AAT1235 High Efficiency White LED Drivers for Backlight and Keypad General Description Features The AAT1235 is a highly integrated, high efficiency power solution for white LED backlight and keypad backlights in mobile/portable devices. It is based on a switching boost converter which steps up the singlecell lithium-ion/polymer battery voltage to drive 5 strings of series-connected white LEDs with precision current regulation. The AAT1235 is capable of driving a total of four LEDs per channel. • • • • • • The boost converter can produce an output drive of up to 24V at 100mA. The high switching frequency (up to 2MHz) provides fast response to load transients and allows the use of small external components. A fully integrated control circuit simplifies the design and reduces total solution size. • AnalogicTech's Advanced Simple Serial Control™ (AS2Cwire™) serial digital input is used to individually turn each output sink on/off and adjust the LED current by group. Unlike conventional pulse width modulation (PWM) control of LED brightness, the AAT1235 drives the LEDs with constant, non-pulsating current. • • • A similar device is also available with an I2C twowire interface; please see the AAT1236 datasheet. Input Supply Voltage Range: 2.7V to 5.5V Maximum Boost Output Drive: Up to 24V at 100mA Up to 85% Efficient Operation Up to 2MHz Switching Frequency with Small Inductor User-Programmable Full-Scale LED Current, Up to 30mA Single-Wire AS2Cwire Serial Interface — Five Addressable Registers • Independent LED Current Control by Group — Backlight Group B1-B2, 16 Settings — Auxiliary Group A1-A3, 16 Settings • Independent LED ON/OFF Control — Fast, 1MHz Serial Interface Non-Pulsating, High-Performance LED Current Drive for Uniform Illumination — 10% Absolute Accuracy — 2% Channel-to-Channel Matching Over-Voltage and Over-Temperature Protection Automatic Soft-Start Minimizes Large Inrush Current at Startup Available in 3x4mm TDFN34-16 Package Applications The AAT1235 is available in a Pb-free, thermallyenhanced 16-pin 3x4mm TDFN package and is specified for operation over the -40°C to +85°C temperature range. • • • • • • • Typical Application L = 2.2µH SwitchReg™ D1 Digital Still Cameras (DSCs) Keypad Backlight Large Panel Displays Mobile Handsets Personal Media Players PDAs and Notebook PCs White LED Backlight Keypad or RGB LEDs Up to 24V Max C OUT 2.2µF Backlight LEDs LIN Input : 2.7V~5.5V SW VIN CIN 2.2µF IN R2 187kΩ B1 B2 A1 EN/SET RSET A2 A3 OV AAT1235 GND AGND R3 12.1kΩ R1 22.6kΩ AS 2Cwire Control 1235.2007.02.1.1 1 AAT1235 High Efficiency White LED Drivers for Backlight and Keypad Pin Descriptions Pin # Symbol 1 VIN 2 OV 3 4 5 6 EN/SET B1 B2 RSET 7 8 IN GND 9 SW 10, 11 12 13 14 15 16 EP N/C AGND A3 A2 A1 LIN Function Input supply for the converter. Connect a 2.2µF or larger ceramic capacitor from VIN to GND. Boost output over voltage detect pin. Use resistor divider to set the circuit's external overvoltage protection. See Applications Information for details. AS2Cwire control or enable pin. Backlight current sink 1. Connect the cathode of the last LED in the string to B1. Backlight current sink 2. Connect the cathode of the last LED in the string to B2. LED current set resistor. A 22.6kΩ resistor from RSET to AGND sets the maximum LED current in A1-A3 and B1-B2 to 20mA. Input bias supply for the internal circuitry. Connect IN to VIN directly at the AAT1235. Ground for the boost converter. Connect GND to AGND at a single point as close to the AAT1235 as practical. Boost converter switching node. A 2.2µH inductor, connected between SW and LIN, sets the boost converter's switching frequency. Not connected. Ground pin. Connect AGND to GND at a single point as close to the AAT1235 as practical. Auxiliary current sink 3. Connect the cathode of the last LED in the string to A3. Auxiliary current sink 2. Connect the cathode of the last LED in the string to A2. Auxiliary current sink 1. Connect the cathode of the last LED in the string to A1. Switched power input. Connect LIN to the external power inductor. Exposed paddle (bottom) Connected internally to SW. Connect to SW or leave floating. Pin Configuration TDFN34-16 (Top View) VIN OV EN/SET B1 B2 RSET IN GND 2 1 16 2 15 3 14 4 13 5 12 6 11 7 10 8 9 LIN A1 A2 A3 AGND N/C N/C SW 1235.2007.02.1.1 AAT1235 High Efficiency White LED Drivers for Backlight and Keypad Absolute Maximum Ratings1 TA = 25°C unless otherwise noted. Symbol VIN, IN SW EN/SET, Bx, Ax, RSET, OV, LIN TS TJ TLEAD Description Value Units Input Voltage Switching Node -0.3 to 6.0 28 V V Maximum Rating VIN + 0.3 V -65 to 150 -40 to 150 300 °C °C °C Value Units 50 2 °C/W W Storage Temperature Range Operating Temperature Range Maximum Soldering Temperature (at leads, 10 sec) Thermal Information2 Symbol θJA PD Description Thermal Resistance Maximum Power Dissipation3 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 circuit board. 