SM8122A White LED Driver IC OVERVIEW The SM8122A is a high efficiency step-up DC/DC converter. Due to high voltage CMOS process realizing 25V output supply as maximum value, 2 to 6 lights of white LED connected in series can be lighted. By connecting in series, current variation among LED is eliminated. Current value sent to white LED can be set by external resistors. In addition, brightness can also be adjusted by control to FB pin or CE pin. Since the SM8122A has an over voltage protection circuit built-in, it dispenses with the existing external ZD (zener diode). Besides, the switching frequency of the SM8122A is higher (2.0MHz) than the existing product (SM8121A), so that it can respond to lower inductance value. FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ PINOUT Boost-up control using PWM 2 to 6 lights of white LED (connected in series) lighted Output current value can be set by external resistors (51Ω: 9.8mA, 33Ω: 15.2mA, 24Ω: 20.8mA) Brightness adjustable by control to FB pin or CE pin Current variation among LED decreased by high precision High efficient drive by step-up model Over voltage protection circuit built-in Supply voltage range: 2.3 to 5.5V Maximum output voltage: 25V Quiescent current: 820µA (typ) Standby current: 1.0µA (max) RON (Switching MOS-Tr): 2Ω (typ) Switching frequency: 2.0MHz (typ) Output current detection accuracy: ± 2% Package: SOT23-6W (SM8122AH) MSON-6 (SM8122AD) (Top view) ■ ■ SOT23-6W SW 1 6 VDD VOUT 2 5 VSS FB 3 4 CE MSON-6 VDD 1 6 VOUT SW 2 5 VSS CE 3 4 FB APPLICATIONS ■ ■ ■ ■ ■ ■ ■ ■ ■ Cellular phone Pager Digital still camera Handy terminal PDAs Portable games White LED drive LCD bias supply Flash memory supply ORDERING INFORMATION Device Package SM8122AH SOT23-6W SM8122AD MSON-6 NIPPON PRECISION CIRCUITS INC.—1 SM8122A PACKAGE DIMENSIONS (Unit: mm) ■ SOT23-6W 2.9 ± 0.2 + 0.1 0.15 − 0.05 (0.95) 0.1MIN 2.8 ± 0.2 1.8 ± 0.2 1.9 ± 0.2 (0.95) 0 to 0.1 1.1 ± 0.1 0.8 ± 0.1 0 to 15 ° 0.1 + 0.1 0.4 − 0.05 MSON-6 0.1 ± 0.05 45 ° 0.6 ± 0.05 4 0.2 ± 0.08 0.018 45 ° 0.14 ± 0.05 0.038 ± 0.02 0.125 45 ° 1 0.8 ± 0.1 R0 .07 5 4 R0 6 ° 3 .1 0.5 ± 0.1 1.4 ± 0.1 45 ° 6 45 MIN1.45 1.8 ± 0.15 1 0.75 ± 0.05 0.8 ± 0.05 MIN1.65 0.2 ± 0.08 0.1 ± 0.06 2.0 ± 0.15 + 0.1 0.75 − 0 ■ 0.2 M 3 1.0 ± 0.1 0.3 ± 0.1 NIPPON PRECISION CIRCUITS INC.—2 SM8122A BLOCK DIAGRAM SW VOUT OVP COMP VDD FB Buff ERR AMP PWM COMP RAMP GENERATOR VREF OSC SOFT START CE VSS PIN DESCRIPTION Number Name I/O 2 SW O Coil switching 2 6 VOUT I Output voltage detection 3 4 FB I Feed back (Output current detection) Chip enable (High active) SOT23-6W MSON-6 1 Description 4 3 CE Ip1 5 5 VSS – GND 6 1 VDD – Power supply 1. Input with built-in pull-down resistor NIPPON PRECISION CIRCUITS INC.—3 SM8122A SPECIFICATIONS Absolute Maximum Ratings Parameter Symbol Rating Unit Supply voltage range VDD −0.3 to 6.5 V Input voltage range VIN VSS – 0.3 to VDD + 0.3 V SW output voltage range VSW –0.3 to 30 V SW input current ISW 500 mA Power dissipation PD 250 (Ta = 25°C) mW Operating temperature range Topr –40 to 85 °C Storage temperature range Tstg −55 to 125 °C Electrical Characteristics VDD = 3.6V, VSS = 0V, Ta = 25°C unless otherwise noted Rating Parameter Pin Symbol Condition Unit min typ max Supply voltage VDD VDD 2.3 3.6 5.5 V Maximum output voltage SW VOUT – – 25 V Standby current VDD ISTB VCE = 0V – – 