30V/1.2A 、High dimming ratio LED constant current driver PT4115 (PT4115B89E,PT4115B89E-B,PT4115BSOH,PT4115BSOH-B)

 PT4115B89E GENERAL DESCRIPTION

  The PT4115 is a continuous conduction mode inductive step-down converter, designed for driving single or multiple series connected LED efficiently from a voltage source higher than the total LED chain voltage. The device operates from an input supply between 6V and 30V and provides an externally adjustable output current of up to 1.2A. Depending upon the supply voltage and external components, the PT4115can provide more than 30 watts of output power.

The PT4115 includes the power switch and a high-side output current sensing circuit, which uses an external resistor to set the nominal average output current, and a dedicated DIM input accepts either a DC voltage or a wide range of pulsed dimming. Applying a voltage of 0.3V or lower to the DIM pin turns the output off and switches the device into a low current standby state.

 
The PT4115 is available in SOT89-5 and ESOP8 packages.

PT4115B89E,PT4115BSOH  FEATURES

● Simple low parts count

● Wide input voltage range: 6V to 30V

● Up to 1.2A output current

● Single pin on/off and brightness control using DC voltage or PWM

● Up to 1MHz switching frequency

● Typical 5% output current accuracy

● Inherent open-circuit LED protection

● High efficiency (up to 97%)

● High-Side Current Sense

● Hysteretic Control: No Compensation

● Adjustable Constant LED Current

●  ESOP8 package for large output power application

●  RoHS compliant  

PT4115B89E,PT4115BSOH APPLICATIONS

● Low voltage halogen replacement LEDs

● Automotive lighting

● Low voltage industrial lighting

● LED back-up lighting

● Illuminated signs

● SELV lighting

● LCD TV backlighting

PT4115B89E,PT4115BSOH ORDERING INFORMATION

PT4115B89E,PT4115BSOH TYPICAL APPLICATION CIRCUIT

PT4115B89E,PT4115BSOH PIN ASSIGNMENT

PT4115B89E,PT4115BSOH  OPERATION DESCRIPTION

    The device, in conjunction with the coil (L1) and current sense resistor (RS), forms a self oscillating continuous-mode buck converter.When input voltage VIN is first applied, the initial current in L1 and RS is zero and there is no output from the current sense circuit. Under this condition, the output of CS comparator is high. This turns on an internal switch and switches the SW pin low, causing current to flow from VIN to ground, via RS, L1 and the LED(s). The current rises at a rate determined by VIN and L1 to produce a voltage ramp (V_CSN) across RS. When (V_IN-V_CSN) > 115mV, the output of CS comparator switches low and the switch turns off. The current flowing on the RS decreases at another rate. When (VIN-VCSN) < 85mV, the switch turns on again and the mean current on the LED is determined by

The high-side current-sensing scheme and on-board current-setting circuitry minimize the number of external components while delivering LED current with±5% accuracy, using a 1% sense resistor.

The PT4115 allow dimming with a PWM signal at the DIM input. A logic level below 0.3V at DIM forces PT4115 to turn off the LED and the logic level at DIM must be at least 2.5V to turn on the full LED current.The frequency of PWM dimming ranges from 100Hz to more than 20 kHz.

  The PT4115 allow dimming with a PWM signal at the DIM input. A logic level below 0.3V at DIM forces PT4115 to turn off the LED and the logic level at DIM must be at least 2.5V to turn on the full LED current.The frequency of PWM dimming ranges from 100Hz to more than 20 kHz. (VDIM) to adjust the output current to a value below the nominal average value defined by RS. The DC voltage is valid from 0.5V to 2.5V. When the dc voltage is higher than 2.5V, the output current keeps constant. The LED current also can be adjusted by a resistor connected to the DIM pin. An internal pull-up resistor (typical 200KΩ) is connected to a 5V internal regulator. The voltage of DIM pin is divided by the internal and external resistor.

The DIM pin is pulled up to the internal regulator (5V) by a 200KΩ resistor. It can be floated at normal working. When a voltage applied to DIM falls below the threshold (0.3V nom.), the output switch is turned off. The internal regulator and voltage reference remainpowered during shutdown to provide the reference for the shutdown circuit. Quiescent supply current during shutdown is nominally 95uA and switch leakage is below 5uA.

Additionally, to ensure the reliability, the PT4115 is built with a thermal shutdown (TSD) protection and a thermal pad. The TSD protests the IC from over temperature (160℃). Also the thermal pad enhances power dissipation. As a result, the PT4115 can handle a large amount of current safely.

