AT mA LED Driver w/ Internal Switch

Transcription

1 FEATURES Up to 95% Efficiency 0.1V Current Sense Threshold Voltage 5V to 36V Input Voltage Range Driving up to 30LEDs (1W 10S3P) at DC36V IN Up to 1MHz Oscillation Frequency Continuous 1A Output Capability (typical 3%) Output Current Accuracy PWM or Analog Dimming Control Thermal Shutdown Protection SOT-25 & SOT-89-5 Package Meet RoHS Standard APPLICATION High power LED lighting Automotive LED lighting Low voltage industrial lighting LED back-up lighting Constant Current Source DESCRIPTION AT7401 is a continuous mode inductive step-down LED driver that is designed for driving single or multiple series connected LEDs. The input voltage range of AT7401 is from 5V to 36V. It is suitable for series-parallel 1W, 3W, or 5W high power LED application due to the high operation voltage and output capability. Depending upon supply voltage and external components, this can provide up to 30W of output power. AT7401 includes the output switch and a high-side output current sensing circuit, which uses an external resistor to set the nominal average output current. The output current can be adjusted below the set value, by applying an external PWM signal or DC level to DIM pin. Additionally, to ensure the system reliability, the AT7401 is with built-in over temperature protection, And LED open circuit short- circuit protection to protect system from being damaged. ORDER INFORMATION PIN CONFIGURATIONS(TOP VIEW) IAT AT KE R Circuit Type Shipping: R: Tape & Reel T: Tube 5 4 KE:SOT-25 KG5: SOT

2 PIN DESCRIPTIONS Pin Symbol SW GND DIM ISENSE VIN Pin Description Switch output. This pin is the Drain of the internal N-MOSFET. Signal and power ground. Connect this pin to the circuit ground. Multi-function for ON/OFF and brightness control. - Leave this pin for normal operation. - Drive it below 0.5V to turn off this device. Current sense input. Connect a resistor (R S ) from this pin to input to set average LED current. I LED =0.1V/R S Power supply input. Decouple to ground with 10uF or higher X7R ceramic capacitor. TYPICAL APPLICATION CIRCUITS C2 ISENCE Figure 1 2

3 BLOCK DIAGRAM ISENCE Figure 2 3

4 ABSOLUTE MAXIMUM RATINGS (Note 1) Parameter Symbol Max. Value Unit Supply Voltage V IN 0~36(40V for 0.5sec) V Output Current I OUT 1.25 A Sustaining Voltage at SW pin V SW -0.5 ~ 36V(40V for 0.5sec) V Power Dissipation, P T A =25 C(Note 2) Thermal Resistance Junction to Ambient Thermal Resistance Junction to Ambient SOT P D SOT SOT-25(Note 3) 333 Θ JA SOT SOT Θ JC SOT mw C/W C/W Junction Temperature T J 150 C Lead Soldering Temperature (5 sec) T LEAD 300 C Note 1: Stresses listed as the above Absolute Maximum Ratings may cause permanent damage to the device. These are for stress ratings. Functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may remain possibility to affect device reliability. Note 2: Thermal Resistance is specified with the component mounted on a low effective thermal conductivity test board in free air at T A =25 C. Note 3: Thermal Resistance is specified with approximately 1 square of 1 oz copper. RECOMMENDED OPERATING CONDITIONS (Note 4) Parameter Symbol Operation Conditions Unit Input Voltage Range V IN 5 ~ 36 V Operating Ambient Temperature Range T A -40 ~ 85 Operating Junction Temperature Range T J -40 ~ 125 o C o C Note 4:These are conditions under which the device functions but the specifications might not be guaranteed. For guaranteed specifications and test conditions, please see the Electrical Specifications. 4

5 ELECTRICAL CHARACTERISTICS TA=25oC, VIN=12V, VOUT=3.6V, L1=68uH, CIN=COUT=10uF; unless otherwise specified. Parameter Symbol Test Conditions Min. Typ. Max. Unit Operating voltage V IN 5-36 V Operating current I IN V IN = 5V~36V ma Output Current I OUT SOT SOT89-5L ma Output current Accuracy I OUT /I OUT 150mA < IOUT < 1000mA - ±3 ±5 % Efficiency VIN = 12V, IOUT=350mA, VOUT=10.8V % SW Dropout voltage V SW IOUT=1000mA V Input Voltage High VIH V Input Voltage Low VIL V Load Regulation %/ V VIN=24V, IOUT=500mA, 3.6V<VOUT<18V %/V Sense threshold hysteresis VSENSE_HYS - ±15 - % Mean current sense threshold voltage VSENSE mv Switch on resistance RDS (ON) VIN=12V, IOUT=350mA,VOUT=10.8V Ω Internal propagation delay tpd ns Minimum switch ON time ton_min ns Minimum switch OFF time toff_min ns Thermal Shutdown Threshold tsd o C Thermal Shutdown Hysteresis tsd_hys o C Recommended duty cycle range of switch Dsw Maximum operating frequency Freq_MAX kh Z 5

6 Duty cycle range of PWM signal applied to the DIM pin Duty_DIM PWM frequency = 1kHz Rise time of output current Tr VOUT=3.6V, IOUT=350mA, fdim=1khz, Duty_DIM=50% ns Fall time of output current tf VOUT=3.6V, IOUT=350mA, fdim=1khz, Duty_DIM=50% ns 6

7 TYPICAL OPERATINGCHARACTERITICS Efficiency vs. Input Voltage at Various LED Cascaded Number Efficiency vs. input IOUT=370mA Efficiency vs. input IOUT=370mA Efficiency vs. input IOUT=370mA 7

