SM8120A White LED Driver IC OVERVIEW The SM8120A is a high efficiency step-up DC/DC converter. Due to high voltage CMOS process realizing 24V output supply as maximum value, white LED of 2–4 lights 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. PINOUT FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Boost-up control using PFM White LED of 2-4 lights (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 Supply voltage range: 2.4 to 5.5V Maximum output voltage: 24V Quiescent current: 80µA (typ) Standby current: 1.0µA (max) RON (Switching MOS-Tr): 2Ω (typ) Maximum switching frequency: 500kHz (typ) Output current detection accuracy: ± 2% Small package: SOT23-5 (Top view) VDD 1 VSS 2 SW 3 ■ ■ ■ ■ FB 0.20MIN + 0.2 1.6 − 0.1 + 0.2 2.8 − 0.3 + 0.1 0.15 − 0.05 1.9 ± 0.2 2.9 ± 0.2 0.4 ± 0.1 0.12 M 0 ~ 0.10 ■ ■ Cellular phone Pager Digital still camera Handy terminal PDAs Portable games White LED drive LCD bias supply Flash memory supply 1.1 ± 0.1 ■ 4 (Unit: mm) 0.8 ± 0.1 ■ CE PACKAGE DIMENSIONS APPLICATIONS ■ 5 0.95 0.1 ORDERING INFORMATION Device Package SM8120AH SOT23-5 NIPPON PRECISION CIRCUITS INC.—1 SM8120A BLOCK DIAGRAM SW VDD FB Buff Q S COMP R VREF SOFT START OSC CE VSS PIN DESCRIPTION Number Name I/O 1 VDD – Power supply 2 VSS – GND 3 SW O Coil switching 4 FB I Feed back (Output current detection) CE Ip1 5 Description Chip enable (High active) 1. Input with built-in pull-down resistor NIPPON PRECISION CIRCUITS INC.—2 SM8120A 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 27 V SW input current ISW 250 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 2.4 3.6 5.5 V Supply voltage VDD VDD Standby current VDD ISTB VCE = 0V – – 1.0 µA Quiescent current VDD IDD VFB = 1.0V – 80 120 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 Maximum switching frequency SW fOSC VFB = 0V 450 500 550 kHz Duty SW Duty VFB = 0V 53 60 67 % – – V CE VIH 2.0 Input voltage VIL – – 0.6 V CE ICE VCE = 3.6V – 5.0 10 µA FB IFB VFB = 0.5V –1.0 – 1.0 µA Soft-start time SW TSS – 500 – µs FB voltage FB VFB 0.49 0.50 0.51 V Input current NIPPON PRECISION CIRCUITS INC.—3 SM8120A OPERATION OVERVIEW VIN 3.0 to 4.5V L 22µH CIN 4.7µF SBD COUT 1.0µF SW LED FB ZD VDD Buff Q S COMP R VREF OSC Enable Disable CE SOFT START VSS R1 The SM8120A basic structure is a step-up DC/DC converter. The booster control employs Pulse Frequency Modulation (PFM) which controls the frequency (500kHz max.) at constant SW-Tr ON time (1.2µs 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 SM8120A 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 SW-Tr ON time is restricted to 0.6µs. Selecting the Current-setting Resistor (R1) The SM8120A 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.—4 SM8120A Selecting the Inductor (L) The recommended inductance for use with the SM8120A is 22µH. 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 22µH, and the SW-Tr ON time T ON is 1.2µs, then the peak inductor current Ipeak is (3.6 × 1.2 × 10-6) / (2.2 × 10-6) = 0.2A = 200mA. Selecting the Capacitors (CIN, COUT) The recommended capacitances for use with the SM8120A are 4.7µF ceramic input capacitor C IN and 1.0µF tantalum output capacitor COUT. The input capacitor ESR ratings affect the ripple voltage, therefore capacitors with low ESR rating are recommended. When the output capacitor ESR ratings are too low, it affect the response to the FB pin, therefore tantalum capacitors are recommended. The input capacitor should be mounted close to the SM8120A 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 VIN COUT LED VDD CIN CE FB VSS R1 NIPPON PRECISION CIRCUITS INC.—5 SM8120A LED OPEN-CIRCUIT PROTECTION When there is no load (LED open-circuit), the FB pin is pulled-down and then switching occurs at maximum frequency. Consequently, the output voltage continues to be boosted and the SW pin voltage may exceed the maximum rating of 27V. A zener diode can be added so that it acts as the output load when the LED is opencircuit, preventing the SW voltage from rising. The zener diode must be selected so that the zener does not breakdown during normal operation. The zener voltage VZD range is given by the following relationship, where N is the number of LEDs connected in series, VF MAX is the maximum LED forward-bias voltage drop, VOUT MAX is the SW pin maximum output voltage, VFB is the FB pin voltage, and VSBD is the Schottky-barrier