20. Switching Frequency vs Input Voltage

8.3.2 LDO Regulator

The LM3492HC device offers an integrated, 5.5-V, LDO regulator. For stability, connect an external capacitor C_VCC of more than 0.47-µF between the VCC and GND pins. The current limit of the LDO is typically 30 mA. The LDO regulator can be used to pullup the open-drain COMM pin with an external resistor, and sources current to the ILIM pin to adjust the current limit of the integrated MOSFET. When the voltage on the VCC pin (V_CC) is higher than the undervoltage lockout (UVLO) threshold of 3.78 V, the device becomes enabled and the CDHC pin sources a current to charge up an external capacitor (C_CDHC) to provide a soft-start function.

8.3.3 Enable and Disable

To enable the LM3492HC device, the voltage on the EN pin (V_EN) must be higher than an enable threshold of typically 1.63 V. If the voltage on the EN pin (V_EN) is lower than 1.43 V, the device shuts down. In this case, the LDO regulator turns off and the CDHC pin becomes internally grounded. The EN pin internally pulls up. After enable, a 40-µA current source pulls up the EN pin. If the EN pin is connected to low such that the device is shutdown, the pullup current is reduced to 2 µA. These advantages allow the device to effectively avoid false disabling by noise during operation, and minimize power consumption during shutdown. The enable threshold is so precise that it can support a UVLO function for the input voltage as shown in Figure 22. The input voltage can be connected to the EN pin through a resistor divider consisting of R_EN1 and R_EN2. This circuitry ensures that the device operates after the input voltage reaches a minimum require value V_IN(EN), as shown in Equation 1.

To maintain the V_EN level below the absolute maximum specification, place a Zener diode (D_EN) between the EN pin and GND pins.

Figure 22. Input Voltage UVLO Implemented by Precision Enable

After the EN pin is pulled low, the device performs the following functions:

• resets IOUT overvoltage and undervoltage indications and the corresponding COMM bit pattern
• resumes the switching frequency tuning to the normal frequency
• resumes channel 1 of the current regulator if it is disabled

Pulling the EN pin low for a short period of approximately 200 ns achieves these same functions with little or no effect on the operation of the boost converter and the current regulator.

8.3.4 Current Limit

The current limit (I_CL) of the integrated MOSFET of the LM3492HC device provides a cycle-by-cycle current limit for protection. This limit can be decreased by injecting a small signal current, I_ILIM into the ILIM pin. The relationship between I_CL and I_ILIM is described in Equation 2.

As shown in Figure 23, create current limit functionality by connecting a resistor (R_ILIM) between the VCC pin and the ILIM pin. The typical voltage on the ILIM pin is 0.7 V. To obtain the maximum current limit, connect the ILIM pin to ground.

Figure 23. Programmable Current Limit

8.3.5 Thermal Protection

An internal thermal shutdown circuit provides thermal protection. The circuit activates at 165°C (typically) to disable the LM3492HC device. In this case, the LDO regulator turns off and the CDHC pin becomes internally grounded. Thermal protection helps prevent catastrophic failures from accidental device overheating. When the junction temperature of the device drops below 145°C (typical hysteresis = 20°C), the device resumes normal operation.

8.3.6 Dynamic Headroom Control, Over-Ride, and Soft-Start

The LM3492HC device uses dynamic headroom control (DHC) to adjust the output voltage (V_OUT) of the boost converter to reduce the power loss of the current regulator and thereby maximize efficiency. To understand this control function, consider V_LED,n the forward voltage of an LED string connecting to the IOUTn pin and V_IOUT,n as the voltage of the IOUTn pin (where n is 1, 2 for channels 1, 2 of the current regulator). V_LED,n normally and gradually decreases (in terms of minutes) as a result of the rise of the LED die temperature during operation. The DHC adjusts the output voltage (V_OUT) by adjusting a threshold that is reflected in the voltage of the FB pin with reference to V_IOUT,n, (the difference between V_OUT and V_LED,n). The capacitor C_CDHC sets the sensitivity of DHC, which affects the response time on adjusting V_OUT. If the capacitance value of C_CDHC is small, V_OUT is more sensitive to the variation of V_LED,n.