8 Efficiency vs. LED Cascaded Number at Various Input Voltage Efficiency vs. LED cascaded IOUT =370mA Efficiency vs. LED cascaded IOUT =370mA Efficiency vs. LED cascaded IOUT =370Ma 8

9 Output Current vs. Input Voltage at Various LED Cascaded Number % 95.00% 90.00% 85.00% 80.00% 75.00% 70.00% 65.00% 30V 24V 12V 9V 60.00% 1LED 2LED 3LED 4LED 5LED 6LED Output current vs. input IOUT =370mA Output current vs. input IOUT =370mA Output current vs. input IOUT =370mA 9

10 Output Currnet vs. Input Voltage at Various Inductor Output current vs. input in cascaded, IOUT =370mA Output current vs. input in cascaded, IOUT =370mA Output current vs. input in cascaded, IOUT =370mA 10

11 Output Current vs. LED Cascaded Number at Various Input Voltage Output current vs. LED cascaded IOUT =370mA Output current vs. LED cascaded IOUT =370mA Output current vs. LED cascaded IOUT =370mA 11

12 Output Current vs. LED Cascaded number at Various Inductor Output Current vs. LED Cascaded VIN =12V, IOUT =370mA Output Current vs. LED Cascaded VIN =24V, IOUT =370mA Output Current vs. LED Cascaded VIN =30V, IOUT =370mA 12

13 Switching Frequency vs. LED Cascaded Number at Various Inductor Switching Frequency vs. LED Cascaded VIN =12V, IOUT =370mA Switching Frequency vs. LED Cascaded VIN =24V, IOUT =370mA Switching Frequency vs. LED Cascaded VIN =30V, IOUT =370mA 13

14 APPLICATION INFORMATION 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 Iout and is given by: IOUTNOM = 0.1/ RS [for RS 0.1) ] Nominal ripple current is ±15mV/ RS The table below gives values of nominal average output current for several preferred values of current setting resistor (RS) in the typical application circuit above: RS (Ω) Nominal average output current (ma) Dimming A Pulse Width Modulated (PWM) signal with duty cycle DPWM can be applied to the DIM pin. A logic low (below 0.5V) at DIM will disable the internal MOSFET and turn off the current flow to the LED array. An internal pull-high circuit ensures that the AT7401 is ON when DIM pin is unconnected. Open / short circuit LED protection When any LED is open-circuit, the output current will be turned off. When any LED is short-circuit, the output current will be limited to its preset value. Over Temperature protection When the junction temperature over range. AT7401 will turn off output current. Minimum Input Voltage The Minimum Input Voltage is the sum of the voltage drops on RSEN, DCR of L1, Rds(ON) of 14 Internal MOS switch and the total forward voltage of LEDS VLED. VIN=VRS+VLED+VL1+VSW. Design Consideration: (1) Switching Frequency For better output current accuracy, the switching frequency should be determined by Minimum on/off time SW waveform. FSW=(1-D)/TOFFMIN, when the duty cycle > 0.5 or FSW=D/TONMIN, when the duty cycle < 0.5, (D=Vout/VIN) The switching frequency is related to efficiency (better at low frequency), the size/cost of components, and the amplitude of output ripple voltage and current ( smaller at high frequency). The slower switching frequency comes from the large value of inductor. In many applications, the Sensitivity of EMI limits the switching frequency. The switching frequency can be ranged from 40kHz to 1.0MHz. (2) LED Ripple Current An LED constant current driver is designed to control the current through the cascaded LEDs, instead of the voltage across it. Higher LED ripple current allows the use of smaller inductance, smaller output capacitance, and even without an output capacitor. The advantages of higher LED ripple current are to minimize PCB size and reduce cost because of no output capacitor. Lower LED ripple current requires large induce and output capacitor. The advantages of lower LED ripple current are to extend LED life time and to reduce heating of LED. The recommended ripple current is from 5% to 20% of normal LED output current.

15 Capacitor Selection A low ESR capacitor should be used for input decoupling, as the ESR of this capacitor appears in series 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 input source is close to the device, but higher values will improve performance at lower input voltages, especially when the source impedance is high. The input capacitor should be placed as close as possible to the IC. Inductor Selection The inductance is determined by two factors : the switching frequency and the inductor ripple Current. The calculation of the inductance, L1, can be described as L1>(VIN-VOUT-VSEN-(Rds(ON)x IOUT))x D/( fswx IL) Higher values of inductance are recommended at higher supply voltages 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. The inductor should be mounted as close to the device as possible with low resistance 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. 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. Switch on time: TON= L I/(VIN-VLED IAVG (RS + rl + Rsw ) ) TOFF= L I/(VLED + VD + IAVG ( Rs + rl) ) Where L is the coil inductance rl is the coil resistance RS is the current sense resistance IAVG is the required LED current I is the coil peak-peak ripple current {Internally set to 0.3 x IAVG} VIN is the supply voltage VLED is the total LED forward voltage Rsw is the switch resistance VD is the diode forward voltage at the required load current 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 operating voltage 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 85oC. Excess leakage will increase 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. 15

16 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. 16

17 PACKAGE OUTLINE DIMENSIONS SOT-25 PACKAGE OUTLINE DIMENSIONS 17

18 PACKAGE OUTLINE DIMENSIONS SOT-89-5L PACKAGE OUTLINE DIMENSIONS Note : Information provided by IAT is believed to be accurate and reliable. However, we cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in an IAT product; nor for any infringement of patents or other rights of third parties that may result from its use. We reserve the right to change the circuitry and specifications without notice. Life Support Policy: IAT does not authorize any IAT product for use in life support devices and/or systems. Life support devices or systems are devices or systems which, (I) are intended for surgical implant into the body or (II) support or sustain life, and whose failure to perform, when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. Typical numbers are at 25 C and represent the most likely norm. 18