DATA SHEET AAT2404 Voltage-Variable Current Sourcing Boost Controller For LED Lighting Applications General Description Features The AAT2404 is a highly integrated high-efficiency variable voltage current sourcing boost controller for white LED backlight applications intended for use in large size LCD panels and LCD TVs. To accommodate various LED backlighting configurations in both direct and edge lighting applications, the device uses a high voltage external power MOSFET. The AAT2404 contains an integrated current sense architecture eliminating the need for an expensive low resistance/high accuracy sense resistor. The device operates ideally from a regulated 12V or 24V DC power supply, but can also operate over a 10.8V to 28V range. • • • • • • • • • • • • • The AAT2404 provides an output voltage up to 100V regulated by the CSFB pin provided by the ICs in Skyworks' family of white LED drivers for TV applications. The CSFB pin is an analog voltage representing the LED string with the highest voltage requirement. Regulating to this voltage allows for a wide range of LED characteristics, while maintaining the lowest possible power dissipation. The CSFB regulation point can be set by adjusting a resistor to ground from the RSET pin. VIN Range: 10.8V – 28.0V Maximum VLED: 100V Up to 95% Boost Conversion Efficiency Integrated Current Sense Eliminates Need for Ballast Resistors Switching Frequency Options ▪ 400KHz Nominal ▪ Adjustable Range from 100kHz to 800kHz Adjustable Regulation Voltage ▪ Analog Input from LED Driver ▪ User Adjustable for Fixed Output Integrated Low Impedance Gate Drive = VCC Flexible Current Sense Feedback Control Power OK Output Integrated Over-Voltage Protection Soft-Start to Minimize Inrush Current TQFN34-24 Low Profile Package -40°C to +85°C Temperature Range Applications • • • • The boost switching frequency is nominally 400kHz to allow for optimum efficiency with the smallest external filter. However, the device switching frequency may be adjusted with an external resistor to optimize system performance. Current mode control provides fast response to line and load transients. Large Size LCD TV, Panels LCD Monitors Video Walls White LED Backlighting Thermal protection circuitry shuts down the boost converter in the event of an over-temperature condition. The AAT2404 is available in the Pb-free, thermally enhanced 24-pin 3 x 4mm TQFN package. Typical Application D1 L1 VIN VCC VIN = 10.8V to 28V CIN CVCC CC RC RFREQ RSET GATE LXS VLED R1 COUT OVP R2 AAT2404 EN COMP FREQ RSET On/Off Enable Input Q1 POK CSFB Power OK Output Current Sense Feedback Input PGND AGND Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202247A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 7, 2012 1 DATA SHEET AAT2404 Voltage-Variable Current Sourcing Boost Controller For LED Lighting Applications Pin Descriptions Pin # Symbol Function 1, 2, 3, 23, 24 Description LXS I Boost converter current sense node. Connect to the source terminal of the external low resistance power MOSFET to this pin. 4 GATE O Output drive pin. Connect directly to the gate terminal of the external low resistance power MOSFET. The gate voltage range is from 0V to VCC 5 VIN I Input power supply. 6 VCC I/O 7, 8 9 10 AGND COMP NC GND I 11 CSFB I 12 FREQ O Boost converter PWM switching frequency adjust pin. Connect a RFREQ resistor between this pin and AGND to set the switching frequency. 13 OVP I Over-voltage protection. Connect a resistive divider between VLED, this pin, and ground. 14 RSET O Current sink regulation voltage set resistor. Connect the RSET resistor between this pin and AGND. 15 EN I Logic High enable pin. Apply a logic high voltage or connect to VIN to enable the device. Use a 10kΩ resistor between this pin and AGND to for a logic pull-down to shut the device off when an enable signal is not applied. 16 POK O Open drain output. Connect to LED cathode with the anode connected via a resistor to VCC or drive an active low logic signal to a system controller. If not used, leave open / not connected. 17, 18, 19, 20, 21, 22 PGND GND Power ground. EP EP GND Exposed paddle. Connect to PCB GND plane. PCB paddle heat sinking should maintain acceptable junction temperature. Internally regulated power supply. Decouple with 2.2µF or greater value capacitor between this pin and AGND. Analog ground. Boost converter compensation. Connect external resistor and capacitor to this pin and AGND. Not connected. Current sink feedback. When used with compatible Skyworks LED driver devices1, connect the driver CSFBO output directly to this pin for the current sink feedback from current sink device. Pin Configuration TQFN34-24 (Top View) PGND PGND PGND LXS LXS 24 LXS LXS LXS GATE VIN VCC AGND 23 22 21 20 1 19 2 18 3 17 EP 4 5 16 15 6 14 7 13 8 9 10 11 PGND PGND PGND POK EN RSET OVP 12 FREQ CSFB N/C COMP AGND 1. Compatible Skyworks LED backlight driver products include the AAT2401, AAT2402 and AAT2403. 2 Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202247A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 7, 2012 DATA SHEET AAT2404 Voltage-Variable Current Sourcing Boost Controller For LED Lighting Applications Absolute Maximum Ratings1 Symbol Description VIN,EN VCC GATE, LXS, POK, OVP, COMP, RSET, FREQ, CSFB TJ TLEAD Input Voltage, EN to GND Low Voltage Pin to GND GATE, LXS, POK, OVP, COMP, RSET, FREQ, CSFB Voltage to GND Maximum Junction Operating Temperature Maximum Soldering Temperature (at leads, 10 sec.) Value Units -0.3 to 32 -0.3 to 6.0 -0.3 to V CC + 0.3 -40 to +150 300 O C Thermal Information2 Symbol θJA PD TA Description Thermal Resistance3 Maximum Power Dissipation Operating Temperature Range Value 50 2.3 -40 to 85 Units O C/W W O C 1. Stresses above those listed in Absolute Maximum Ratings may cause permanent damage to the device. Functional operation at conditions other than the operating conditions specified is not implied. Only one Absolute Maximum Rating should be applied at any one time. 2. Mounted on an FR4 board. 