Constant Current LED Driver

feature

Current source with overvoltage protection

Input voltage range. 1.8V to 6V

Internal 30V switch

Efficiency up to 85%

Precise brightness control using PWM signal or analog signal

On-off level. up to 1 MHz

Internal power MOSFET switch. 500 mA

Operates with small output capacitors as low as 100 nF

Disconnect LED during shutdown

No-load quiescent current. 38µA typical

Turn off the current. 0.1µA typical

× Available in 3mm 3mm QFN small package

application

White LED power supply for backlight/side lights

monitor

– PDAs, PDAs, Smartphones

– Handheld device

-cell phone

Level 1

illustrate

The TPS61042 is a high frequency boost converter with constant current output to drive white LEDs or similar. The LED current is set by an external sense resistor (RS) and is directly regulated by the feedback pin (FB), which regulates the voltage across the sense resistor RS to 252 mV (typ). To control the LED brightness, the LED current can be pulsed by applying a PWM (pulse width modulation) signal with a frequency range of 100hz to 50khz to the control pin (CTRL). As described in the application information section, to allow for more flexibility, the device can be configured in a position where the brightness can also be controlled by an analog signal. To avoid leakage current that may pass through the LED during shutdown, the control pin (CTRL) disables the device and disconnects the LED from ground. For maximum safety during operation, the outputs have integrated overvoltage protection to prevent damage to the device in the event of high impedance outputs such as faulty LEDs.

Functional block diagram

Detailed description

operate

The TPS61042 works like a standard boost converter, but regulates the voltage across the sense resistor (RS) instead of the output voltage. This provides a precisely regulated LED current, independent of input voltage and the number of LEDs connected. With integrated overvoltage protection (OVP), the TPS61042 is configured as a current source with overvoltage protection, ideal for driving LEDs. With a 30V internal switch, the device can generate output voltages up to 27.5V and has an internal 500mA MOSFET switch (Q1). This allows multiple LEDs to be connected in series to the output. The internal LED switch (Q2) in series with the LED has a maximum current rating of 60mA and disconnects the LED 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 only on the PWM duty cycle, independent of the PWM frequency and amplitude.

boost converter

The boost converter adopts a pulse frequency modulation (PFM) scheme with constant peak current control. This control scheme maintains high efficiency over the entire load current range, switching frequencies up to 1 MHz, and enables the use of small external components. The converter monitors the induced voltage on RS through the feedback pin (FB), when the feedback voltage is lower than the reference voltage (252 mV typical), the main switch turns on and the current rises. When the inductor current reaches the internally set 500mA (typ) peak current, the main switch turns off. See the Peak Current Control section for more information. The second criterion for closing the main switch is a maximum on-time of 6 microseconds (typ.). This limits the maximum turn-on time of the converter under extreme conditions. When the switch is closed, the external Schottky diode is forward biased, delivering the stored inductive energy to the output. The main switch remains off until a minimum off time of 400 ns (typ) has elapsed and the feedback voltage is again below the reference voltage. With 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 voltages, and LED current. Lower LED current reduces switching frequency, enabling high efficiency over the entire LED current range. This regulation scheme is inherently stable, allowing a wide range of inductor and output capacitor choices.

Peak Current Control (Boost Converter)

The internal switch is turned on until the inductor current reaches the DC current limit (ILIM) of 500 mA (typ). Due to the internal current limit delay of 100 ns (typ), the actual current slightly exceeds the DC current limit threshold. The typical peak current limit can be calculated as:

II 100 ns I

P (typical) (limit) L

I 500mA 100mA (typ) l I

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

soft start

All inductive boost converters will exhibit high inrush currents during startup unless special precautions are taken. This can cause the voltage at the input rail to drop during start-up, causing the system to shut down unexpectedly or prematurely.

The TPS61042 limits the inrush current during startup by increasing the current limit in two steps, from ILIM/4 for 256 switching cycles, to ILIM/2 for the next 256 switching cycles, to the full current limit.

Detailed Description (continued) Control (Control)

The CTRL pin has two functions. One is enabling and disabling of devices. The other is PWM control of the internal LED switch (Q2). If no PWM signal is applied to the control pin, the control pin can be used as a standard enable pin for the device. To enable the device, the control pin must be pulled high for a period of at least 50 microseconds. The device starts with a soft-start cycle. Pulling the control pin to GND for ≥ 32 ms will disable the device, disconnect the LED from GND by opening the LED switch (Q2) to avoid any LED leakage current. See Figure 16 for control pin timing.