3. Derate 20mW°C above 40°C ambient temperature. 1235.2007.02.1.1 3 AAT1235 High Efficiency White LED Drivers for Backlight and Keypad Electrical Characteristics1 VIN = 3.6V; CIN = 2.2µF;TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = 25°C. Symbol Power Supply VIN VOUT(MAX) VUVLO ICC ISHDN(MAX) IOX IDX IDX-Matching TSS VOV Description Conditions Input Voltage Range Maximum Output Voltage UVLO Threshold Operating Current (No Switching) IN Pin Shutdown Current Maximum Continuous Output Current Current Sink Accuracy Current Matching Between Any Sink Channels Soft-Start Time OVP Threshold Voltage OVP Threshold Hysteresis Low Side Switch On Resistance Input Disconnect Switch Current Set Ratio Input Switch Current Limit RDS(ON)N RDS(ON)IN ISET ILIMIT EN/SET Input VEN(L) Enable Threshold Low VEN(H) Enable Threshold High TEN/SET LO EN/SET Low Time TEN/SET HI EN/SET High Time TOFF EN/SET Off Timeout TLAT EN/SET Latch Timeout IEN/SET EN/SET Input Leakage Thermal Protection TJ-TH TJ Thermal Shutdown Threshold TJ-HYS TJ Thermal Shutdown Hysteresis Min Typ 2.7 VIN Rising Hysteresis VIN Falling B1 = B2 = A1 = A2 = A3 = 1.2V, 2mA Setting, RSET = 226kΩ EN = GND VO = 24V RSET = 22.6kΩ RSET = 22.6kΩ, A1 = A2 = A3 = B1 = B2 = 0.4V From Enable to Output Regulation; VFB = 300mV VOUT Rising Max Units 5.5 24 2.7 V V V mV V 300 µA 1.0 µA mA mA % 150 1.8 100 18 20 2 22 5 300 1.1 IOUT = 100mA IOUT = 100mA ISINK/IRSET, VRSET = 0.6V 1.2 100 80 200 760 µs 1.3 V mV mΩ mΩ A/A A 0.4 V V µs µs µs µs µA 1.2 1.4 0.3 VEN/SET = VIN = 5V 75 75 500 500 1 -1 140 15 °C °C 1. The AAT1235 is guaranteed to meet performance specifications over the -40°C to +85°C operating temperature range is assured by design, characterization, and correlation with statistical process controls. 4 1235.2007.02.1.1 AAT1235 High Efficiency White LED Drivers for Backlight and Keypad Typical Characteristics Efficiency vs. LED Current Efficiency vs. LED Current (Group B On; Group A Off) (Group B Off; Group A On) 83 84 VIN = 5V 81 80 VIN = 3.6V 79 VIN = 4.2V 78 VIN = 5V 83 Efficiency (%) Efficiency (%) 82 77 82 81 VIN = 3.6V 80 79 VIN = 4.2V 78 76 77 1.6 3.9 6.2 8.5 10.8 13.1 15.4 17.7 20 1.6 3.9 Output LED Current (mA) 6.2 8.5 10.8 13.1 15.4 17.7 20 Output LED Current (mA) Efficiency vs. LED Current LED Current Accuracy vs. Supply Voltage 86 3 85 2 84 Accuracy (%) Efficiency (%) (Group A and B On) VIN = 5V 83 82 VIN = 3.6V 81 VIN = 4.2V 80 79 1.6 IB1, B2, A1, A2, A3 1 0 -1 -2 -3 3.9 6.2 8.5 10.8 13.1 15.4 17.7 2.7 20 3.1 3.4 Output LED Current (mA) Shutdown Current (µA) LED Current (mA) I A1 19.4 19.2 19.0 I A3 I A2 I B2 18.6 18.4 2.7 3.1 3.4 3.8 4.1 4.5 Supply Voltage (V) 1235.2007.02.1.1 4.8 5.2 5.5 0.7 I B1 18.8 4.5 Shutdown Current vs. Supply Voltage and Temperature 19.8 19.6 4.1 Supply Voltage (V) LED Current vs. Supply Voltage 20.0 3.8 4.8 5.2 5.5 0.6 25°C 0.5 85°C 0.4 0.3 0.2 0.1 0.0 2.7 -40°C 3.1 3.5 3.9 4.3 4.7 5.1 5.5 Supply Voltage (V) 5 AAT1235 High Efficiency White LED Drivers for Backlight and Keypad Typical Characteristics LED Current vs. Temperature LED Current Accuracy vs. Temperature 21.2 LED Current (mA) 21.0 (All Channels = 20mA) LED Current Accuracy (%) (All Channels = 20mA) IA3 20.8 20.6 20.4 20.2 IB1 IA2 IB2 20.0 19.8 19.6 19.4 IA1 19.2 19.0 -40 -15 10 35 60 4 3 2 1 0 IB1 -1 IA2 IA3 -2 -3 IB2, A1 -4 -5 -6 -40 85 -15 10 Temperature (°°C) Shutdown Operation Output Ripple (All Channels) (All Channels = 20mA) Output Voltage (top) (V) Switching Node (middle) (V) 0 50 IGROUP_B (mA) IINDUCTOR (A) 0 0.5 0 14.5 14.0 13.5 16V 0V 1.0 0.5 0.0 Time (50µs/div) Time (200ns/div) Switching Frequency vs. Supply Voltage and Temperature Output Ripple 13.0 12.5 14V 0V 0.5 0.0 6 Switching Frequency (MHz) 13.5 Inductor Current (bottom) (A) Output Voltage (top) (V) Switching Node (middle) (V) (All Channels = 10mA) Time (200ns/div) 85 Inductor Current (bottom) (A) 0 50 IGROUP_A (mA) 60 Temperature (°°C) 5 Enable (V) 35 2.5 25°C 2.0 1.5 -40°C 1.0 +85°C 0.5 0.0 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5 Supply Voltage (V) 