1.0 µA 200 400 VDD IDD VFB = 1.0V – Quiescent current µA VFB = 0V – 820 1600 µA SW-Tr ON resister SW RON ISW = 100mA, VDD = 3.6V – 2.0 3.0 Ω SW-Tr leak current SW ILEAK VSW = VDD – – 1.0 µA Switching frequency SW fOSC VFB = 0V 1.8 2.0 2.2 MHz Maximum duty SW Duty VFB = 0V 75 85 90 % – – V CE VIH 2.0 Input voltage VIL – – 0.6 V Input current Soft-start time CE ICE VCE = 3.6V – 5.0 10 µA FB IFB VFB = 0.5V –1.0 – 1.0 µA VOUT IVOUT VOUT = 25V 60 82 120 µA TSS1 Switching stop time 10 20 70 µs TSS2 Maximum duty restriction time – 500 – µs SW FB voltage FB VFB 0.49 0.50 0.51 V Coil inductance SW LSW – 4.7 10 µH VOV 25 30.5 36 V VOVR 23 28.5 – V Over voltage detection VOUT Over voltage detection release NIPPON PRECISION CIRCUITS INC.—4 SM8122A OPERATION OVERVIEW L 4.7µH SBD VIN 2.3 to 5.5V VOUT SW CIN 4.7µF COUT 1.0µF LED OVP COMP VDD Buff Enable Disable CE VSS ERR AMP PWM COMP FB RAMP GENERATOR VREF OSC SOFT START R1 The SM8122A basic structure is a step-up DC/DC converter. The booster control employs Pulse Width Modulation (PWM) which controls the pulse duty cycle (85% max.) at constant frequency (2.0MHz typ.). The LED current is set by a current-setting resistor R1 connected between pins FB (with stable voltage of 0.5V typ.) and VSS. When the switching transistor SW-Tr is ON, energy is stored in the inductor L. When SW-Tr is rapidly switched OFF, the energy stored in the inductor generates a voltage across the terminals of the inductor. The induced voltage, after being added to the input voltage, turns ON the Schottky barrier diode SBD and the stored energy is transferred to the output capacitor. This sequence of events continues repeatedly, boosting the output voltage. The SM8122A features a built-in soft-start function. The soft-start time is approximately 500µs from after the chip enable input CE rising edge. During this interval, the maximum duty is restricted. NIPPON PRECISION CIRCUITS INC.—5 SM8122A OVP (Over Voltage Protection) SM8122A is always monitoring the VOUT terminal voltage in order to protect itself from the stress of VOUT over voltage. If SM8122A detects the VOUT over voltage, it immediately stop the switching of the inductor drive transistor. After the VOUT terminal voltage decreases below the release voltage, SM8122A restarts switching the inductor drive transistor. The over voltage is set as approximately 30.5V, the release voltage is approximately 28.5V. Over voltage detection Over voltage detection 30.5V VOUT 28.5V Over voltage detection release SW Tr = OFF Over voltage detection release SW Tr Switching SW Tr = OFF Selecting the Current-setting Resistor (R1) The SM8122A control stabilizes the voltage on pin FB (0.5V typ.). Hence, the current-setting resistor R1 connected between FB and VSS sets the LED current ILED, where the resistance R1 is given by the following equation. R1 = 0.5 / ILED FB VFB=0.5V ILED=0.5/R1 R1=0.5/ILED NIPPON PRECISION CIRCUITS INC.