Adjustable LED brightness DC-DC converter TPS61042

Abstract: TPS61042 is a DC-DC converter produced by TI. The device can also be used as a LED driver, and the driver output voltage up to 27.5V. This paper introduces the working principle of TPS61042 and the typical application design method.

Keywords: DC-DC converter; pulse frequency modulation PFM; pulse width modulation PWM; TPS61042

1, characteristics

TPS61042 is a LED drive control high frequency DC output. By adjusting the pulse width modulation (PWM) signal to the control terminal (CTRL), the LED brightness can be adjusted. LED current can be set by external detection resistor. Since the TPS61042 built-in N channel 500mAMOSFET, which can effectively limit the spike, the conversion frequency of up to 1MHz. In addition, the chip CTRL pin control signal can also be in the shutdown state of LED and ground off, which can effectively avoid leakage current loss.

TPS61042 can provide drive to applied PDA, palmtop computer, handheld devices and mobile phones and other devices for LED backlight. Its main features are as follows:

PWM signal can be used for precise brightness control;

Built in 500mA, N channel switch MOSFET tube;

The input voltage range is 1.8V ~ 6.0V;

Max quiescent current is 38 A;

Conversion efficiency up to 85%;

QFN small package.

2, TPS61042 limit value and pin function

The electrical parameters of TPS61042 are as follows:

The power supply voltage (Vin to GND): 0.3V ~ 7V;

SW, LED, OVP to ground voltage (VSW, VLED, VOVP): less than 30V;

VRS, VCTRL, VFB:Vin + 0.3V;

3.7mW the continuous dissipation power of TA is less than 25 DEG C: 370mW, while in the TA greater than 25 DEG C, the drop rate is;

The operating temperature range: - 40 ~ 85 c;

Junction temperature: 158;

The storage temperature: 65 150 DEG c.;

Welding temperature (10 seconds): 260.

The TPS61042 pin arrangement is shown in Figure 1 and the pin functions are listed in table 1.

Table 1 TPS61042 pin name and function

Tube feet

The name said

I/O

Power can 1

LED

I

LED crystal switch tube Q2 input. Connect LED to the crystal switch 2

RS

O

LED crystal switch Q2 output. External connection resistance detection 3

VIN

I

The input power 4

FB

I

LED current feedback input. By adjusting the voltage of Rs (the 252mV) to adjust the LED current 5

CTRL

I

PWM signal control terminal. The high level of effective, when the LED switch is turned on, the input signal of 50kHz PWM 100Hz to control LED brightness 6

GND

To the end of 7

OVP

I

Overvoltage protection. Connect the output filter capacitor 8

SW

I

Crystal switch Q1 input 3, TPS61042 working principle

The structure diagram of TPS61042 as shown in figure 2. The main functions of the device are DC - DC converter, LED brightness control and overvoltage protection and under voltage lockout.

3.1DCDC conversion

TPS61042 voltage conversion using pulse frequency modulation (PFM) mode. Conversion efficiency up to 1MHz. When the voltage on the detection resistance is lower than 252mV, the main switch Q1 is switched on and the current rises. When the current reaches 500mA or the switching time exceeds 6 s, the main switch Q1 is turned off. An external Schottky diode transmits inductance energy to the output. The main switch is switched on again after maintaining a shutdown time of at least 400ns and the feedback voltage drops below the reference voltage (252mV). The chip PFM current control switch frequency and inductance value, input and output voltage, LED current and so on.

3.2 brightness control

Application frequency range 100Hz ~ PWM signal 50kHz will make LED current ripple with periodic PWM signals. The effective period of the PWM signal can be from 1% to 100%, when the effective period is less than 1%, if the PWM signal is no more than 10ms, the converter will stop. In addition, the control side of the TPS61042 and enable function. When CTRL is set to a high level of 500 s, the conversion is enabled, and when the CTRL is set to a low 32ms, the conversion is disabled. The timing of the PWM signal is shown in figure 3.

3.3 over voltage protection and under voltage lockout

To prevent the 30V limit voltage output voltage is higher than the value of Q1, the chip design of overvoltage protection circuit. When the output voltage exceeds 30V, the Q1 turns off. When the input voltage is lower than 1.5V, the converter is turned off, then the Q1 and LED switch tube Q2 is in high impedance state.

4, application circuit design

4.1 current limit value is set

Is shown in Figure 4 the typical application circuit, TPS61042, before the inductor current reaches 500mA, the internal switch connected. In fact, when the current reaches the 500mA limit, the current may be slightly more than 500mA in the continuous 100ns. Specific limit current is as follows:

IP = I (LIM) VI / L * 100ns

That is