1235.2007.02.1.1 AAT1235 High Efficiency White LED Drivers for Backlight and Keypad Typical Characteristics Enable Threshold Low vs. Supply Voltage and Temperature Line Transient Input Voltage (top) (V) 4.0 3.5 14.2 3.0 14.1 14.0 13.9 13.8 Output Voltage (bottom) (V) 4.5 Enable Threshold Low (V) (All Channels = 20mA) 1.1 1.0 -40°C 0.9 25°C 0.8 0.7 +85°C 0.6 2.7 3.1 3.5 4.3 4.7 5.1 5.5 Supply Voltage (V) Time (50µs/div) Enable Threshold High vs. Supply Voltage and Temperature Input Disconnect Switch Resistance vs. Supply Voltage and Temperature 280 1.2 260 1.1 -40°C 25°C RDS(ON)IN (mΩ Ω) Enable Threshold High (V) 3.9 1.0 0.9 +85°C 0.8 +120°C +100°C 240 220 200 +85°C 180 +25°C 160 0.7 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5 140 2.5 3.0 Supply Voltage (V) 3.5 4.0 4.5 5.0 5.5 6.0 Supply Voltage (V) Low Side Switch On Resistance vs. Supply Voltage and Temperature Soft Start Operation (All Channels = 20mA) 160 RDS(ON)N (mΩ Ω) 2 EN/SET (V) 140 0 +120°C 120 20 +100°C 10 100 VOUT (V) 80 +25°C 60 40 2.5 0 0.5 +85°C IINDUCTOR (A) 3.0 3.5 4.0 4.5 Supply Voltage (V) 1235.2007.02.1.1 5.0 5.5 0 6.0 Time (200µs/div) 7 AAT1235 High Efficiency White LED Drivers for Backlight and Keypad EN/SET Off Timeout (TOFF) (µs) EN/SET Off Timeout vs. Supply Voltage and Temperature 400 -40°C 350 300 250 +85°C 200 150 +25°C 100 50 2.7 3.1 3.5 3.9 4.3 4.7 5.1 5.5 EN/SET Latch Timeout (TLAT) (µs) Typical Characteristics EN/SET Latch Timeout vs. Supply Voltage and Temperature 400 350 250 200 +85°C 150 +25°C 100 50 2.7 Supply Voltage (V) 3.1 3.5 3.9 4.3 4.7 5.1 5.5 Supply Voltage (V) Transition of LED Current (Group B = 1.8mA to 20mA; Group A = 20mA) 4 4 2 2 0.02 0 0 0.02 0 0.02 0 IA1 (middle) (A) IB1 (bottom) (A) 0 EN/SET (top) (V) Transition of LED Current (Group B = 1.8mA; Group A = 20mA to 1.8mA) IA1 (middle) (A) IB1 (bottom) (A) EN/SET (top) (V) -40°C 300 0 Time (250µs/div) Time (250µs/div) Transition of LED Current (Group B = 20mA; Group A = 20mA to 1.8mA) 4 0 0.02 0 0.02 IA1 (middle) (A) IB1 (bottom) (A) EN/SET (top) (V) 2 0 Time (250µs/div) 8 1235.2007.02.1.1 AAT1235 High Efficiency White LED Drivers for Backlight and Keypad Functional Block Diagram LIN SW VIN IN OV ROM Boost Converter Control V(A1, A2, A3) VREF V(B1, B2) AS2Cwire Control D/A A2 D/A A3 D/A B1 D/A B2 Max Current Adjustment GND AGND Functional Description The AAT1235 consists of a controller for the step-up switching converter and its power switch, and five regulated current sinks each programmable at 16 levels into two groups, which can be turned on/off individually. An external Schottky diode, a power inductor, an output capacitor, and a resistor divider are required to complete the solution. The AAT1235's boost controller is designed to deliver 100mA up to 24V. The AAT1236 is capable of driving a total of five channels divided into two groups with four white LEDs connected in series at each channel. The output load current can be programmed by the current sink magnitudes. AS2Cwire programming allows independent control of two current sink groups (A1 to A3 and B1 to B2) and control on/off 1235.2007.02.1.1 A1 ROM VREF EN/SET D/A RSET with a different configuration on each channel. Unused sink channel(s) must be connected to AGND to ensure proper function of the AAT1235. Control Loop The AAT1235 provides the benefits of current mode control with a simple hysteretic output current loop providing exceptional stability and fast response with minimal design effort. The device maintains exceptional constant current regulation, transient response, and cycle-by-cycle current limit without additional compensation components. The AAT1235 modulates the power MOSFET switching current to maintain the programmed sink current through each channel. The sink voltage at each channel is monitored and the controller provides direct feedback in order to maintain the desired LED current. 9 AAT1235 High Efficiency White LED Drivers for Backlight and Keypad 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 current level. During the OFF interval, the input current decays until the 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. Peak current is adjusted by the controller until the desired LED output current level is met. The magnitude of the feedback error signal determines the average input current. Therefore, the AAT1235 controller implements a programmed current source connected to the output capacitor, parallel with the LED channels. There is no right-half plane zero, and loop stability is achieved with no additional compensation components. The controller responds by increasing the peak inductor current, resulting in higher average current in the inductor and LED channels. Under light load conditions, the inductor OFF interval current goes below zero and the boost converter enters discontinuous mode operation. Further reduction in the load current results in a corresponding reduction in the switching frequency. The AAT1235 provides a pulsed frequency operation which reduces switching losses and maintains high efficiency under light load conditions. 