3. Derate 20mW/°C above 25°C. Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202247A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 7, 2012 3 DATA SHEET AAT2404 Voltage-Variable Current Sourcing Boost Controller For LED Lighting Applications Electrical Characteristics1 VIN = 24V; CIN = 4.7µF, COUT = 4.7µF; CVCC = 2.2µF; L1 = 10µH; RSET =10.2kΩ; TA = -40°C to 85°C unless otherwise noted. Typical values are at TA = 25°C. Symbol Description Power Supply, Current Sinks VIN Input Voltage Range VCC Linear Regulator Output Voltage VUVLO Under Voltage Threshold VLED IQ ISD Output Voltage Range Quiescent Current VIN Pin Shutdown Current Over-Voltage Threshold Over-Voltage Hysteresis Sense Device ON Resistance Low Side Switch Current Limit Oscillator Frequency Soft-Start Duty Cycle2 VOVP RSENSE ILIMIT FOSC TSS D Gate Drive RDS_P Driver High Side ON Resistance RDS_N Driver Low Side ON Resistance tR Gate Rise Time tF Gate Fall Time Logic Level Inputs: EN VI(L) Input Logic Threshold Low VI(H) Input Logic Threshold High IEN Input Enable Leakage Currrent Logic Level Outputs: POK VPOK(LOW) POK Logic Output Low ISINK POK Logic High Leakage Thermal Protection TJ (SD) TJ Thermal Shutdown Threshold TJ (HYS) TJ Thermal Shutdown Hysteresis Conditions Min Typ 10.8 0mA < ILOAD < 15mA VIN Rising Hysteresis VIN Falling VIN = 10.8V to 28.0V Not switching EN = Logic Low VLED Rising VLED Falling RFREQ = 10kΩ VLED = 35V RFREQ = 10kΩ VCC VCC VCC VCC = = = = Max Units 28 V V V mV V V mA µA V mV mΩ A kHz ms % 5.0 10 200 8.5 VIN + 3V 1.1 320 5V 5V 5V, CLOAD = 0.5nF 5V, CLOAD = 0.5nF 1 10 1.2 100 60 10 400 1.5 80 1.3 480 2 1 10 10 Ω ns ns 0.4 2 V V µA 0.4 1 V µA 2.5 ISINK = -1mA VPOK = 5.5V 140 15 °C 1. The AAT2404 is guaranteed to meet performance specifications over the -40°C to +85°C operating temperature range and is assured by design, characterization, and correlation with statistical process controls. 2. The boosted output voltage, VLED, cannot exceed 100V. 4 Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202247A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 7, 2012 DATA SHEET AAT2404 Voltage-Variable Current Sourcing Boost Controller For LED Lighting Applications Typical Characteristics UVLO vs. Temperature Quiescent Current vs. Temperature (VIN = 24V; VEN = VIN; Non-switching) 9.8 Quiescent Current (mA) 1.4 9.6 UVLO (V) 9.4 9.2 9.0 8.8 8.6 Rising 8.4 8.2 -40 Falling -15 10 35 60 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 -40 85 -15 10 Shutdown Current vs. Temperature 85 (VIN = 24V; VLED = 31V; COUT = 20µF; L = 10µH) 16 100 14 96 12 92 Efficiency (%) Shutdown Current (µA) 60 Efficiency vs. Load Current (VIN = 24V; VEN = GND) 10 8 6 4 88 84 80 76 72 2 0 -40 35 Temperature (°C) Temperature (°C) 68 -15 10 35 60 85 Temperature (°C) 0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6 4.0 Output Current (A) Input Logic Threshold vs. Temperature Input Logic Threshold (V) (VIN = 24V) 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1.0 0.9 -40 Logic Threshold High Logic Threshold Low -15 10 35 60 85 Temperature (°C) Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202247A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 7, 2012 5 DATA SHEET AAT2404 Voltage-Variable Current Sourcing Boost Controller For LED Lighting Applications Typical Characteristics POK Logic High Leakage vs. Temperature POK Logic Output Low vs. Temperature (VIN = 24V; IPOK = -1mA) Logic Output Low Voltage (mV) (VIN = 24V; VPOK = 5.5V) Leakage Current (nA) 80 70 60 50 40 30 20 10 0 -40 -15 10 35 60 85 Temperature (°C) 50 45 40 35 30 25 20 15 10 -40 Current Limit (A) Frequency Accuracy (%) 11.5 0.5 0.0 -0.5 -1.0 11.0 10.5 10.0 9.5 9.0 8.5 -1.5 -15 10 35 60 8.0 -40 85 -15 10 35 60 Gate Rise and Fall Time vs. CLOAD (VIN = 24V; VCC = 5V) 80 90 70 Rise/Fall Time (ns) 100 80 70 60 50 40 30 60 50 40 30 20 Rise Fall 10 0 -15 10 35 Temperature (°C) 60 85 Temperature (°C) Sense Device On Resistance vs. Temperature RSENSE (mΩ) 85 12.0 Temperature (°C) 6 60 (VIN = 24V) (VIN = 24V; fOSC = 400kHz; RFREQ = 10kΩ) 1.0 20 -40 35 Current Limit vs. Temperature 1.5 -2.0 -40 10 Temperature (°C) Oscillator Frequency Accuracy vs. Temperature 2.0 -15 85 0.1 1 Capacitance (nF) Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202247A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 7, 2012 10 DATA SHEET AAT2404 Voltage-Variable Current Sourcing Boost Controller For LED Lighting Applications Functional Block Diagram VIN Linear Regulator VCC EN OVP GATE COMP FREQ RSET Logic and PWM Control LXS I-Precise™ CSFB POK AGND Functional Description The AAT2404 is a high voltage DC-DC boost converter that functions as a voltage variable current source that is designed to complement Skyworks' family of white LED drivers for TV applications PGND Regulating to this voltage allows for a wide range of LED characteristics, while maintaining the lowest possible power dissipation for the system. The CSFB regulation voltage point can be set by adjusting an external resistor to ground from the RSET pin. Operating from a 10.8V to 28V input supply range, the AAT2404 can supply a compliance voltage up to 100V with the output power limited only by the size and selection of the external switching MOSFET, inductor and schottky diode. Input voltage sources common to LCD monitors and TV display panels are 12V or 24V with a maximum effective switching duty cycle of 80%. The AAT2404 provides