When applied to the control pin, the internal LED switch (Q2) is driven by a PWM signal. 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 control pin accepts a PWM duty cycle from D=1% to 100%. Duty cycles below 1% are also possible because the device is forced to shut down because the applied PWM signal has an off time of more than 10 ms.

When a PWM signal is applied to the control pin, the LED switch (Q2) turns on immediately. The internal error comparator is disabled for 400 ns. After the LED switch (Q2) is turned off, a delay time of 400 ns is required to establish the correct voltage level across the sense resistor RS.

For good LED current accuracy and linearity, the switching frequency of the converter must be higher than the PWM frequency applied to the CONTROL pin.

To enable the device, the control signal must be up to 50 microseconds. The PWM signal can then be applied with a pulse width (tp) greater or less than tON. 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 32ms before the device goes into shutdown enables PWM dimming frequencies as low as 100Hz. When the control signal is up to at least 50 microseconds, the device is enabled again.

This CTRL pin must be terminated.

Apply a PWM signal on the control pin, and the turn-on time is tp2.5s ≤ μ In this case, the turn-on time of the PWM signal tp must be greater than 2.5 microseconds before the soft-start is completed. Soft-start is completed after 512 switching cycles. Simply put, to estimate the soft-start time, multiply the period of the PWM signal by 512 and add 50 microseconds to it. For a 50 kHz signal, the minimum soft-start period is 10.3ms. After the soft-start time is complete, the PWM on-time can be reduced to tp>400ns.

Detailed Description (continued) Over Voltage Protection (OVP)

As with any current source, the output voltage rises as the output impedance is increased or disconnected. To prevent the output voltage from exceeding the maximum main switch (Q1) rated voltage of 30V, an overvoltage protection circuit is integrated. When the output voltage exceeds the OVP threshold voltage, (Q1) turns off. The converter switch remains off until the output voltage is below the OVP threshold voltage. As long as the output voltage is below the OVP threshold, the converter continues to operate normally until the output voltage exceeds the OVP threshold again.

undervoltage lockout

An undervoltage lockout function prevents malfunction of the device at input voltages below 1.5 V (typ). As long as the input voltage is below the undervoltage threshold, the device remains off, with the main MOSFET switch (Q1) and LED switch (Q2) on.

Thermal shutdown

An internal thermal shutdown is performed in the TPS61042, which 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 LED switch (Q2) are turned on.

application information

Inductor Selection, Maximum Load Current and Switching Frequency

The PFM peak current control scheme of the TPS61042 is inherently stable. The inductor value does not affect the stability of the regulator. The choice of inductor, along with the nominal LED current, input and output voltages of the application, determine the switching frequency of the converter.

The first step is to calculate the maximum load current the converter can support using the selected inductor. The inductance value has a small effect on the maximum effective load current and is only second order. A good inductance value starts at 4.7µH. Depending on the application, inductor values as low as 1.0 μH can be used. The maximum inductance value is determined by the switch's maximum on-time of 6 microseconds (typ). During these 6 microseconds, a peak current limit of 500 mA (typ) must be reached for proper operation. The maximum load current of the converter is determined at the operating point at which the converter begins to enter continuous conduction mode. The converter must always operate in discontinuous conduction mode to maintain regulation.

The maximum load current can be calculated based on the time period of the inductor current fall time being greater or less than the converter's minimum off time (400ns typical).

(Peak inductor current as described in the Peak Current Control section)

The above formula contains the desired converter efficiency, which allows to calculate the desired maximum load current that the converter can support.

If the converter can support the required LED current, the next step is to calculate the converter switching frequency at the operating point, which must be less than 1 MHz. At the same time, to avoid nonlinear brightness control, the switching frequency of the converter should be much higher than the PWM frequency applied at the control pin. Then the switching frequency of the operating point can be calculated as:

Application Materials (continued)

The smaller the inductance value, the higher the switching frequency of the converter, but the lower the efficiency.

The saturation current of the selected inductor must meet the maximum peak current of the converter calculated in the peak current control section. This calculation uses the maximum value of I(LIM) (600mA).

Another important inductance parameter is the DC resistance. The lower the DC resistance, the higher the efficiency of the converter.

Output Capacitor Selection and Line Adjustment

For better output voltage filtering, low ESR output capacitors are recommended. Ceramic capacitors have lower ESR values, but depending on the application, tantalum capacitors can be used.