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 LED current will not significantly change the operating frequency. A small 2.2µH (±20%) inductor is selected to maintain high frequency switching (up to 2MHz) and high efficiency operation for outputs up to 24V. Soft Start / Enable The input disconnect switch is activated when a valid supply voltage is present and the EN/SET pin is strobed high. Slew rate control on the input disconnect switch ensures minimal inrush current as 10 the output voltage is charged to the input voltage, prior to switching of the N-channel power MOSFET. A monotonic turn-on is guaranteed by the built-in soft-start circuitry, which eliminates output current overshoot across the full input voltage range and all load conditions. Current Limit and Over-Temperature Protection The switching of the N-channel MOSFET terminates when a current limit of 1.5A (typical) is exceeded. This minimizes power dissipation and component stresses under overload and short-circuit conditions. Switching resumes when the current decays below the current limit. Thermal protection disables the AAT1235 when internal power dissipation becomes excessive, as it disables both MOSFETs. The junction over-temperature threshold is 140°C with 15°C of temperature hysteresis. The output voltage automatically recovers when the over-temperature fault condition is removed. Over-Voltage Protection Over-voltage protection prevents damage to the AAT1235 during open-circuit causing high output voltage conditions. An over-voltage event is defined as a condition where the voltage on the OV pin exceeds the over-voltage threshold limit (VOV = 1.2V typical). When the voltage on the OV pin has reached the threshold limit, the converter stops switching and the output voltage decays. Switching resumes when the voltage on the OV pin drops below the lower hysteresis limit, maintaining an average output voltage between the upper and lower OV thresholds multiplied by the resistor divider scaling factor. Under-Voltage Lockout Internal bias of all circuits is controlled via the VIN input. Under-voltage lockout (UVLO) guarantees sufficient VIN bias and proper operation of all internal circuitry prior to soft start. 1235.2007.02.1.1 AAT1235 High Efficiency White LED Drivers for Backlight and Keypad Constant Current Output Level Settings and AS2Cwire Serial Interface The LED current sink level of each group and the on/off status of each channel is controlled by AnalogicTech's AS2Cwire serial digital input. Since each current sink is programmable, no PWM or additional control circuitry is needed to control LED brightness. This feature greatly reduces the burden on a microcontroller or system IC to manage LED or display brightness, allowing the user to "set it and forget it." With its high-speed serial interface (1MHz data rate), the input sink current can be changed quickly and easily. Also the non-pulsating LED current reduces system noise and improves LED reliability. AS2Cwire relies on the number of rising edges of the EN/SET pin to address and load the registers. AS2Cwire latches data or address after the EN/SET pin has been held high for time TLAT (500µs). Address or data is differentiated by the number of EN/SET rising edges. Since the data registers are 4 bits each, the differentiating number of pulses is 24 or 16, so that Address 0 is signified by 17 rising edges, Address 1 by 18 rising edges, and so forth. Data is set to any number of rising edges between 1 and including 16. A typical write protocol is a burst of EN/SET rising edges, signifying a particular address, followed by a pause with EN/SET held high for the TLAT timeout period, a burst of rising edges signifying data, and a TLAT timeout for the data registers. Once an address is set, then multiple writes to that address are allowed where only data is issued. When EN/SET is held low for an