8.3.1 Operation

The TPS61043 operates like a standard boost converter but regulates the voltage across the sense resistor (R_S) instead of the output voltage. This gives an accurate regulated LED current independent of the input voltage and number of LEDs connected. With integrated overvoltage protection (OVP) the TPS61043 is configured as a current source with overvoltage protection ideally suited to drive LEDs. The device can generate output voltages of up to 16.9 V if the OVP-function is used and has an internal 400 mA MOSFET switch (Q1). This allows up to four LEDs to be connected in series to the output. The internal LED switch (Q2) in series with the LEDs has a maximum current rating of 60 mA and disconnects the LEDs from ground during shutdown. The LED switch is driven by a PWM signal applied to the control pin (CTRL), which directly controls the LED brightness. With this control method the LED brightness depends on the PWM duty cycle only and is independent of the PWM frequency and amplitude. If the OVP-function is not needed, the device can be used to generate output voltages up to 28V.

8.3.2 Boost Converter

The boost converter operates in a pulse frequency modulation (PFM) scheme with constant peak current control. This control scheme maintains high efficiency over the entire load current range and with a switching frequency of up to 1 MHz, enables the use of small external components. The converter monitors the sense voltage across R_S with the feedback pin (FB) and, when the feedback voltage falls below the reference voltage (252 mV typ), the main switch turns on and the current ramps up. The main switch turns off when the inductor current reaches the internally set peak current of 400 mA (typ). Refer to the Peak Current Control (Boost Converter) section for more information. The second criteria that turns off the main switch is the maximum on-time of 4.5 µs (typ). This limits the maximum on-time of the converter in extreme conditions. As the switch is turned off the external Schottky diode is forward biased, delivering the stored inductor energy to the output. The main switch remains off until the minimum off time of 400 ns (typ) has passed and the feedback voltage is below the reference voltage again. Using this PFM peak current control scheme, the converter operates in discontinuous conduction mode (DCM) where the switching frequency depends on the inductor, input and output voltage, and LED current. Lower LED currents reduce the switching frequency, which results in high efficiency over the entire LED current range. This regulation scheme is inherently stable, allowing a wide range for the selection of the inductor and output capacitor.

8.3.3 Peak Current Control (Boost Converter)

The internal switch is turned on until the inductor current reaches the DC current limit (I_LIM) of 400 mA (typ) . Due to the internal current limit delay of 100 ns (typ) the actual current exceeds the DC current limit threshold by a small amount. The typical peak current limit can be calculated:

Equation 1.

Equation 2.

The higher the input voltage and the lower the inductor value, the greater the current limit overshoot.

8.3.4 Softstart

All inductive step-up converters exhibit high in-rush current during start-up if no special precautions are taken. This can cause voltage drops at the input rail during start-up, which may result in an unwanted or premature system shutdown.

The TPS61043 limits this in-rush current during start-up by increasing the current limit in two steps starting from I_LIM/4 for 256 switch cycles to I_LIM/2 for the next 256 switch cycles and then full current limit. See Figure 16 for typical start-up behavior.

8.3.5 Control (CTRL)

The CTRL pin serves two functions. One is the enable and disable of the device. The other is the PWM control of the internal LED switch (Q2). The CTRL pin can be used as a standard enable pin for the device if no PWM signal is applied to the CTRL pin. To enable the device, the CTRL pin must be pulled high for a time period of at least 500 µs. The device starts with the Softstart cycle. Pulling the CTRL pin to GND for a time period ≥32 ms disables the device, disconnecting the LEDs from GND by opening the LED switch (Q2) to avoid any LED leakage current. See Figure 9 for the CTRL pin timing.

Figure 9. CTRL Timing Diagram

To enable the device, the CTRL signal must be high for 500 µs (see Figure 9). The PWM signal can then be applied with a pulse width (t_p) greater or smaller than t_ON. To force the device into shutdown mode, the CTRL signal must be low for at least 32 ms. Requiring the CTRL pin to be low for 32 mS before the device enters shutdown allows for PWM dimming frequencies as low as 100 Hz. The device is enabled again when a CTRL signal is high for a period of 500 µs minimum. See Figure 7 for the PWM duty cycle versus LED current characteristic.

The internal LED switch (Q2) is driven by the PWM signal when applied to the CTRL pin. Applying a PWM signal in the range of 100 Hz to 50 kHz allows the LED current to be pulsed with the duty cycle of the PWM signal. The CTRL pin accepts a PWM duty cycle from D = 1% to 100%. Duty cycles below 1% are also possible with the restriction that the device is forced into shutdown as the off time of the applied PWM signal exceeds 10 ms.

When a PWM signal is applied to the CTRL pin the LED switch (Q2) turns on immediately. The internal error comparator is disabled for 400 ns. This 400 ns delay time is required to establish the correct voltage level across the sense resistor R_S after the LED switch (Q2) is closed.

To achieve good LED current accuracy and linearity, the switching frequency of the converter must be higher than the PWM frequency applied to the CTRL pin. This CTRL pin must be terminated.

8.4.1 Overvoltage Protection (OVP)

As with any current source, the output voltage rises as the output impedance increases as for example with a disconnected load. To prevent the output voltage from exceeding the maximum main switch (Q1) voltage rating, an overvoltage protection (OVP) circuit is integrated. With an OVP threshold voltage of 19 V maximum, up to 4 LEDs can be connected in series. This allows the use of a cheaper output capacitor with a 25 V voltage rating. When the output voltage exceeds the OVP threshold voltage, (Q1) turns off. The converter switch remains off until the output voltage falls below the OVP threshold voltage. As long as the output voltage is below the OVP threshold the converter continues its normal operation, until the output voltage exceeds the OVP threshold again. If overvoltage protection is not needed, then the OVP pin should be connected to GND. In this case the TPS61043 can be used to generate output voltages up to 28 V.

8.4.2 Undervoltage Lockout

An undervoltage lockout feature prevents mis-operation of the device at input voltages below 1.5 V (typical). As long as the input voltage is below the undervoltage threshold the device remains off, with the main MOSFET switch (Q1) and the LED switch (Q2) open.

8.4.3 Thermal Shutdown

An internal thermal shutdown is implemented in the TPS61043 that shuts down the device if the typical junction temperature of 160°C is exceeded. If the device is in thermal shutdown mode, the main MOSFET switch (Q1) and the LED switch (Q2) are open.