PT4115B89E,PT4115BSOH APPLICATION NOTES

     Setting nominal average output current with external resistor RS

The nominal average output current in the LED(s) is determined by the value of the external current sense resistor (RS) connected between VIN and CSN and is given by:  

I_{OUT＝0.1/Rs(Rs≥0.082Ω)}

This equation is valid when DIM pin is float or applied with a voltage higher than 2.5V (must be less than 5V). Actually, RS sets the maximum average current which can be adjusted to a less one by dimming.

Output current adjustment by external DC control voltage

The DIM pin can be driven by an external dc voltage (VDIM), as shown, to adjust the output current to a value below the nominal average value defined by RS.

The average output current is given by:

Note that 100% brightness setting corresponds to: 2.5V≤V_DIM≤5V

PT4115B89E,PT4115BSOH Output current adjustment by PWM control

A Pulse Width Modulated (PWM) signal with duty cycle PWM can be applied to the DIM pin, as shown below, to adjust the output current to a value below the nominal average value set by resistor RS:

PWM dimming provides reduced brightness by modulating the LED’s forward current between 0% and 100%. The LED brightness is controlled by adjusting the relative ratios of the on time to the off time. A 25% brightness level is achieved by turning the LED on at full current for 25% of one cycle. To ensure this switching process between on and off state is invisible by human eyes, the switching frequency must be greater than 100 Hz. Above 100 Hz, the human eyes average the on and off times, seeing only an effective brightness that is proportional to the LED’s on-time duty cycle. The advantage of PWM dimming is that the forward current is always constant, therefore the LED color does not vary with brightness as it does with analog dimming. Pulsing the current provides precise brightness control while preserving the color purity.The dimming frequency of PT4115 can be as high as 20 kHz.

  Shutdown mode

  Taking the DIM pin to a voltage below 0.3V will turn off the output and the supply current will fall to a low standby level of 95μA nominal.

  Soft-start

  An external capacitor from the DIM pin to ground will provide additional soft-start delay, by increasing thetime taken for the voltage on this pin to rise to the turn-on threshold and by slowing down the rate of rise of the control voltage at the input of the comparator. Adding capacitance increases this delay byapproximately 0.8ms/nF.

Inherent open-circuit LED protection

If the connection to the LED(s) is open-circuited, the coil is isolated from the SW pin of the chip, so the device and LED will not be damaged.

  Capacitor selection

  A low ESR capacitor should be used for input decoupling, as the ESR of this capacitor appears inseries with the supply source impedance and lowers overall efficiency. This capacitor has to supply the relatively high peak current to the coil and smooth the current ripple on the input supply. A minimum value of 4.7uF is acceptable if the DC input source is close to the device, but higher values will improve performance at lower input voltages, especially when the sourceimpedance is high. For the rectified AC input, the capacitor should be higher than 100uF and the tantalumcapacitor is recommended. The input capacitor should be placed as close as possible to the IC.For maximum stability over temperature and voltage, capacitors with X7R, X5R, or better dielectric are recommended. Capacitors with Y5V dielectric are not suitable for decoupling in this application and should NOT be used.A suitable Murata capacitor would be GRM42-2X7R475K-50.

  Inductor selection

  Recommended inductor values for the PT4115 are in the range 27uH to 100uH.Higher values of inductance are recommended at lower output current in order to minimize errors due to switching delays, which result in increased ripple and lower efficiency. Higher values of inductance also result in a smaller change in output current over the supply voltage range. (See graphs). The inductor should be mounted as close to the device as possible with lowresistance connections to the SW and VIN pins.The chosen coil should have a saturation current higher than the peak output current and a continuous current rating above the required mean output current.Following table gives the guideline on inductor selection:

Suitable coils for use with the PT4115 are listed in the table below:

The inductor value should be chosen to maintain operating duty cycle and switch 'on'/'off' times within the specified limits over the supply voltage and load current range.The following equations can be used as a guide.SW Switch 'On' time

SW Switch 'Off' time

Where:

L is the coil inductance(H)

rL is the coil resistance (Ω )

RS is the current sense resistance (Ω )

Iavg is the required LED current (A)

Δ I is the coil peak-peak ripple current (A) {Internally set to 0.3 x Iavg}

VIN is the supply voltage (V)

VLED is the total LED forward voltage (V)

RSW is the switch resistance (Ω ) {=0.6Ω nominal}

VD is the diode forward voltage at the required load current (V)