The choice of the output capacitor value directly affects the output voltage ripple of the converter, and also affects the regulation of the circuit. The greater the output voltage ripple, the greater the line regulation, which means that if the input voltage changes, the LED current will also change. If a certain change in LED current results in a noticeable change in LED brightness, it depends on the LED manufacturer and application. Applications requiring good line regulation ≤1%/V (typ) must use an output capacitor value of ≥1µF.

use:

I(LIM) = Minimum switch current limit (400mA typical)

L = selected inductance value

I(load) = rated load current fS = switching frequency at rated load current calculated previously.

VF = rectifier diode forward voltage (0.3 V typical)

CO = selected output capacitor

ESR = output capacitor ESR value

Input Capacitor Selection

For good input voltage filtering, low ESR ceramic capacitors are recommended. A 4.7µF ceramic input capacitor is sufficient for most applications. For better input voltage filtering, the capacitor value can be increased.

efficiency

The overall efficiency of an application depends on the specific application conditions, mainly the choice of inductor. The lower the inductance value, the higher the switching frequency and the lower the switching losses. The lower the DC resistance of the inductor, the lower the copper losses and the higher the efficiency. Therefore, depending on the inductor chosen, the efficiency can typically vary by ±5%. As a guide to application efficiency. These curves show the typical efficiency of powering four LEDs using a 4.7 μH inductor that is only 1.2 mm high. The efficiency curve shows the efficiency of delivering power to the LEDs, not the total converter efficiency, calculated as:

Set LED current

The converter regulates the LED current by regulating the voltage across the current sense resistor (RS). The voltage across the sense resistor is regulated to the internal reference voltage V(FB) = 252 mV.

The current programming method is used when the brightness of the LEDs is fixed or controlled by a PWM signal applied to the control pin. When a PWM signal is used on the control pin, the brightness of the LED depends only on the PWM duty cycle, independent of the PWM frequency or amplitude, simplifying the system.

Alternatively, an analog voltage can be used to control the LED brightness.

The LED current is determined by the voltage applied to R2 (V(adj)) and the choice of R1, R2 and the sense resistor (RS). In this configuration, the LED current is controlled linearly instead of the pulsed current in the previous configuration. To choose a resistor value, the following steps are required.

1. Select the voltage V(adjmax) to turn off the LED. →V (adjusted maximum value) (eg 3.3 V)

2. Select the voltage V (adjust the minimum value) to fully turn on the LED. →V (adjustment minimum value) (e.g. 0.0 V)

3. Select the maximum and minimum LED currents IO(max) and IO(min). (eg IO(max) = 20mA, IO(min) = 0mA)

4. Calculate R2 to obtain a feedback current in the range of I1 = 3µA to 10µA when the LED is fully on:

Individually enabled PWM control

The control pin (CTRL) combines the enable function and the PWM brightness control function in one pin. For some systems, a separate enable function is required.

Independent Enable and PWM Control Using Schottky Diodes

PWM brightness control 100 Hz to 50 kHz

enable (EN)

Separate enable and PWM control using transistors

Separate enable and PWM control using AND gate

Layout Considerations

In all switching power supplies, layout is an important design step, especially at high peak currents and switching frequencies. If not carefully laid out, the regulator can exhibit noise issues and duty cycle jitter.

Input capacitors should be placed as close as possible to the input pins for good input voltage filtering. Inductors and diodes must be placed as close as possible to the switch pins to minimize noise coupling to other circuits. Since the feedback pin and network are a high impedance circuit, the feedback network should be kept away from the inductor.

Thermal factor

The TPS61042 is available in a thermally enhanced QFN package. The package includes thermal pads that enhance the thermal performance of the package. See QFN/SON PCB Accessory Application Note (SLUA271).

The thermal resistance of a QFN package to the surrounding environment is highly dependent on the layout of the PCB. Thermal resistance, R, can be increased by using thermal vias and wide PCB traces. Under normal operating conditions, the thermal pad does not require PCB vias. However, the thermal pad must be soldered to the PCB. Θ armor Θ armor

TPS61042, the total system height is 1.0 mm. Efficiency=82.7%@VI=3.0v/19ma

TPS61042 features low LED ripple current and higher accuracy using 4.7µF output capacitor

TPS61042 powers 6 LEDs, efficiency=84%@VI=3.6V/19mA

TPS61042 powers 8 LEDs, efficiency=81%@VI=3.6V/18.6MA

Adjustable brightness control using analog voltage

Alternately Adjustable Brightness Control Based on PWM Signal