4 Channels Constant Current LED Driver with PWM Brightness Control Description The SN3214 provides four regulated current sources, delivers up to 20mA of load current to accommodate four White LEDs. It requires no charge pump, has no noise and significantly improved the efficiency. Brightness can be controlled PWM techniques. The constant current source is set with an external sense resistor. Alternatively, a PWM signal applied to The PWM pin can vary the perceived brightness of the LED. The device is in shut down mode when the PWM Input is logic low. Features Ultra Low Headroom Voltage Cost Effective LED Driver Four Channel Constant Current Output Ideal for Driving LEDs Current Adjustable Via an External Resistor Best Noise and Efficiency Performance Ultra Low Quiescent Supply Current: 122uA (typ) Highly Integrated Design, Minimal Component Less than 1μA (max.) Shutdown Current Small Package QFN-16 3mm 3mm, 4mm 4mm Application White LED Display Backlights White LED Keypad Backlights 1-Cell Li-Ion Battery-operated Equipment Including PDAs, Hand-held PCs, Cellular Phone Typical application 16 15 14 13 PWM 1 2 12 11 3 10 4 9 5 6 7 8 VIN RSET 7.5 k Ω 1μF C IN 0.1μF Note: If less than four LEDs connected, the left pin can be floating or connected to GND. Connecting to GND is recommended. The current calculating method is explained in APPLICATION INFORMATION 1
Pin Configurations Package Pin Configurations (Top View) SN3214 LED4 QFN-16 ISET 5 16 15 6 VIN LED1 LED2 LED3 14 7 VIN 13 8 NC Pin Description Ordering Information Pin Name Pin No. Description PWM 1 PWM brightness control NC 2-4,8-11 No connection I SET 5 Current Set Input V IN 6,7 Input Voltage GND 12 Ground LED1-LED4 16-13 Regulated output current sinks 1~4 Order Number Package Type Operating Temperature range SN3214I316E QFN-16-40 C to 85 C SN3214I416E QFN-16-40 C to 85 C SN3214 Environmental Code E: Lead Free Pin Code 16:16pins Package Type 3: QFN, 3mm 3mm; 4: QFN, 4mm 4mm Temperature Code I: Industrial, -40 C to +85 C Absolute Maximum Ratings 2
Input Voltage (V IN ).......................-0.3V to 6.0V PWM......................-0.3V to (VIN+0.3V) / 6.0V θ JA of QFN-16 3mm 3mm................ 34.45 C/W θ JA of QFN-16 4mm 4mm................ 30.12 C/W Junction Temperature (T J )............-40 C to 105 C Storage Temperature.................. -65 C to 150 C Operating Ratings T MIN T A T MAX................... 40 C T A 85 C Input Voltage (V IN ).................. 2.7V V IN 5.5V Electrical Characteristics Unless otherwise specified, V IN = 3.6V, R SET =7.5kΩ, V LEDX =0.5V. Symbol Parameter Condition Min Typ. Max Units I LEDX Output Current Regulation 0.4V V LEDX 2.0V 16.0 ma V HR V HR Minimum Header Room Voltage(LEDX) Minimum Header Room Voltage(LEDX) V IN =3.3V, I LEDX =16.0mA 44.0 mv V IN =3.3V, R SET =6.0kΩ, I LEDX =20.0mA 53.0 mv V SET I SET PIN Voltage 1.2 V I LEDX /I SET Output Current to Current Set Ratio 100.0 I Q Quiescent Supply Current I LEDX =0mA, I SET =Float 122.0 μa I PWM Shutdown Supply Current V(PWM) = 0 V 0.8 μa V PWM-IH PWM Input Logic High 3.0V V IN 5.5V 1.4 V V PWM-IL PWM Input Logic Low 3.0V V IN 5.5V 0.4 V I PWM PWM Pin Current V(PWM) = 1.5V 2.8 μa 3
Typical Performance Characteristics I LED vs. Output Voltage LED Current vs. Power Supply LED Current (ma) RSET=6.0k RSET=7.5k RSET=12.0k LED Current (ma) RSET=6.0k RSET=7.5k RSET=12.0k VIN=3.3V VIN=3.3V VLEDX=0.2V Output Voltage(VLEDX)(mV) Figure 1 Power Supply(V) Figure 2 Quiescent Current vs. Power Supply LED Current vs. R SET Quiescent Current ( A LED Current (ma) VIN=3.3V RSET=7.5k Power Supply(VIN)(V) Figure 3 RSET(kΩ) Figure 4 4
Circuit Description The SN3214 is a parallel white-led driver with four matched current outputs. The matched current regulators each have a 100:1 current ratio between the LEDX outputs and the I SET currents. The mirrors control the current through the LEDs without the use of external ballast resistors. With a total of 80mA of total output current available, the SN3214 is easily capable of supplying 20mA per each of the four outputs through the proper selection of the R SET resistor. LED brightness control can be achieved on the SN3214 with a PWM signal. Application Information Shutdown When the voltage on the active-high-logic PWM pin is low, the SN3214 will be in shutdown mode. While disabled, the SN3214 typically draws 0.1μA form the power supply. There is no internal pull-up or pull-down on the PWM pin of the SN3214. Output Current Capability The SN3214 is capable of providing up to 20mA of current to each of the four outputs given an input voltage of 2.7V to 5.5V. An external resistor can be used to set the output current, as approximated with the following the equation: R SET =100 (V RSET / I LEDX ) V RSET is the voltage of R SET resistance, it is 1.20V typically. In order for the output currents to be regulated properly, sufficient headroom voltage (V HR ) is required. The headroom voltage refers to the minimum amount of voltage that must be present across the current source in order to ensure the desired current is realizable. To ensure the desired current is obtained, apply the following equations to find the minimum input voltage required: V IN - V LEDX V HR V LEDX is the diode forward voltage; V HR is typically as shown in table1. I LED R SET V HEADROOM 10.0mA 12.0kΩ 27 mv(typ.) 16.0mA 7.5kΩ 44 mv(typ.) 20.0mA 6.0kΩ 53 mv(typ.) Table1. I LED, R SET and V HR-MIN, V CC =3.3V PWM Brightness Control Brightness control can be implemented by pulsing a signal at the PWM pin. The R SET value should be selected using the R SET equation. The LED brightness is proportional to the duty cycle (D) of the PWM signal. The PWM frequency (f) should be limited to accommodate the turn-on time (T ON =50μs) of the device. D (1/f) > T ON f MAX = D MIN / T ON If the PWM frequency is much less than 100Hz, flicker may be seen in the LEDs. For the SN3214, zero duty cycle will turn off the LEDs and a 50% duty cycle will result in an average I LED being half of the programmed LED current. For example, if R SET is set to program 16mA, a 50% duty cycle will result in an average I LED of 8mA, I LED being half the programmed LED current. R SET should be chosen not to exceed the maximum current delivery capability of the device. LED Selection The SN3214 has ultra low Headroom Voltage and it is designed to drive white-leds with a typical forward voltage up to 3.5V. As the voltage drop the recommend HR voltage, the drive current will automatically scale down to reduce the LED current consumption. Parallel LEDx Outputs for Increased Current Drive Outputs LED1 through LED4 may be connected together in any combination to drive higher currents through fewer LEDs. For example in Figure 5, outputs LED1 and LED2 are connected together to drive one LED while LED3 and LED4 are connected together to drive a second LED. C IN 0.1μF 1μF V IN PWM GND LED1 LED2 LED3 LED4 I SET R SET Figure 5 Two Parallel Connected LEDs With this configuration, two parallel current sources of equal value provide current to each LED. RSET should therefore be chosen so that the current through each output is programmed to 50% of the desired current through the parallel connected LEDs. For example, if 32mA is the desired drive current for 2 parallel connected LEDs, R SET should be selected so that the current through each of the outputs is 16mA. Other combinations of parallel outputs may be implemented in similar fashions, such as in Figure 6. C IN 0.1μF 1μF V IN PWM GND LED1 LED2 LED3 LED4 I SET R SET Figure 6 One Parallel Connected LED 5
Connecting outputs in parallel does not affect internal operation of the SN3214 and has no impact on the Electrical Characteristics and limits previously presented. The available diode output current, maximum diode voltage, and all other specifications provided in the Electrical Characteristics table apply to parallel output configurations, just as they do to the standard 4-LED application circuit. Power Dissipation The maximum allowable power dissipation that this package is capable of handling can be determined as follows: P DMax = (T JMax - T A ) /θ JA Where T JMAX is the maximum junction temperature, T A is the ambient temperature and θ JA is the junction-to-ambient thermal resistance of the specified package. The SN3214 come in the QFN-16 3mm 3mm and 4mm 4mm package, 3mm 3mm package has a junction-to-ambient thermal resistance (θ JA ) equal to 34.45 C/W, and 4mm 4mm package has a junction-to-ambient thermal resistance (θ JA ) equal to 30.12 C/W. This value of θ JA is highly dependant upon the layout of the PC board. The actual power dissipated by the SN3214 follows the equation: P DISS = (V IN I CC ) - N(V LEDX I LEDX ) Where N equals the number of active outputs, V LEDX is the LED forward voltage, and I LEDX is the current supplied to the LED. Input Capacitor Selection The SN3214 is designed to run off of a fixed input voltage. Depending on the stability and condition of this voltage rail, it may be necessary to add some small input capacitors to help filter out any noise that may be present on the line. In the event that filtering is needed, surface-mount multi-layer ceramic capacitors are recommended. These capacitors are small and inexpensive. Two capacitances, a 0.1μF and a 1μF are typically sufficient. 6
Package Information QFN-16: 3mm 3mm Symbol Dimension (mm) MIN NOM MAX A 0.70 0.75 0.80 A1 0.00 0.02 0.05 C 0.20 REF b 0.18 0.25 0.30 D 2.90 3.00 3.10 D2 1.55 1.70 1.80 E 2.90 3.00 3.10 E2 1.55 1.70 1.80 e 0.50BSC L 0.30 0.40 0.50 y 0.00 0.075 7
QFN-16: 4mm 4mm Symbol Dimension (mm) MIN NOM MAX A 0.70 0.75 0.80 A1 0.00 0.02 0.05 b 0.25 0.30 0.35 C 0.20 REF. D 3.90 4.00 4.10 D2 2.00 2.65 2.80 E 3.90 4.00 4.10 E2 2.00 2.65 2.80 e 0.65 L 0.30 0.425 0.65 y 0.00 0.076 8