a low gate impedance driver to minimize the switching losses of the external boost power MOSFET and can attain boost conversion efficiencies up to 95%. The boost switching frequency is nominally 400kHz to allow for optimum efficiency with the smallest external filter. Alternatively, the device switching frequency may be adjusted over a 100kHz to 800kHz range by an external resistor if required by a specific application. The AAT2404 uses a unique internal current scheme, and relies on a current sense feedback loop (CSFB) integrated into the AAT2401/02S/03 LED driver ICs which eliminates the need for low resistance, 1% tolerance current sense resistors for each LED backlight string. The CSFB function is an analog voltage feedback system that represents the LED string with the highest voltage requirement. For reliability and protection of the application system, the AAT2404 has a thermal protection circuit to shut down the boost converter in the event of an over-temperature condition. An output over voltage protection circuit (OVP) constantly monitors the boost output voltage and will terminate the boost switching cycle if the output exceeds a user set threshold. Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202247A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 7, 2012 7 DATA SHEET AAT2404 Voltage-Variable Current Sourcing Boost Controller For LED Lighting Applications Input Supply and Control Loop Current Sink Feedback (CSFB) and RSET The AAT2404 has specially designed input stages to permit operation and control over a 10.8V to 28V input range. This device is intended to function as a voltage variable current source to drive large strings of backlight LEDs. The system current limit is based on the programming of the downstream LED controller constant current sinks. The current sink feedback to the AAT2404 maintains a compliance voltage to support the varying demands based on the LED combined forward voltage at any given forward current setting. The AAT2404 utilizes a current sink feedback (CSFB) function that directly interfaces to Skyworks LED controllers such as the AAT2401, AAT2402S and AAT2403. When used with these devices, their integrated CSFBO output can be connected directly to the AAT2404 CSFB pin. The voltage level of this feedback system represents the proper regulation point for the LED array to support a programmed LED drive current. The range of the CSFB signal should be from 0.5V to 2.5V under normal operating conditions. The AAT2404 has the benefit of current mode control with a simple voltage feedback loop providing exceptional stability and fast response with minimal design effort. The device modulates the external power MOSFET switching current to maintain the programmed feedback voltage that is user adjustable via the RSET resistor. The switching cycle initiates when the N-channel MOSFET is turned ON and current ramps up in the inductor. The ON interval is terminated when the inductor current reaches the programmed peak level. During the OFF interval, the input current decays until a lower threshold, or zero inductor current, is reached. The lower current is equal to the peak current minus a preset hysteresis threshold, which determines the inductor ripple current. The peak current is adjusted by the controller until the output voltage requirement of the LED array is met as determined by the voltage on the CSFB input pin. The feedback voltage threshold is user adjustable by programming the RSET resistor, simplifying integration with other Skyworks's devices. The feedback threshold voltage for the AAT2404 should be greater than the current sink dropout voltage to prevent ILED from going out of regulation. However if the feedback voltage threshold is much higher than the dropout voltage, the VLED voltage will be higher than the optimum voltage required to drive the white LED strings. This will result in unwanted power being dissipated by the LED driver. Set the feedback voltage threshold between 10% and 20% higher than the dropout voltage to maintain current regulation and avoid excessive power dissipation. Control Loop Compensation The COMP pin is the output of the transconductance error amplifier. The AAT2404 is a current mode boost controller and as such has eliminated the double pole of the LC filter. The magnitude of the feedback error signal determines the average input current to the AAT2404; the internal control circuit implements a programmed current source connected to the output capacitor and load impedance. Regulator stability is achieved with a simple RC compensation network from the COMP pin to ground. If the ESR of the output capacitor is high, then an additional capacitor in parallel with the RC network may be needed. 8 CSFB Threshold (V) Operating frequency varies with changes in the input voltage, output voltage, and inductor size. Once the boost converter has reached continuous mode, further increases in the output current will not significantly change the operating frequency. 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0 5 10 15 20 25 30 RSET (kΩ) Figure 1: CSFB Threshold vs. RSET (VIN = 24V, FOSC = 200kHz). Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202247A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 7, 2012 35 DATA SHEET AAT2404 Voltage-Variable Current Sourcing Boost Controller For LED Lighting Applications When using the AAT2404 in a given application, one must first determine what the maximum combined LED string voltage will be at the specified maximum forward current. The maximum practical operating duty cycle for the DC-DC boost function is approximately 80%. The maximum output voltage can be approximated using Equation 1: Eq. 1: VLED = VIN 1-D value between the FREQ pin and ground. To set a recommend 400kHz switching frequency, the nominal value RFREQ value is 10kΩ. Refer to Figure 2 for resistor values to program a specific switching frequency. 