—6 SM8122A Selecting the Inductor (L) The inductor DC resistance affects the power efficiency, therefore a low DC resistance inductor is recommended. Note also that the peak inductor current Ipeak should not exceed the inductor maximum current rating. In pulsed current mode control, the peak inductor current Ipeak is given by the following equation. Ipeak = (VIN × TON) / L For example, if the input voltage VIN is 3.6V, the inductance L is 4.7µH, and the SW-Tr ON time TON is 2MHz × 85% = 0.425µs, then the peak inductor current Ipeak is (3.6 × 0.425 × 10-6) / (4.7 × 10-6) = 0.326A = 326mA. Selecting the Capacitors (CIN, COUT) The recommended capacitances for use with the SM8122A are 4.7µF ceramic input capacitor CIN and 1.0µF ceramic output capacitor COUT. The capacitor ESR ratings affect the ripple voltage, therefore capacitors with low ESR rating are recommended. The input capacitor should be mounted close to the SM8122A IC. Note that the capacitor voltage ratings should be selected to provide sufficient margin for the applied input and output voltages. For example, if a lithium-ion battery (2.5 to 4.5V) is connected to the input and 3 white LEDs connected in series at the output draw 20mA, then the maximum input voltage is 4.5V and the maximum output voltage is (4.0V × 3 LEDs) + 0.5V = 12.5V. Therefore, the input capacitor should have a voltage rating of 6V, and the output capacitor should have a voltage rating of 16V. Selecting the Rectifier Schottky Barrier Diode (SBD) The rectifier schottky barrier diode forward-direction voltage drop affects the power efficiency, therefore a Schottky barrier diode with low forward-direction voltage drop is recommended. Note that the diode should be selected to provide sufficient margin for the rated current and reverse-direction withstand voltage. Board Layout Notes The following precautions should be followed for stable device operation. ■ ■ ■ ■ The inductor L and Schottky barrier diode SBD should be connected close to the pin SW using thick, short circuit wiring. The input capacitor CIN should be mounted close to the IC. The IC supply voltage VDD wiring and inductor supply wiring should be isolated, reducing any common impedances. The ground wiring should be connected at a single point, reducing any common impedances. SBD L SW VOUT LED VIN COUT VDD CIN CE FB VSS R1 NIPPON PRECISION CIRCUITS INC.—7 SM8122A BRIGHTNESS ADJUSTMENT Brightness Adjustment using FB Pin The LED brightness can be adjusted using an input DC control voltage connected through resistor R3 to the FB pin. Alternatively, the brightness can be controlled by a PWM signal by adding a low-pass filter comprising resistor R4 and capacitor C1. The PWM signal frequency range is determined by the low-pass filter coefficients. For example, the recommended values for resistor R4 (50kΩ) and capacitor C1 (0.1µF) provide a PWM signal frequency range of 1kHz to 1MHz. Brightness adjustment using FB pin (DC voltage input) 20 SBD COUT 1.0µF LED current [mA] 15 L 4.7µH SW VIN 3.6V CIN 4.7µF VOUT VDD SM8122 LED VSS 5 CE FB 10 R2 20kΩ DC Voltage 0 to 3V R1 30Ω R3 100kΩ 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 DC voltage [V] DC voltage vs. LED current Brightness adjustment circuit using FB pin (DC voltage input) When the brightness is controlled by DC voltage (VDC) connected to resistor R3, the LED current (ILED) is given by equation 1. VFB − ILED = R2 × (VDC − VFB) R3 ... (1) R1 If the values R1 = 30Ω, R2 = 20kΩ, R3 = 100kΩ, VFB = 0.5V, and VDC = 0V are inserted in equation 1, the LED current ILED = 20mA, as shown in equation 2. 0.5 − ILED = 20,000 × (0 − 0.5) 100,000 30 = 0.6 = 20mA 30 ... (2) If the values R1 = 30Ω, R2 = 20kΩ, R3 = 100kΩ, VFB = 0.5V, and VDC = 3V are inserted in equation 1, the LED current ILED = 0mA, as shown in equation 3. 