SM8122AD

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 I I PINOUT (Top view) I I I I I I I I I I I I I I 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) SOT23-6W SW 1 6 VDD VOUT 2 5 VSS FB 3 4 CE I MSON-6 VDD 1 6 5 4 VOUT VSS FB APPLICATIONS I I I I I I I I I Cellular phone Pager Digital still camera Handy terminal PDAs Portable games White LED drive LCD bias supply Flash memory supply SW 2 CE 3 ORDERING INFORMATION Device SM8122AH SM8122AD Package SOT23-6W MSON-6 NIPPON PRECISION CIRCUITS INC.—1 SM8122A PACKAGE DIMENSIONS (Unit: mm) I SOT23-6W 2.9 ± 0.2 1.9 ± 0.2 + 0.1 0.15 − 0.05 1.8 ± 0.2 2.8 ± 0.2 (0.95) (0.95) 0 to 15 ° 0.8 ± 0.1 1.1 ± 0.1 0 to 0.1 0.1 + 0.1 0.4 − 0.05 0.2 M I MSON-6 0.1MIN 0.1 ± 0.06 MIN1.65 45 ° 0.8 ± 0.05 0.2 ± 0.08 2.0 ± 0.15 1.8 ± 0.15 1.4 ± 0.1 0.5 ± 0.1 MIN1.45 1 6 6 R0 .1 1 0.8 ± 0.1 4 45 ° 3 4 0.6 ± 0.05 3 0.14 ± 0.05 0.038 ± 0.02 0.2 ± 0.08 45 ° 45 ° R0 .07 5 0.125 1.0 ± 0.1 0.3 ± 0.1 + 0.1 0.75 − 0 0.018 NIPPON PRECISION CIRCUITS INC.—2 0.75 ± 0.05 0.1 ± 0.05 45 ° SM8122A BLOCK DIAGRAM SW VOUT OVP COMP VDD Buff PWM COMP ERR AMP FB RAMP GENERATOR VREF SOFT START CE OSC VSS PIN DESCRIPTION Number Name SOT23-6W 1 2 3 4 5 6 MSON-6 2 6 4 3 5 1 SW VOUT FB CE VSS VDD O I I Ip1 – – Coil switching Output voltage detection Feed back (Output current detection) Chip enable (High active) GND Power supply I/O Description 1. Input with built-in pull-down resistor NIPPON PRECISION CIRCUITS INC.—3 SM8122A SPECIFICATIONS Absolute Maximum Ratings Parameter Supply voltage range Input voltage range SW output voltage range SW input current Power dissipation Operating temperature range Storage temperature range Symbol VDD VIN VSW ISW PD Topr Tstg Rating −0.3 to 6.5 VSS – 0.3 to VDD + 0.3 –0.3 to 30 500 250 (Ta = 25°C) –40 to 85 −55 to 125 Unit V V V mA mW °C °C Electrical Characteristics VDD = 3.6V, VSS = 0V, Ta = 25°C unless otherwise noted Rating Parameter Supply voltage Maximum output voltage Standby current Quiescent current SW-Tr ON resister SW-Tr leak current Switching frequency Maximum duty Input voltage Pin VDD SW VDD VDD SW SW SW SW CE CE Input current FB VOUT Soft-start time FB voltage Coil inductance Over voltage detection VOUT Over voltage detection release SW FB SW Symbol VDD VOUT ISTB IDD RON ILEAK fOSC Duty VIH VIL ICE IFB IVOUT TSS1 TSS2 VFB LSW VOV VOVR VCE = 3.6V VFB = 0.5V VOUT = 25V Switching stop time Maximum duty restriction time VCE = 0V VFB = 1.0V VFB = 0V ISW = 100mA, VDD = 3.6V VSW = VDD VFB = 0V VFB = 0V Condition min 2.3 – – – – – – 1.8 75 2.0 – – –1.0 60 10 – 0.49 – 25 23 typ 3.6 – – 200 820 2.0 – 2.0 85 – – 5.0 – 82 20 500 0.50 4.7 30.5 28.5 max 5.5 25 1.0 400 1600 3.0 1.0 2.2 90 – 0.6 10 1.0 120 70 – 0.51 10 36 – V V µA µA µA Ω µA MHz % V V µA µA µA µs µs V µH V V Unit NIPPON PRECISION CIRCUITS INC.—4 SM8122A OPERATION OVERVIEW L 4.7µH SBD VIN 2.3 to 5.5V CIN 4.7µF SW VOUT LED OVP COMP COUT 1.0µF VDD Buff PWM COMP ERR AMP FB Enable Disable CE VSS RAMP GENERATOR OSC VREF 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 30.5V Over voltage detection 28.5V VOUT Over voltage detection release Over voltage detection release SW Tr = OFF 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. I I I I 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 VIN CIN VOUT LED VDD CE VSS FB COUT 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 L 4.7µH SW VOUT SM8122 VDD VSS CE R2 20kΩ DC Voltage 0 to 3V R3 100kΩ R1 30Ω LED 15 LED current [mA] 10 VIN 3.6V CIN 4.7µF FB 5 0 0.0 0.5 1.0 1.5 DC voltage [V] 2.0 2.5 3.0 Brightness adjustment circuit using FB pin (DC voltage input) DC voltage vs. LED current When the brightness is controlled by DC voltage (VDC) connected to resistor R3, the LED current (ILED) is given by equation 1. R2 × (VDC − VFB) R3 R1 VFB − ILED = ... (1) 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 15 SW VOUT SM8122 VDD VSS CE LED VIN 3.6V CIN 4.7µF LED current [mA] R1 30Ω 10 FB 5 R3 50kΩ Duty [%] VPWM [V] PWM signal R4 50kΩ R2 20kΩ C1 0.1µF 0 0.0 0.5 1.0 1.5 VPWM × Duty [V] 2.0 2.5 3.0 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. R2 × (VPWM × Duty − VFB) R3 +R4 R1 VFB − ILED = ... (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 SBD Average LED current [mA] COUT 1.0µF L 4.7µH SW VOUT SM8122 VDD VSS CE LED 15.0 100 [Hz] 400 [Hz] 1000 [Hz] 1400 [Hz] 10.0 VIN 3.6V CIN 4.7µF 5.0 FB 0.0 PWM signal R1 25Ω 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 COUT 20mA 3.5V 3.5V 3.5V 0.5V COUT 0mA 2.7V 2.7V 2.7V 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 VOUT SM8122 VDD VSS CE LED Select signal 2 Low Low Select signal 1 Low High Low High 2mA ILED VIN 3.6V CIN 4.7µF 2 + 5 = 7mA 2 + 12.5 = 14.5mA 2 + 5 + 12.5 = 19.5mA FB High R3 40Ω Select signal 1 Select signal 2 Tr2 R2 100Ω Tr1 R1 250Ω High NIPPON PRECISION CIRCUITS INC.—11 SM8122A RECOMMEND PATTERN SOT23-6W 0.7 1.0 0.95 0.95 Footprint pattern MSON-6 ç 2.4 2.4 2.3 0.225 0.225 ° 45 0.5 0.5 45 ° 0.25 0.25 0.6 1.4 2.0 0.5 0.5 0.5 0.4 0.8 1.0 0.5 0.8 1.0 0.4 Footprint pattern Metalmask pattern NIPPON PRECISION CIRCUITS INC.—12 1.4 1.9 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: sales@npc.co.jp NC0323AE 2005.05 NIPPON PRECISION CIRCUITS INC.—13