1 Frequency (MHz) Maximum Output Voltage Compliance Where D = DC-DC boost switching duty cycle. The AAT2404 has an internal linear regulator to produce 5V from the VIN high voltage input for internal logic, clock, and control functions. The regulator output is connected to the VCC pin and should be bypassed with a 2.2µF or larger ceramic capacitor. The 5V may be used as a logic pull-up reference termination for all AAT2404 logic functions such as a pull up for the open drain power OK (POK) function. This output is not intended to support external loads from circuits other than low current logic terminations. IC Enable and Soft Start An enable pin is provided as a master on/off function that may be toggled by an external system controller or connected directly to VIN. This is a logic active high function. If the IC enable is not needed, connect the EN pin to VCC to turn the AAT2404 on. The slew rate limited turn-on is guaranteed by the built-in soft-start circuitry. Soft start eliminates output current overshoot across the full input voltage range and all load conditions. After the soft start sequence has terminated, the initial output voltage is determined by the level sensed on the CSFB pin. Boost Converter Switching Frequency The AAT2404 is designed to operate over a wide input to output voltage range with a nominal 400kHz switching frequency. However, if a specific system or application demands a different operating switching frequency, the frequency can be user adjusted by changing the resistor 0.6 0.4 0.2 0 0 However, the maximum output voltage should not exceed 100V. Internal Linear Voltage Regulator 0.8 5 10 15 20 25 30 35 40 45 RFREQ (kΩ) Figure 2: AAT2404 DC_DC Boost Switching Frequency vs. RFREQ Resistor Value. OVP The over-voltage protection (OVP) circuit is provided to shut down the boost control switching to the external N-channel MOSFET if the output voltage exceeds a user preset level, which can occur if the load circuit becomes disconnected (open). The OVP pin input threshold is 1.2V and the OVP shutdown voltage should be selected so that the circuit is active within a reasonable margin above the normal output operation voltage. To program a desired OVP output limit level, place a resistor divider between the voltage output node, the OVP pin, and ground. Set the OVP voltage using Equation 2: Eq. 2: R1 = R2 VOUTPUT PROTECTION -1 VOVP Where: VOVP = OVP threshold = 1.2V VOUTPUT PROTECTION = Desired output protection voltage level R2 should typically be set to 12.1kΩ (Nominal range for R2 should be between 10kΩ and 47kΩ). The maximum OVP is 120V. Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202247A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 7, 2012 9 DATA SHEET AAT2404 Voltage-Variable Current Sourcing Boost Controller For LED Lighting Applications Thermal Protect Shutdown Power OK Flag Output If an operating condition causes excess power dissipation in the AAT2404, the device will shut down when the die temperature exceeds 140°C. When the die cools or when the source of the over-temperature condition is removed, the AAT2404 will automatically restart. There is 15°C of shutdown restart hysteresis. A power OK (POK) flag is provided to inform the system when the output supply voltage is turned on and has reached 80% regulation. The POK output is an open drain N-channel MOSFET switch connected to ground internally. A 10KΩ or greater value pull-up resistor should be connected between the POK pin and VCC. The POK flag can function as an active low logic signal that can be used to alert system logic or as an enable signal to a downstream load circuit to sequence the load power-on after the boost supply is operating. 10 Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202247A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 7, 2012 DATA SHEET AAT2404 Voltage-Variable Current Sourcing Boost Controller For LED Lighting Applications Application Information Selecting the Switching Frequency The AAT2404 is a highly integrated high-efficiency variable voltage current sourcing boost controller and like all boost controllers care must be taken in the selection of external components during the design stage to ensure a stable and reliable system. The design is an iterative process since design parameters are mutually dependant. As an example of this iterative process please refer to the step-by-step design example application note. Selecting the optimal switching frequency is an iterative process since component and electrical parameters are all interrelated. For example, to reduce the inductor value and hence its size a higher switching frequency is desired. However, too high of a switching frequency may cause the switching losses in the external N-channel boost MOSFET to become dominant and exceed the power dissipation. A good starting point for the switching frequency is between 200kHz and 400kHZ. To set the switching frequency, please refer to the Boost Converter Switching Frequency section of this product datasheet. DC-DC Boost Duty Cycle Calculation In order to correctly determine the characteristics for external component selection, the switching duty cycle for the boost converter function should be calculated. If the input voltage to the AAT2404 is constant, then there is only one maximum duty cycle condition to be concerned with (Equation 3). In the case of varied input supply, the minimum, maximum, and nominal duty cycle should be calculated (Equations 3, 4, and 