amount of time longer than TOFF (500µs), the AAT1235 enters into shutdown mode and draws less than 1µA from the input. Data and Address registers are cleared (reset to 0) during shutdown. Address EN/SET Rising Edges Data Register 0 1 2 3 4 17 18 19 20 21 B1-B2 current A1-A3 current B1-B2 on/off A1-A3 on/off A1-A3, B1-B2 on Address 0 and 1: LED Brightness Control Outputs A1, A2, A3, B1, B2 are each capable of sinking up to 30mA. The maximum current can be programmed by an external resistor at the RSET pin. It is suggested to connect up to four white LEDs in series for each channel and to keep the same number of LEDs in each channel. Outputs B1 and B2 are intended to drive the white LEDs for the backlight in a phone, while A1:A3 can supply the keypad LEDs. For large displays, all five outputs can be used. Data All Outputs (%) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 100 84 71 60 51 43 35 31 26 21 18 15 13.5 12.0 10.5 9.0 Table 2: Address 0 and 1 for Current Setting as Percentage of the Maximum Level Set by RSET. Address 2: LED Group B ON/OFF Control Data B1 B2 1 2 3 4 Off Off On On Off On Off On Table 3: Address 2 for Group B ON/OFF State. Table 1: AS2Cwire Serial Interface Addressing. 1235.2007.02.1.1 11 AAT1235 High Efficiency White LED Drivers for Backlight and Keypad Address 3: LED Group A ON/OFF Control Address 4: LED Groups A & B ON Control Data A1 A2 A3 B2 B1 A1 A2 A3 1 2 3 4 5 6 7 8 Off Off Off Off On On On On Off Off On On Off Off On On Off On Off On Off On Off On On On On On On Table 5: Address 4 for Turning Group A and B All On. Address 4 turns on all channels with the current programmed by Addresses 0 and 1. No DATA needs to be provided. Table 4: Address 3 for Group A ON/OFF State. AS2Cwire Serial Interface Timing Address Data THI TLO TLAT TLAT EN/SET 1 Address 12 2 17 18 1 0 2... n ≤ 16 1 Data Reg 1 0 Data Reg 2 0 n 1235.2007.02.1.1 AAT1235 High Efficiency White LED Drivers for Backlight and Keypad Application Information Channel Disable Tie all unused channels to AGND. On start-up, these channels will be automatically disabled. LED Selection LED Current (mA) 35 30 25 20 15 10 5 0 Although the AAT1235 is specifically designed to drive white LEDs, the device can also be used to drive most types of LEDs with forward voltages ranging between 2.0V and 4.7V. Since the A1, A2, A3, and B1, B2 input current sinks are matched with low voltage dependence, the LED-to-LED brightness will be matched regardless of the individual LED forward voltage (VF) levels. In some instances, it may be necessary to drive high-VF type LEDs. The low dropout (~0.1V @ 20mA ILED) current sinks in the AAT1235 make it capable of driving LEDs with forward voltages as high as 4.7V from an input supply as low as 3.0V. LED outputs A1-A3 and B1-B2 can be combined to drive highcurrent LEDs without complication, making the AAT1235 a perfect application for large LCD display backlighting and keypad LED applications. Constant Current Setting The LED current is controlled by the RSET resistor. For maximum accuracy, a 1% tolerance resistor is recommended. Table 6 shows the RSET resistor value for AAT1235 for various LED full-scale current levels. ILED (mA) Ω) RSET (kΩ 30 25 20 15 10 5 14.7 17.4 22.6 29.4 44.2 93.1 Table 6: Maximum LED Current and RSET Resistor Values (1% Resistor Tolerance). 10 36 62 88 114 140 166 192 218 244 270 RSET (kΩ Ω) Figure 1: LED Current vs. RSET Values. Over-Voltage Protection The over-voltage protection circuit consists of a resistor network connected from the output voltage to the OV pin (see Figure 2). This over voltage protection circuit prevents damage to the device when one of the five channels has an open LED circuit. The AAT1235 continues to operate; however, the LED current in the remaining channels is no longer regulated and the actual LED current will be determined by the externally programmed over-voltage protection threshold, the inductor value, and the switching frequency. The resistor divider can be selected such that the over-voltage threshold occurs prior to the output reaching 24V (VOUT(MAX)). The value of R3 should be selected from 10kΩ to 20kΩ to minimize switching losses without degrading noise immunity: R2 = R 3 · ⎛ VOUT(PROTECTION) ⎞ -1 VOV ⎝ ⎠ VOUT AAT1235 R2 COUT OV GND R3 Figure 2: Over-Voltage Protection Circuit. Maximum LED current per channel versus RSET value is shown in Figure 1. 