diode forward-bias voltage drop. (VF MAX × N) ≤ VZD ≤ (VOUT MAX − VFB − VSBD) When the load is applied using a connector (SM8120A and LEDs on separate boards), the zener diode should be mounted on the same board as the SM8120A device so that the SW boost prevention function can operate when the load is disconnected. Zener Diode (ZD) Only Connection When the load is removed (LEDs open circuit), the output voltage is determined by the zener voltage, and the output current is determined by the output current-setting resistance. Consequently, the output current when the load is removed is not limited, and thus the input current cannot be controlled. VOUT=VZD+0.5=15.5V SBD COUT 1.0µF IIN=(VOUT IZD)/VIN=65mA L 22µH SW VIN 3.6V CIN 4.7µF VDD LED Open ZD (15V) VSS CE FB 33Ω IZD=0.5/33=15.15mA Zener Diode (ZD) and Current-Limiting Resistance Connection When the load is removed (LEDs open circuit), the output voltage is determined by the zener voltage, and the output current is determined by the sum of the output current-setting resistance and the current-limiting resistance. Consequently, the output current is limited when the load is removed, and the input current can be controlled. VOUT=VZD+0.5=15.5V SBD COUT 1.0µF IIN=(VOUT IZD)/VIN=2mA L 22µH SW VIN 3.6V CIN 4.7µF VDD FB LED Open ZD (15V) VSS CE 1kΩ 33Ω IZD=0.5/1033=0.48mA NIPPON PRECISION CIRCUITS INC.—6 SM8120A 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 22µH SW VIN 3.6V CIN 4.7µF VDD LED VSS CE FB 10 5 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.—7 SM8120A Brightness adjustment using FB pin (PWM signal input) 20 SBD COUT 1.0µF 15 SW VIN 3.6V VDD LED VSS CIN 4.7µF LED current [mA] L 22µH 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.—8 SM8120A 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 COUT 1.0µF Average LED current [mA] 100Hz L 22µH SW VIN 3.6V VDD LED VSS CIN 4.7µF FB CE 15.0 400Hz 10.0 5.0 1000Hz 1400Hz R1 25Ω PWM signal 0.0 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 SM8120A 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.—9 SM8120A 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 22µH SW VIN 3.6V VDD LED VSS CIN 4.7µF FB 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 Select signal 2 R2 100Ω R1 250Ω Tr1 Tr2 NIPPON PRECISION CIRCUITS INC.—10 SM8120A TYPICAL APPLICATION CIRCUITS 2 LEDs 3 LEDs SBD SBD COUT 2.2µF COUT 2.2µF L 22µH SW L 22µH ILED VDD SW ILED VDD LED VSS CIN 4.7µF VIN LED FB VSS CIN 4.7µF VIN CE FB CE R CIN COUT L SBD LED R : 2012Y5VIC475Z (TDK) :16MCM225MA (Nippon Chemi-con) : LQH32CN220K21 (Murata) : RB551V-30 (ROHM) : NSCW455 (NICHIA) CIN COUT L SBD LED 100 100 95 95 : 2012Y5VIC475Z (TDK) :16MCM225MA (Nippon Chemi-con) : LQH32CN220K21 (Murata) : RB551V-30 (ROHM) : NSCW455 (NICHIA) 90 90 VIN = 4.5V VIN = 4.5V 85 VIN = 3.6V VIN = 2.4V 80 Efficiency [%] Efficiency [%] 85 75 70 VIN = 3.6V VIN = 2.4V 80 75 70 65 65 60 60 55 55 50 50 0 5 10 15 0 20 5 10 100 100 95 95 90 ILED = 15mA ILED = 5mA ILED = 2mA 85 Efficiency [%] Efficiency [%] 20 90 ILED = 15mA ILED = 5mA ILED = 2mA 85 80 75 70 80 75 70 65 65 60 60 55 55 50 50 2.5 3.0 3.5 4.0 4.5 5.0 5.5 2.5 3.0 3.5 VIN [V] 4.0 4.5 5.0 5.5 VIN [V] 25 25 15 R = 25Ω 20 R = 25Ω R = 33Ω 15 R = 33Ω 10 R = 50Ω 5 R = 100Ω 10 R = 50Ω 5 R = 100Ω ILED [mA] 20 ILED [mA] 15 ILED [mA] ILED [mA] R = 250Ω 0 2.5 3.0 3.5 4.0 VIN [V] 4.5 5.0 5.5 R = 250Ω 0 2.5 3.0 3.5 4.0 VIN [V] 4.5 5.0 5.5 NIPPON PRECISION CIRCUITS INC.—11 SM8120A 4 LEDs SBD COUT 2.2µF L 22µH SW ILED VDD LED VSS CIN 4.7µF VIN FB CE R CIN COUT L SBD LED : 2012Y5VIC475Z (TDK) :16MCM225MA (Nippon Chemi-con) : LQH32CN220K21 (Murata) : RB551V-30 (ROHM) : NSCW455 (NICHIA) 100 95 90 VIN = 4.5V Efficiency [%] 85 VIN = 3.6V VIN = 2.4V 80 75 70 65 60 55 50 0 5 10 15 20 ILED [mA] 100 95 90 ILED = 15mA ILED = 5mA ILED = 2mA Efficiency [%] 85 80 75 70 65 60 55 50 2.5 3.0 3.5 4.0 4.5 5.0 5.5 VIN [V] 25 R = 25Ω 15 R = 33Ω 10 R = 50Ω 5 R = 100Ω ILED [mA] 20 R = 250Ω 0 2.5 3.0 3.5 4.0 VIN [V] 4.5 5.0 5.5 NIPPON PRECISION CIRCUITS INC.—12 SM8120A FOOTPRINT PATTERN SOT23-5 2.4 1.0 0.7 0.95 NIPPON PRECISION CIRCUITS INC.—13 SM8120A 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. 4-3, Fukuzumi 2-chome, Koto-ku, Tokyo 135-8430, Japan Telephone: +81-3-3642-6661 Facsimile: +81-3-3642-6698 http://www.npc.co.jp/ Email: [email protected] NC0203BE 2004.01 NIPPON PRECISION CIRCUITS INC.—14