5) and the use of the maximum value should be carried forward for the selection of the inductor, N-channel MOSFET, and reverse blocking diode. Eq. 3: DNOM = VLED - VIN(NOM) + VD VLED + VD Eq. 4: DMIN = VLED - VIN(MAX) + VD VLED + VD Eq. 5: DMAX = VLED - VIN(MIN) + VD VLED + VD Selecting the Boost Inductor The first parameter to be considered in the selection of the boost inductor is the inductance value. In a fixedfrequency boost converter like the AAT2404, this value is based on the desired peak-to-peak ripple current ∆IL, which flows in the inductor along with the average or DC inductor current IL. In continuous conduction mode (CCM) IL is greater than the current output of the boost regulator, ILED. Taking into account the conservation of power and neglecting efficiency losses, the two currents are related by the following: Conservation of power: Eq. 6: VIN · IL = VLED · ILED Eq. 7: VLED = VIN (1 - D) Rearranging for IL: Where: DMIN = Minimum boost switching duty cycle DMAX = Maximum boost switching duty cycle (must be ≤ 80%) DNOM = nominal boost switching duty cycle VIN[MAX] = Maximum input supply voltage for the application VIN[MIN] = Minimum input supply voltage for the application VLED = Voltage output of the boost regulator (estimate the maximum summed VF for the LED string for the backlighting application) VD = Reverse blocking diode forward voltage. A Schottky diode is recommended for this application due to their low forward voltage characteristic. The VF of a Schottky diode is typically between 0.5V and 0.7V. Eq. 8: IL = VLED · ILED VIN Substituting VIN for VLED: VIN ·I (1 - D) LED Eq. 9: IL = VIN Eq. 10: IL = ILED (1 - D) Where: VIN = Input supply voltage VLED = Voltage output of the boost regulator IL = Average inductor current or input supply current ILED = Current output of the boost regulator D = Boost switching duty cycle Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202247A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 7, 2012 11 DATA SHEET AAT2404 Voltage-Variable Current Sourcing Boost Controller For LED Lighting Applications The inductance value chosen is a tradeoff between size and cost. Larger inductance means lower input ripple current, however because the inductor is connected to the output during the off-time, there is a limit to the reduction in output ripple voltage. Lower inductance results in smaller, less expensive magnetics. An inductance that gives a ripple current of 30% of IL is a good starting point for a CCM boost converter: Eq. 11: ∆iL(MAX) ≈ 0.3 · overheat the inductor and/or push the AAT2404 into current limit. In a boost converter, peak inductor current, IPK, is equal to the maximum average inductor current plus one half of the ripple current. First, the ripple current, ∆iL, must be determined under the conditions that give maximum average inductor current: ILED 1-D Where: ∆iL(MAX) = Maximum desired inductor peak to peak current ripple ILED = Current output of the boost regulator D = Boost switching duty cycle Minimum inductance should be calculated at the extremes of input voltage to find the operating condition with the highest requirement. Depending on the amount the input voltage is boosted, the duty cycle term (D) can become the dominant term. The minimum inductor value can be established by one of the following equations, whichever produces the larger minimum inductor value: Eq. 12: LMIN = VIN(MAX) 1 ∆iL(MAX) · DMIN · fSW Eq. 13: LMIN = VIN(MIN) 1 ∆iL(MAX) · DMAX · fSW Where: LMIN = Minimum inductance VIN(MAX) = Maximum input supply voltage VIN(MIN) = Minimum input supply voltage ∆iL(MAX) = Maximum inductor peak to peak current ripple DMIN = Minimum boost switching duty cycle DMAX = Maximum boost switching duty cycle fSW = Switching frequency Figure 3: CCM Inductor Current. 12 VIN(MIN) 1 · DMAX · f L SW Eq. 15: ∆iL = VIN(MAX) 1 · DMIN · f L SW Eq. 16: IL(PK) = IL + ∆iL 2 Where: ∆iL = Nominal ripple current in the inductor VIN(MIN) = Minimum input voltage L = Inductance D = Boost switching duty cycle fSW = Switching frequency IL(PK) = Peak inductor current IL = Average inductor current IPK should be less than the saturation current specification of the selected inductor. The final parameter of an inductor to consider is the DC resistance (DCR), which contributes to the power loss of the inductor and degrades the boost converter efficiency and increases the inductor's operating temperature. Based on the inductor value calculation, the next higher standard value inductor should be used. The second parameter that should be taken into consideration when selecting the boost inductor is the peak current capability. This is the level above which the inductor will saturate and the inductance can drop severely, resulting in a higher peak current that may Eq. 14: ∆iL = Eq. 17: PLOSS(L) = I2RMS · DCR Where: Eq. 18: IRMS = (IL)2 + 2 1 ∆iL 12 is the RMS current in the inductor for continuous conduction mode operation. Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202247A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 7, 2012 DATA SHEET AAT2404 Voltage-Variable Current Sourcing Boost Controller For LED Lighting Applications Selecting the Schottky Diode A low forward voltage drop Schottky diode is used as a rectifier diode to reduce its power dissipation and improve efficiency. The average current through diode is the average load current ILED, and the peak current through the diode is the peak current through the inductor IPK. The diode should be rated to handle more than its peak current. Eq. 19: ID(PK) = IL(PK) = IL + ∆iL 2 Where: ID(PK) = Peak diode current IL(PK) = Peak inductor current IL = Average inductor current ∆iL = Nominal ripple current in the inductor The peak reverse voltage for the boost converter is equal to the regulator output voltage. The diode must be capable of handling this voltage. Using 80% derating on VLED for ringing on the switch node, the rectifier diode minimum reverse breakdown voltage is: Eq. 20: VBRR(MIN) ≥ VLED 0.8 Where: Eq. 22: D = TON = TON · FS TON + TOFF The maximum duty cycle can be estimated from the relationship for a continuous mode boost converter. Maximum duty cycle (DMAX) is the duty cycle at minimum input voltage (VIN(MIN)): Eq. 23: DMAX = VLED - VIN(MIN) VLED The average diode current during the OFF time can be estimated: Eq. 24: IAVG(OFF) = ILED 1 - DMAX The VF of the Schottky diode can be estimated from the average current during the off time. The average diode current is equal to the output current: Eq. 25: IAVG(TOT) = ILED The average output current multiplied by the forward diode voltage determines the loss of the output diode: Eq. 26: PLOSS(DIODE) = IAVG(TOT) · VF = ILED · VF VBRR(MIN) = Minimum voltage breakdown of the Schottky diode VLED = Voltage output of the boost regulator To assure the rectifier diode is rated for the power dissipation requirement for a given application, the Schottky diode power dissipation can be estimated. The switching period is divided between ON and OFF time intervals: Eq. 21: 1 = TON + TOFF = D + D’ FS During the ON time, the N-channel power MOSFET is conducting and storing energy in the boost inductor. During the OFF time, the N-channel power MOSFET is not conducting. Stored energy is transferred from the input battery and boost inductor to the output load through the output diode. Duty cycle is defined as the ON time divided by the total switching interval: For continuous LED currents, the diode junction temperature can then be estimated: Eq. 27: TJ(DIODE) = TAMB + θJA · PLOSS(DIODE) The external Schottky diode junction temperature should be below 110°C, and may vary depending on application and/or system guidelines. The diode θJA can be minimized with additional metal PCB area on the cathode. However, adding additional heat-sinking metal around the anode may degrade EMI performance. The reverse leakage current of the rectifier must be considered to maintain low quiescent (input) current and high efficiency under light load, the rectifier reverse current increases dramatically at elevated temperatures. Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202247A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 7, 2012 13 DATA SHEET AAT2404 Voltage-Variable Current Sourcing Boost Controller For LED Lighting Applications Selecting the External N-Channel Boost MOSFET Then the gate drive power required to turn on a MOSFET is: Selection of the external power MOSFET is controlled by tradeoffs among efficiency, cost and size. The critical parameters for the selection of a MOSFET are: minimum threshold voltage, VGSth(MIN), minimum drain to source breakdown voltage, BVDSS, on-resistance, RDS(ON), and total gate charge, QG. VGS, Gate-Source Voltage (V) The peak-to-peak gate drive level is set by the VCC voltage, which is typically 5V for the AAT2404 under normal operating conditions. This requires the minimum threshold voltage of the MOSFET to be less than 5V; logic level MOSFETS have minimum threshold voltages less than 5V. However, in switch mode operation the gate-to-drain (“Miller”) charge parameter of the MOSFET QGD will affect the VGSTH parameter. Consult the Gate Charge Characteristics plot found in the datasheet of the MOSFET and ensure that the QGD plateau is less than 4.5V (the lower the better). 12 Where: PG = Gate charge loss in the linear regulator of the AAT2404 VG = Gate drive voltage VG = VCC = 5V QG = Total gate charge of the MOSFET fSW = Switching frequency During the off state of the boost controller the voltage across the MOSFET is equal to the output voltage, VLED, when neglecting the intrinsic diode voltage drop. The BVDSS parameter of the MOSFET must be greater than the voltage output. Using 80% derating on VLED for ringing on the switch node, the minimum BVDSS voltage of the MOSFET is Eq. 30: BVDSS ≥ VLED 0.8 BVDSS = Minimum drain to source breakdown voltage of the MOSFET VLED = Voltage output of the boost regulator 10 8 First order power losses in a MOSFET can be attributed to conduction loss, switching loss, and the gate drive loss. Although the gate drive loss is not strictly in the MOSFET it is included in the MOSFET power loss calculation. 6 VDS = 40V VDS = 100V VDS = 160V 4 QGD plateau 2 0 Eq. 29: PG = VG · QG · fSW 0 10 20 30 40 50 Eq. 31: PMOSFET = PC + PSW + PG 60 QG, Total Gate Charge (nC) Figure 4: Example Gate Charge Characteristics (ID = 21A). Estimating gate drive power required to turn the MOSFET on and the power losses in the MOSFET is a good way of balancing the tradeoffs and comparing the relative merit between MOSFET devices. The amount of current needed to turn on a MOSFET is: Where: PMOSFET = Power dissipated by the MOSFET PC = Conduction loss of the MOSFET PSW = Switching loss of the MOSFET PG = Gate charge loss in the linear regulator of the AAT2404 Conduction loss is the I2R loss when the MOSFET is turned on and is approximated by the following equation: Eq. 32: PC = D · Eq. 28: IG = QG · fSW ILED 1-D 2 · RDS(ON) Where Where: IG = Required current to turn on a MOSFET QG = Total Gate charge of the MOSFET fSW = Switching frequency PC = Conduction loss of the MOSFET D = Boost switching duty cycle ILED = Current output of the boost regulator RDS(ON) = Maximum high temperature on-resistance of the MOSFET 14 Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202247A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 7, 2012 DATA SHEET PRODUCT