1235.2007.02.1.1 13 AAT1235 High Efficiency White LED Drivers for Backlight and Keypad If four LEDs are connected in series on one channel, the total VF from the WLEDs could be as high as 18.8V. Therefore, using R3 = 12.1kΩ and setting VOUT(PROTECTION) = 20V is recommended. Selecting a 1% resistor, this results in R2 = 187kΩ (rounded to the nearest standard 1% value). It is always recommended to use the same number of WLEDs on each channel and set the appropriate over-voltage protection. Failure to do so may cause any one of the (5) sink pins to exceed the absolute maximum rating voltage and permanently damage the device in case the channel is disconnected (open circuit failure). Examples of over voltage settings for various strings of series-connected LEDs are shown in Table 7. Number of WLEDs on Each Channel Total Maximum VF (V) VOUT(PROTECTION) (V) Ω R3 = 12.1kΩ Ω) R2 (kΩ 4 3 2 18.8 14.1 9.4 20 15 10 187 140 88.7 Table 7: Over-Voltage Protection Settings. VOUT JP1 0 LED1 R2 187k VIN LED7 LED3 LED8 LED4 LED9 3 2 1 Enable/Set C1 2.2µF R3 12.1k R1 22.6k JP3 0 LED6 LED2 JP6 0 JP6 JP2 0 D1 MBR0530T1 SW_Node U1 AAT1235 TDFN3X4 JP7 0 1 2 3 4 5 6 7 8 VIN LIN OV A1 EN/SET A2 B1 A3 B2 AGND RSET NC IN NC GND SW JP4 0 JP5 0 LED11 LED16 LED21 LED12 LED17 LED22 LED13 LED18 LED23 LED14 LED19 LED24 C2 2.2µF 25V L1 2.2µH 16 15 14 13 12 11 10 9 JP8 0 JP9 0 JP10 0 RTN L1: 2.2uH Taiyo Yuden NR4018T2R2M C1: 0805 10V 2.2µF X7R GRM21BR71A225KA01 C2: 0805 25V 2.2µF X7R GRM21BR71E225KA73L LED1-24: OSRAM LW M673 or equivalent Figure 3: A AAT1235-based High Efficiency White LED Driver Schematic. 14 1235.2007.02.1.1 AAT1235 High Efficiency White LED Drivers for Backlight and Keypad LED Brightness Control The AAT1235 uses AS Cwire programming to control LED brightness. The output current of the AAT1235 can be changed successively to brighten or dim the LEDs in smooth transitions (i.e., to fade in or fade out) or in discrete steps, giving the user complete programmability and real-time control of LED brightness. 2 Selecting the Schottky Diode To ensure minimum forward voltage drop and no recovery, high voltage Schottky diodes are recommended for the AAT1235 boost converter. The output diode is selected to maintain acceptable efficiency and reasonable operating junction temperature under full load operating conditions. Forward voltage (VF) and package thermal resistance (θJA) are the dominant factors in selecting a diode. The diode’s non-repetitive peak forward surge current rating (IFSM) should be considered for high pulsed load applications, such as camera flash. IFSM rating drops with increasing conduction period. Manufacturers’ datasheets should be reviewed carefully to verify reliability under peak loading conditions. The diode's published current rating may not reflect actual operating conditions and should be used only as a comparative measure between similarly rated devices. 20V rated Schottky diodes are recommended for output voltages less than 15V, while 30V rated Schottky diodes are recommended for output voltages higher than 15V. Estimating Schottky Diode Power Dissipation The switching period is divided between ON and OFF time intervals: 1 = TON + TOFF FS tor. 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: D= TON TON + TOFF = TON ⋅ FS 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)): DMAX = VOUT - VIN(MIN) VOUT The average diode current during the OFF time can be estimated: IAVG(OFF) = IOUT 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: IAVG(TOT) = IOUT The average output current multiplied by the forward diode voltage determines the loss of the output diode: PLOSS(DIODE) = IAVG(TOT) · VF During the ON time, the N-channel power MOSFET is conducting and storing energy in the boost induc- 1235.2007.02.1.1 = IOUT · VF 15 AAT1235 High Efficiency White LED Drivers for Backlight and Keypad For continuous LED currents, the diode junction temperature can be estimated: TJ(DIODE) = TAMB + θJA · PLOSS(DIODE) External Schottky diode junction temperature should be below 110ºC, and may vary depending Manufacturer Part Number Rated IF(AV) Current (A)1 Diodes, Inc. ON Semi ON Semi B0520WS MBR130LSFT MBR0530T 0.50 1.00 0.50 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. Rated Voltage (V) Thermal Resistance θJA, °C/W)1 (θ Case 20 30 30 426 325 206 SOD-323 SOD-123 SOD-123 Table 8: Typical Surface Mount Schottky Rectifiers for Various Output Loads. (select TJ < 110°C in application circuit). Selecting the Boost Inductor The AAT1235 controller utilizes hysteretic control and the switching frequency