0.5 − ILED = 20,000 × (3 − 0.5) 100,000 30 = 0 = 0mA 30 ... (3) Taking the above diagram as an example, inserting the values R1 = 30Ω, R2 = 20kΩ, R3 = 100kΩ, VFB = 0.5V, and VDC = 0 to 3V into equation 1 gives the maximum LED current ILED of 20mA when VDC = 0V (equation 2) and the minimum LED current ILED of 0mA when VDC = 3V (equation 3). NIPPON PRECISION CIRCUITS INC.—8 SM8122A Brightness adjustment using FB pin (PWM signal input) SBD 20 COUT 1.0µF L 4.7µH VIN 3.6V VOUT CIN 4.7µF LED current [mA] 15 SW VDD SM8122 LED VSS CE FB 10 5 R3 50kΩ PWM signal Duty [%] R4 50kΩ R1 30Ω R2 20kΩ C1 0.1µF 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 VPWM × Duty [V] VPWM [V] Brightness adjustment circuit using FB pin (PWM signal input) PWM signal vs. LED current When the brightness is controlled by PWM signal (VPWM × Duty), the LED current (ILED) is given by equation 4. VFB − ILED = R2 × (VPWM × Duty − VFB) R3 +R4 R1 ... (4) If the values R1 = 30Ω, R2 = 20kΩ, R3 = 50kΩ, R4 = 50kΩ, VFB = 0.5V, VPWM = 3V, and Duty = 0% are inserted in equation 4, the LED current ILED = 20mA, as shown in equation 5. 0.5 − ILED = 20,000 × (3 × 0 − 0.5) 50,000 + 50,000 30 = 0.6 = 20mA 30 ... (5) If the values R1 = 30Ω, R2 = 20kΩ, R3 = 50kΩ, R4 = 50kΩ, VFB = 0.5V, VPWM = 3V, and Duty = 100% are inserted in equation 4, the LED current ILED = 0mA, as shown in equation 6. 0.5 − ILED = 20,000 × (3 × 1 − 0.5) 50,000 + 50,000 30 = 0 = 0mA 30 ... (6) Taking the above diagram as an example, inserting the values R1 = 30Ω, R2 = 20kΩ, R3 = 50kΩ, R4 = 50kΩ, VFB = 0.5V, VPWM = 3V, and Duty = 0 to 100% into equation 4 gives the maximum LED current ILED of 20mA when Duty = 0% (equation 5) and the minimum LED current ILED of 0mA when Duty = 100% (equation 6). NIPPON PRECISION CIRCUITS INC.—9 SM8122A Brightness Adjustment using CE Pin The LED average current can be adjusted by controlling the duty of a PWM signal input on the CE pin. When CE goes from LOW to HIGH, the soft start function operates (with 500µs constant soft start time) and, therefore, the LED average current ratio for a given PWM signal duty falls with increasing PWM signal frequency. Taking this into consideration, the recommended PWM control signal has a frequency range of 100 to 400Hz with duty cycle range of 10 to 90%. 20.0 100 [Hz] 400 [Hz] 1000 [Hz] 1400 [Hz] Average LED current [mA] SBD COUT 1.0µF L 4.7µH SW VIN 3.6V VOUT CIN 4.7µF VDD SM8122 LED VSS 15.0 10.0 5.0 CE FB 0.0 R1 25Ω PWM signal 0 10 20 30 40 50 60 70 80 90 100 PWM signal duty [%] Brightness adjustment circuit using CE pin PWM signal duty vs. LED average current When adjusting the brightness using the CE pin, a ripple voltage synchronized to the PWM signal is generated across the output capacitor COUT. The amplitude of the ripple voltage is determined by the number of LEDs and their forward-bias voltage drop characteristics. If a ceramic capacitor is used for the output capacitor COUT, an audible noise may be generated due to the ceramic capacitor’s piezoelectric effect. The audible noise level depends on the ceramic capacitor (capacitance, bias dependency, withstand voltage etc.), LEDs (number, forward-bias voltage drop etc.), and mounting board (thickness, mounting conditions etc.), and thus should be verified under actual conditions. Alternatively, a tantalum capacitor or film capacitor with low piezoelectric effect can be used as the output capacitor COUT to minimize the noise level, or the brightness can be adjusted using the FB pin as described earlier. The audible noise generated when using the CE pin is not an inherent phenomena of the SM8122A device, but of the brightness adjustment method employed. 