DATASHEET AAT2404 AAT2404 TM SwitchReg Voltage Variable Current Sourcing Controller LED LightingApplications Applications Voltage-Variable Current Sourcing BoostBoost Controller ForFor LED Lighting Switching loss occurs during the transition between the MOSFET being turned on and then turned off. 1 ILED Eq. 33: PSW = 2 · VIN · (1 - D) · (tR + tF) · fSW Where: PSW = Switching loss of the MOSFET VIN = Minimum input voltage ILED = Current output of the boost regulator D = Boost switching duty cycle tR = Rise time of the MOSFET (refer to the selected MOSFET's datasheet) tF = Fall time of the MOSFET (refer to the selected MOSFET's datasheet) fSW = Switching frequency After selecting the MOSFET the package power dissipation in the operating circuit can be estimated. Eq. 34: PD(TOTAL) = POUT 1 η - 1 = VLED · ILED · 1 η -1 Where: PD(TOTAL) = Total power dissipation for the system, (output power plus power loss of the switching MOSFET) η = Boost efficiency (refer to the efficiency curve for the given output load current in the Typical Characteristics section of this datasheet) VLED = Voltage output of the boost regulator ILED = Current output of the boost regulator The power that will be dissipated by the MOSFET should be determined; the package PD rating of the MOSFET selected should exceed this value: Eq. 35: PMOSFET < PD(TOTAL) - PL - PD - (VIN · IQ ) Where: PMOSFET = Power dissipated by the MOSFET PD(TOTAL) = Total system power calculated in Equation 34 PL = Power dissipation of the inductor based on the DC resistance (DCR) PD = Power dissipation of the reverse blocking Schottky diode VIN = Input supply voltage IQ = Device quiescent supply current Selecting the Output Capacitor The output capacitor in a current regulator is selected to control the output ripple current (ΔiF) when the inductor is charging as opposed to a voltage regulator where ΔVO is controlled. As a result, the output capacitor is subjected to much larger ripple currents. Assuming a constant discharging current when the MOSFET switch is on, the voltage ripple across the capacitor is: Eq. 36: ∆VPK-PK = ILED · DMAX COUT · fSW Solving for COUT: Eq. 37: COUT = ILED · DMAX ∆VPK-PK · fSW Where ∆VPK-PK = VLED voltage ripple ILED = Output supply current DMAX = Maximum boost switching duty cycle fSW = Switching frequency The output capacitor must be capable of handling the maximum output RMS current. Use Equation 38 to estimate the ICLED(RMS) value. Eq. 38: ICLED(RMS) = D (1 - D) · ILED2 · (1 - D)2 + ∆iL2 3 Where ILED = Current output of the boost regulator ∆iL = Nominal ripple current in the inductor D = Boost switching duty cycle The equivalent series resistance (ESR) and the equivalent series inductance (ESL) of the output capacitor directly control the output ripple. Use capacitors with low ESR and ESL specification at the output for high efficiency and low ripple voltage. Surface mount tantalum polymer electrolytic, and polymer tantalum SanyoOSCON capacitors are recommended at the output. Selecting the Input Capacitor The input capacitors in a boost regulator control the input voltage ripple (ΔVIN) and prevent impedance mismatch (also called power supply interaction) between the AAT2404 and the stray inductance of the input wire connections. Selection of input capacitors is based on their capacitance, ESR, and RMS current rating. The minimum Skyworks Solutions, Inc. • Phone [781] 376-3000 Fax • www.skyworksinc.com w w w•. a n a[781] l o g 376-3100 i c t e c h . •c [email protected] m 202247A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 7, 2012 15 DATA SHEET AAT2404 Voltage-Variable Current Sourcing Boost Controller For LED Lighting Applications input capacitance is based on ΔVIN or prevention of power supply interaction. In general, the requirement for the greatest capacitance comes from the power supply interaction. To stabilize the regulator, ensure that the regulator crossover frequency is less than or equal to one-tenth of the right-half plane zero or less than or equal to onetenth of the switching frequency whichever is lower. The stray inductance, LS, and resistance, RS, of the input source must be estimated, and if this information is not available, good design practice may assume the inductance and resistances to be 1μH and 0.1Ω, respectively. The regulator loop gain is determined by Equation 41: Minimum input capacitance is then estimated as: 2 · LS · VLED · ILED Eq. 39: CIN(MIN) = VIN2 · RS Where: LS = Power supply parasitic inductance (assumed to be 1μH) VLED = Voltage output of the boost regulator ILED = Current output of the boost regulator VIN = Input supply voltage RS = Power supply stray resistance (assumed to be 0.1Ω) Eq. 41: VREF VIN 1 | AVL | = V · V · GMEA · RC · GCS · 2π · f · C =1 LED LED C OUT Where VREF = Feedback voltage reference set by RSET VIN = Input supply voltage VLED = Voltage output of the boost regulator RC = Compensation resistor GMEA = Error amplifier transconductance: 176µA/V GCS = Current sense amplifier transconductance: 3.0A/V fC = Selected crossover frequency COUT = Output capacitor The AAT2404 regulator loop solving for compensation resistor, RC: Selecting the Compensation Resistor and Capacitor Eq. 42: RC = Regulator stability is achieved with a simple RC compensation network from the COMP pin to ground. Once the boost regulator design requirements have been established and the inductor and output capacitor values have been chosen, the LC filter must be compensated for to stabilize the boost regulator. The AAT2404 senses the inductor current and eliminates the double pole LC filter and simplifies the compensation to a single pole RC caused by the output capacitance and the output load resistance. However, since