Override the DHC functionality by adding internal pullup resistance or external pullup resistance by connecting the CDHC and VCC pins with a resistor. Use a value of approximately 10 MΩ. In this case, the voltage of the CDHC pin rises above 2.5 V, and the voltage of the FB pin rises until the voltage reaches the overvoltage protection threshold. Because the pullup is weak, DHC override occurs only at a low contrast ratio (approximately < 1%).

The C_CDHC capacitor acts to control the soft-start functionality. During the start-up period, the voltage of the CDHC pin rises from 0 V to 2.25 V at a rate that depends on the value of the C_CDHC capacitor. This limitation ensures that the voltage of the FB pin (as well as the output voltage) ramps up in a controlled manner, and effectively implements a soft-start function.

An internal switch grounds the CDHC pin during any of the following cases:

• V_VCC is below the VCC UVLO threshold
• a thermal shutdown occurs
• the EN pin is pulled low

The CDHC pin cannot be connected to the ground externally.

8.3.7 Current Regulator

The LM3492HC device integrates a two-channel current regulator for controlling the current of two LED strings. The two LED strings dim individually by applying individual dimming signals to the DIM1 and DIM2 pins for LED strings 1 and 2, which are connected from the VOUT pin to the IOUT1 and IOUT2 pins. The device pulls the DIM1 and DIM2 pins low internally. The lowest contrast ratio is 10000:1. The finest pulse width of the dimming signal for the DIM1 and DIM2 pins is 300 ns.

The device sets the current of an LED string (I_LED) from 50 mA to 250 mA by using an external resistor R_IREF connected between the IREF pin and ground. Figure 24 describes the relationship between I_LED and R_IREF. The two channels of the current regulator can work in parallel for only one LED string by connecting the IOUT1 and IOUT2 pins together to provide an LED current of up to 500 mA. In this case, connect the DIM1 and DIM2 pins together.

Figure 24. LED Current vs Current Reference Resistance (R_IREF)

Figure 25. Over-Power Protection

If the voltage on the IOUTn (n = 1, 2) pin is higher than 24 V when channel n is on, the regulated current of channel n reduces linearly if the voltage further increases (as shown in Figure 25). The regulated current of another channel is not affected. This over-power protection feature avoids damaging the current regulator owing to the shorting of many LEDs in one string.

8.3.8 Output Voltage Feedback

The device feeds the output voltage back to the FB pin through a feedback circuit consisting of R_FB1, R_FB2, and C_FB as shown in Figure 26. To assist the feeback functionality, maintain a value of 10 pF for C_FB. The DC component of the output voltage feedback uses R_FB1 and R_FB2. The voltage of the FB pin V_FB can be adjusted by DHC. When V_FB reaches V_FB-OVP, the maximum output voltage of the boost converter V_OUT(max) reaches its maximum, as shown in Equation 3.

During DHC operation, maintain the output voltage at a nominal voltage but not the maximum. The nominal output voltage (V_OUT(nom)) is described in Equation 4.

The minimum value of V_IOUT,n is approximately 5 Ω × I_LED. The nominal voltage of the FB pin (V_FB(nom)) is recommended to be from 1.05 V to 2 V. Equation 5 describes the relation between V_OUT(max), V_OUT(nom), and V_FB(nom):

Figure 26. Output Voltage Feedback Circuit

8.3.9 Overvoltage Protection

When V_FB is higher than the FB pin overvoltage protection (OVP) threshold V_FB-OVP (typically 2.76 V and maximum 2.88 V), the on-period of the integrated MOSFET stop immediately, and the MOSFET keeps off until V_FB falls back below below 2.545 V (typical hysteresis 0.215 V).