varies with output load and input voltage. The value of the inductor determines the maximum switching frequency of the boost converter. Increased output inductance decreases the switching frequency, resulting in higher peak currents and increased output voltage ripple. To maintain 2MHz maximum switching frequency and stable operation, an output inductor selected between 1.5µH and 2.7µH is recommended. A better estimate of DMAX is possible once VF is known: DMAX = (VOUT + VF - VIN(MIN)) (VOUT + VF) Where VF is the Schottky diode forward voltage. If not known or not provided by the manufacturer, a starting value of 0.5V can be used. Manufacturer’s specifications list both the inductor DC current rating, which is a thermal limitation, and peak inductor current rating, which is determined by the saturation characteristics. Measurements at full load and high ambient temperature should be performed to ensure that the inductor does not saturate or exhibit excessive temperature rise. 16 The output inductor (L) is selected to avoid saturation at minimum input voltage and maximum output load conditions. Peak current may be estimated using the following equation, assuming continuous conduction mode. Worst-case peak current occurs at minimum input voltage (maximum duty cycle) and maximum load. Switching frequency (FS) can be estimated at 500kHz with a 2.2µH inductor: IPEAK = IOUT D · VIN(MIN) + MAX (1 - DMAX) (2 · FS · L) At light load and low output voltage, the controller reduces the operating frequency to maintain maximum operating efficiency. As a result, further reduction in output load does not reduce the peak current. Minimum peak current can be estimated between 0.5A and 0.75A. At high load and high output voltages, the switching frequency is somewhat diminished, resulting in higher IPEAK. Bench measurements are recommended to confirm actual IPEAK and to ensure that the inductor does not saturate at maximum LED current and minimum input supply voltage. The RMS current flowing through the boost inductor is equal to the DC plus AC ripple components. Under worst-case RMS conditions, the current 1235.2007.02.1.1 AAT1235 High Efficiency White LED Drivers for Backlight and Keypad waveform is critically continuous. The resulting RMS calculation yields worst-case inductor loss. The RMS current value should be compared against the inductor manufacturer's temperature rise, or thermal derating, guidelines: IRMS = IPEAK 3 For a given inductor type, smaller inductor size leads to an increase in DCR winding resistance and, in most cases, increased thermal impedance. Winding resistance degrades boost converter efficiency and increases the inductor’s operating temperature: To ensure high reliability, the inductor case temperature should not exceed 100ºC. In some cases, PCB heatsinking applied to the LIN node (nonswitching) can improve the inductor's thermal capability. However, as in the case of adding extra metal around the Schottky's anode, adding extra PCB metal around the AAT1235's SW pin for heatsinking may degrade EMI performance. Shielded inductors provide decreased EMI and may be required in noise sensitive applications. Unshielded chip inductors provide significant space savings at a reduced cost compared to shielded (wound and gapped) inductors. In general, chiptype inductors have increased winding resistance (DCR) when compared to shielded, wound varieties. PLOSS(INDUCTOR) = IRMS2 · DCR Manufacturer Sumida Sumida Sumida Murata Murata Taiyo Yuden Taiyo Yuden Coiltronics Coiltronics Coiltronics Part Number Inductance (µH) Max DC ISAT Current (A) DCR Ω) (Ω Size (mm) LxWxH Type CDRH4D22/HP-2R2 CDR4D11/HP-2R4 CDRH4D18-2R2 LQH662N2R2M03 LQH55DN2R2M03 NR4018T2R2 NR3015T2R2 SD3814-2R2 SD3114-2R2 SD3112-2R2 2.2 2.4 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.2 2.50 1.70 1.32 3.30 3.20 2.70 1.48 1.90 1.48 1.12 35 105 75 19 29 60 60 77 86 140 5.0x5.0x2.4 4.8x4.8x1.2 5.0x5.0x2.0 6.3x6.3x4.7 5.0x5.7x4.7 4.0x4.0x1.8 3.0x3.0x1.5 3.8x3.8x1.4 3.1x3.1x1.4 3.1x3.1x1.2 Shielded Shielded Shielded Shielded Non-Shielded Shielded Shielded Shielded Shielded Shielded Table 9: Typical Surface Mount Inductors for Various Output Loads (select IPEAK < ISAT). Selecting the Boost Capacitors The high output ripple inherent in the boost converter necessitates the use of low impedance output filtering. Multi-layer ceramic (MLC) capacitors provide small size and adequate capacitance, low parasitic equivalent series resistance (ESR) and equivalent