the AAT2404 is designed to work in the continuous conduction mode (CCM) an undesirable right-half plane zero is produced in the regulation feedback loop. This requires compensating the AAT2404 such that the crossover frequency occurs well below the frequency of the right-half plane zero. Eq. 40: FzRHP = VIN VLED 2 · RL 2π · L Once the compensation resistor is known, set the zero formed by the compensation capacitor and resistor to one-tenth of the crossover frequency, or: Eq. 43: CC = 16 10 2π · fC · RC If the zero of the ESR of the output capacitor is near fC, then it needs to be cancelled out by putting and an extra cap in parallel with RC and CC. To determine the zero of the ESR of the output capacitor: Eq. 44: fESR = 1 2π · RESR · COUT To cancel the ESR zero: Eq. 45: C2 = Where: VIN = Input supply voltage VLED = Voltage output of the boost regulator RL = Output load resistance L = Inductance 2π · fC · COUT · VLED · VLED VREF · VIN · GMEA · GCS RESR · COUT RC Where: RESR = ESR of the output capacitor COUT = Output capacitor RC = Compensation resistor Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202247A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 7, 2012 DATA SHEET AAT2404 Voltage-Variable Current Sourcing Boost Controller For LED Lighting Applications Layout Fundamentals 1. Minimize the length of both traces in series with the output capacitor terminals to avoid high dV/dt (fast changing voltages) and reduce capacitive coupling and electric fields. One trace is from the cathode of the rectifying diode to the positive terminal of the capacitor, the other trace is from PGND to the negative terminal. 2. Minimize the loop area of high di/dt (fast charging currents) to reduce inductance and magnetic field. Use wide traces for high current traces. 3. Maintain a ground plane and connect to the IC PGND pin(s) as well as the PGND connections of CIN and COUT. 4. Consider additional PCB exposed area for the AAT2404 to maximize heat sinking capability. Connect the exposed paddle (bottom of the die) to PGND or GND. Connect AGND as close as possible to the package and maximize the overall heat sinking space. 5. To maximize package thermal dissipation and power handling capacity of the AAT2404's TQFN34-24 and external MOSFET and diode packages (Q1 and D1), solder the exposed paddle of the IC onto the thermal landing of the PCB, where the thermal landing is connected to the ground plane. If heat is still an issue, multi-layer boards with dedicated ground planes are recommended. Also, adding more thermal vias on the thermal landing helps transfer heat to the PCB effectively. The MOSFET and diode can also be mounted upright and connected to heat sinks. Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202247A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 7, 2012 17 DATA SHEET AAT2404 Voltage-Variable Current Sourcing Boost Controller For LED Lighting Applications Typical System Configurations for Direct and Edge Backlighting Applications VIN L1 D1 C5 Q1 EN GATE LXS VIN VCC C1 R4 AAT2404 C2 OVP R5 POK CSFB FREQ COMP RSET C3 R3 R1 R2 10x16 PGND 10x16 10x16 AGND VIN 24V VCC VIN 24V AAT2403 VIN CSFBI CSn CSFBO 16 VIN 24V AAT2403 VIN CSFBI CSn CSFBO 16 AAT2403 VIN CSn 16 CSFBI CSFBO Figure 5: Direct LCD TV LED Backlight System Using the AAT2404 to Drive Three 10Sx16P LED Arrays with Three AAT2403 Current Sink Controllers. 18 Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202247A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 7, 2012 DATA SHEET AAT2404 Voltage-Variable Current Sourcing Boost Controller For LED Lighting Applications Ordering Information Package Marking1 Part Number (Tape and Reel)2 TQFN34-24 9UXYY AAT2404IMK-T1 Skyworks Green™ products are compliant with all applicable legislation and are halogen-free. For additional information, refer to Skyworks Definition of Green™, document number SQ04-0074. Package Information TQFN34-243 3.000 ± 0.050 1.700 ± 0.050 Index Area 0.400 ± 0.050 R(5x) 2.700 ± 0.050 4.000 ± 0.050 0.210 ± 0.040 0.400 BSC Detail “A” Bottom View Detail “A” 0.750 ± 0.050 Top View 0 + 0.10 - 0.00 0.203 REF Side View ALL DIMENSIONS IN MILLIMETERS. 1. XYY = assembly and date code. 2. Sample stock is generally held on part numbers listed in BOLD. 3. The leadless package family, which includes QFN, TQFN, DFN, TDFN, and STDFN, has exposed copper (unplated) at the end of the lead terminals due to the manufacturing process. A solder fillet at the exposed copper edge cannot be guaranteed and is not required to ensure a proper bottom solder connection. Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202247A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 7, 2012 19 DATA SHEET AAT2404 Voltage-Variable Current Sourcing Boost Controller For LED Lighting Applications Copyright © 2012 Skyworks Solutions, Inc. All Rights Reserved. Information in this document is provided in connection with Skyworks Solutions, Inc. (“Skyworks”) products or services. These materials, including the information contained herein, are provided by Skyworks as a service to its customers and may be used for informational purposes only by the customer. Skyworks assumes no responsibility for errors or omissions in these materials or the information contained herein. Skyworks may change its documentation, products, services, specifications or product descriptions at any time, without notice. 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Additional information, including relevant terms and conditions, posted at www.skyworksinc.com, are incorporated by reference. 20 Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com 202247A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • August 7, 2012