An alternative method to implement OVP is to directly monitor V_OUT instead of V_FB. An external circuit as shown in Figure 27 is required. Current is injected to the ILIM pin to drive the LM3492HC device to the current limit mode once V_OUT is higher than the avalanche voltage of the Zener diode D_OVP plus 0.7 V, the typical voltage on the ILIM pin. In this case, the device imporses a maximum limit on V_OUT. However, at the maximum limit of V_OUT, V_FB must be higher than 2.25 V to avoid affecting the start-up of the device.

Figure 27. External OVP Circuit

8.3.10 Bidirectional Communication Pin

The COMM pin of the LM3492HC device is an open-drain bidirectional I/O pin for interfacing with an external MCU for the following functions:

• power-good indication
• overtemperature indication
• output current overvoltage and undervoltage indications
• switching frequency tuning
• channel 1 disabling

Except for the power good indication and the overtemperature alerts, all data flow through the COMM pin is serial and is latched by the falling edge of the signal applying to the DIM1 pin, even when channel 1 of the current regulator is disabled. If the DIM1 pin remains only low or only high, either by an external circuit or by allowing it to open and pull low internally, data does not flow. Figure 28 and Figure 29 show timing diagrams of reading and writing a bit from and to the device through the COMM pin.

Pull up the COMM pin by an MCU I/O pin, which has pullup capability, or an external resistor R_COMM connected to the VCC pin. Without this capability, the voltage of the COMM pin remains at zero. The rise time of the output signal of the COMM pin depends on the pullup power. If the rise time is long (R_COMM is too large or pullup power from the connecting MCU I/O pin is too weak), data may be ready after a longer duration after the falling edge. In this case, the design requires a longer delay between the falling edge latching and the (input or output) bit.

Figure 28. Read from the COMM Pin

Figure 29. Write to the COMM Pin

8.5.1 Output Current Overvoltage Indication

The LM3492HC device gives an IOUTn (n = 1, 2) overvoltage indication if the voltage of the IOUTn pin when DIMn is higher than a threshold of typically 6.5 V. These conditions remain while the device applies 508 consecutive dimming signals on the DIMn pin. The IOUT overvoltage indication can be regarded as a short fault of the LED string n except the following two cases:

• powering up the device at a very low dimming ratio such that V_OUT maintains at a maximum and DHC is not fast enough to reduce V_OUT
• during DHC override condition, a bit pattern (see Table 1) can be read from the COMM pin

The device does not latch off and continues to operate in the presence of the IOUT overvoltage indication.

8.5.2 COMM Pin Bit Pattern

Table 1 summarizes all COMM bit patterns of output current overvoltage and undervoltage indications. An existing COMM bit pattern is cleared if one of the following condition occurs:

• the LM3492HC device is shutdown
• the LM3492HC device is disabled by pulling the EN pin low
• the overtemperature indication is appearing

Apply the clock signal on both DIM1 and DIM2 pins when the COMM bit pattern is read by an external MCU. Before reading the COMM bit pattern, pull the EN pin low for approximately 200 ns to reset the COMM bit pattern. This situation does not affect the operation of the boost converter and the current regulator. After EN is reset, if the IOUT overvoltage or undervoltage condition lasts for 508 consecutive clock cycles, the COMM pin sends the COMM bit pattern for the MCU to read.

In case of overtemperature, the device pulls the COMM pin low to give an overtemperature indication overriding any other pattern. After the overtemperature indication disappears, the COMM bit pattern appears before the over-temperature indication appears again.

8.5.3 Channel 1 Disable

After a power good verification, channel 1 of the current regulator can be disabled by writing a command (see Table 2) to the COMM pin. If LED string 1 is malfunctioning, channel 1 can be disabled and the signal applied on the DIM1 pin can serve as only a clock signal for the data flow of the COMM pin. Channel 1 is by default enabled after reset. If the overtemperature indication or any COMM bit pattern has already presented, no command can be written to the LM3492HC device.