11.0V 8.1V 3.5V COUT 20mA 3.5V 2.7V COUT 0mA 2.7V 3.5V 2.7V 0.5V 0V Output voltage with LEDs ON Output voltage with LEDs OFF CE input signal and output ripple voltage NIPPON PRECISION CIRCUITS INC.—10 SM8122A Current Switching using External Transistors If only a few brightness steps are required, the LED current can be adjusted by switching the LED current setting resistance using external transistors (Tr). SBD COUT 1.0µF L 4.7µH SW VIN 3.6V CIN 4.7µF VOUT FB VDD SM8122 Select signal 1 ILED Low Low 2mA Low High 2 + 5 = 7mA High Low 2 + 12.5 = 14.5mA High High 2 + 5 + 12.5 = 19.5mA CE R3 40Ω Select signal 1 Select signal 2 LED VSS Select signal 2 R2 100Ω R1 250Ω Tr1 Tr2 NIPPON PRECISION CIRCUITS INC.—11 SM8122A RECOMMEND PATTERN SOT23-6W 2.4 1.0 0.7 0.95 0.95 Footprint pattern MSON-6 ç 2.3 ° 0.5 1.9 1.4 0.6 0.5 2.0 1.4 0.25 0.25 0.5 45 0.225 ° 45 0.5 0.225 2.4 0.5 0.4 0.8 0.5 1.0 Footprint pattern 0.8 0.4 1.0 Metalmask pattern NIPPON PRECISION CIRCUITS INC.—12 SM8122A Please pay your attention to the following points at time of using the products shown in this document. The products shown in this document (hereinafter “Products”) are not intended to be used for the apparatus that exerts harmful influence on human lives due to the defects, failure or malfunction of the Products. Customers are requested to obtain prior written agreement for such use from NIPPON PRECISION CIRCUITS INC. (hereinafter “NPC”). Customers shall be solely responsible for, and indemnify and hold NPC free and harmless from, any and all claims, damages, losses, expenses or lawsuits, due to such use without such agreement. NPC reserves the right to change the specifications of the Products in order to improve the characteristic or reliability thereof. NPC makes no claim or warranty that the contents described in this document dose not infringe any intellectual property right or other similar right owned by third parties. Therefore, NPC shall not be responsible for such problems, even if the use is in accordance with the descriptions provided in this document. Any descriptions including applications, circuits, and the parameters of the Products in this document are for reference to use the Products, and shall not be guaranteed free from defect, inapplicability to the design for the mass-production products without further testing or modification. Customers are requested not to export or re-export, directly or indirectly, the Products to any country or any entity not in compliance with or in violation of the national export administration laws, treaties, orders and regulations. Customers are requested appropriately take steps to obtain required permissions or approvals from appropriate government agencies. NIPPON PRECISION CIRCUITS INC. 15-6, Nihombashi-kabutocho, Chuo-ku, Tokyo 103-0026, Japan Telephone: +81-3-6667-6601 Facsimile: +81-3-6667-6611 http://www.npc.co.jp/ Email: [email protected] NC0323AE 2005.05 NIPPON PRECISION CIRCUITS INC.—13