Diode selection

  For maximum efficiency and performance, the rectifier (D1) should be a fast low capacitance Schottky diode with low reverse leakage at the maximum operatingvoltage and temperature.They also provide better efficiency than silicon diodes, due to a combination of lower forward voltage and reduced recovery time.It is important to select parts with a peak current rating above the peak coil current and a continuous current rating higher than the maximum output load current. It is very important to consider the reverse leakage of the diode when operating above 85°C. Excess leakage willincrease the power dissipation in the device and if close to the load may create a thermal runaway condition.The higher forward voltage and overshoot due to reverse recovery time in silicon diodes will increase the peak voltage on the SW output. If a silicon diode is used, care should be taken to ensure that the total voltage appearing on the SW pin including supply ripple, does not exceed the specified maximum value.The following web sites are useful when finding alternatives: www.onsemi.com

 Reducing output ripple

 Peak to peak ripple current in the LED(s) can be reduced, if required, by shunting a capacitor CLEDacross the LED(s) as shown below:

A value of 1uF will reduce the supply ripple current by a factor three (approx.). Proportionally lower ripple can be achieved with higher capacitor values. Note that the capacitor will not affect operating frequency or efficiency, but it will increase start-up delay and reduce the frequency of dimming, by reducing the rate of rise of LED voltage.By adding this capacitor the current waveform through the LED(s) changes from a triangular ramp to a more sinusoidal version without altering the mean current value.

 Operation at low supply voltage

The internal regulator disables the drive to the switch until the supply has risen above the startup threshold (VUVLO). Above this threshold, the device will start to operate. However, with the supply voltage below the specified minimum value, the switch duty cycle will be high and the device power dissipation will be at a maximum. Care should be taken to avoid operating thedevice under such conditions in the application, in order to minimize the risk of exceeding the maximum allowed die temperature. (See next section on thermal considerations). The drive to the switch is turned off when the supply voltage falls below the under-voltage threshold (VUVLO-0.5V).This prevents the switch working with excessive 'on' resistance under conditions where the duty cycle is high.

Thermal considerations

  When operating the device at high ambient temperatures, or when driving maximum load current, care must be taken to avoid exceeding the package power dissipation limits. The graph below gives details for power derating. This assumes the device to be mounted on a 25mm2 PCB with 1oz copper standing in still air.

  Note that the device power dissipation will most often be a maximum at minimum supply voltage. It will also increase if the efficiency of the circuit is low. This may result from the use of unsuitable coils, or excessive parasitic output capacitance on the switch output. When the application is limited by the internal power dissipation of the device, the ESOP8 package is recommended because of its enhanced power dissipation ability.
 

Thermal compensation of output current

High luminance LEDs often need to be supplied with a temperature compensated current in order to maintain stable and reliable operation at all drive levels. The LEDs are usually mounted remotely from the device so，for this reason, the temperature coefficients of the internal circuits for the PT4115 have been optimized to minimize the change in output current when no compensation is employed. If output current compensation is required, it is possible to use an external temperature sensing network - normally using Negative Temperature Coefficient (NTC) thermistors and/or diodes, mounted very close to the LED(s). The output of the sensing network can be used to drive the DIM pin in order to reduce output current with increasing temperature.

 Thermal shutdown protection(TSD)

  To ensure the reliability, the PT4115 is built with a thermal shutdown (TSD) protection function. The TSD protests the IC from over temperature (160℃). When the chip temperature decreases (140℃), the IC recovers again.

  Layout considerations

  Careful PCB layout is critical to achieve low switchinglosses and stable operation. Use a multilayer boardwhenever possible for better noise immunity. Minimizeground noise by connecting high-current ground returns, the input bypass-capacitor ground lead, and the output-filter ground lead to a single point (star ground configuration).SW pinThe SW pin of the device is a fast switching node, so PCB tracks should be kept as short as possible. To minimize ground 'bounce', the ground pin of the device should be soldered directly to the ground plane.Coil and decoupling capacitors and current sense resistorIt is particularly important to mount the coil and the input decoupling capacitor as close to the device pins as possible to minimize parasitic resistance and inductance, which will degrade efficiency. It is also important to minimize any track resistance in series with current sense resistor RS. It’s best to connect VIN directly to one end of RS and CSN directly to the opposite end of RS with no other currents flowing in these tracks. It is important that the cathode current of the Schottky diode does not flow in a track between RS and VIN as this may give an apparent higher measure of current than is actual because of track resistance.