series inductance (ESL), and are well suited for use with the AAT1235 boost regulator. MLC capacitors of type X7R or X5R are recommended to ensure good capacitance stability over the full operating temperature range. The output capacitor is selected to maintain the output load without significant voltage droop (ΔVOUT) 1235.2007.02.1.1 during the power switch ON interval, when the output diode is not conducting. A ceramic output capacitor between 2.2µF and 4.7µF is recommended (see Table 8). Typically, 25V rated capacitors are required for the 24V maximum boost output. Ceramic capacitors selected as small as 0805 are available which meet these requirements. MLC capacitors exhibit significant capacitance reduction with applied voltage. Output ripple measurements should confirm that output voltage droop and operating stability are within acceptable limits. Voltage derating can minimize this factor, but results may vary with package size and among specific manufacturers. 17 AAT1235 High Efficiency White LED Drivers for Backlight and Keypad Output capacitor size can be estimated at a switching frequency (FS) of 500kHz (worst case): COUT = Manufacturer Murata Murata Murata Murata Murata To maintain stable operation at full load, the output capacitor should be selected to maintain ΔVOUT between 100mV and 200mV. The boost converter input current flows during both ON and OFF switching intervals. The input ripple current is less than the output ripple and, as a result, less input capacitance is required. IOUT · DMAX FS · ΔVOUT Part Number Value (µF) Voltage Rating Temp Co Case Size GRM188R60J225KE19 GRM21BR71A225KA01 GRM219R61E225KA12 GRM21BR71E225KA73L GRM21BR61E475KA12 2.2 2.2 2.2 2.2 4.7 6.3 10 25 25 25 X5R X7R X5R X7R X5R 0603 0805 0805 0805 0805 Table 10: Recommended Ceramic Capacitors. PCB Layout Guidelines Boost converter performance can be adversely affected by poor layout. Possible impact includes high input and output voltage ripple, poor EMI performance, and reduced operating efficiency. Every attempt should be made to optimize the layout in order to minimize parasitic PCB effects (stray resistance, capacitance, and inductance) and EMI coupling from the high frequency SW node. A suggested PCB layout for the AAT1235 boost converter is shown in Figures 4 and 5. The following PCB layout guidelines should be considered: 1. Minimize the distance from Capacitor C1 and C2’s negative terminals to the GND pins. This is especially true with output capacitor C2, which conducts high ripple current from the output diode back to the GND pins. 18 2. Minimize the distance between L1 to D1 and switching pin SW; minimize the size of the PCB area connected to the SW pin. 3. Maintain a ground plane and connect to the IC GND pin(s) as well as the GND connections of C1 and C2. 4. Consider additional PCB metal on D1’s cathode to maximize heatsinking capability. This may be necessary when using a diode with a high VF and/or thermal resistance. 5. Do not connect the exposed paddle (bottom of the die) to either AGND or GND because it is connected internally to SW. Connect the exposed paddle to the SW pin or leave floating. 1235.2007.02.1.1 AAT1235 High Efficiency White LED Drivers for Backlight and Keypad Figure 4: AAT1235 Evaluation Board Top Side Layout. Figure 5: AAT1235 Evaluation Board Bottom Side Layout. Figure 6: Exploded View of AAT1235 Evaluation Board Top Side Layout Detailing Plated Through Vias. 1235.2007.02.1.1 19 AAT1235 High Efficiency White LED Drivers for Backlight and Keypad Ordering Information Package Marking1 Part Number (Tape and Reel)2 TDFN34-16 TKXYY AAT1235IRN-T1 All AnalogicTech products are offered in Pb-free packaging. The term “Pb-free” means semiconductor products that are in compliance with current RoHS standards, including the requirement that lead not exceed 0.1% by weight in homogeneous materials. For more information, please visit our website at http://www.analogictech.com/pbfree. Package Information3 TDFN34-16 3.000 ± 0.050 1.600 ± 0.050 Detail "A" 3.300 ± 0.050 4.000 ± 0.050 Index Area 0.350 ± 0.100 Top View 0.230 ± 0.050 Bottom View C0.3 (4x) 0.050 ± 0.050 0.450 ± 0.050 0.850 MAX Pin 1 Indicator (optional) 0.229 ± 0.051 Side View Detail "A" 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. 20 1235.2007.02.1.1 AAT1235 High Efficiency White LED Drivers for Backlight and Keypad © Advanced Analogic Technologies, Inc. AnalogicTech cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in an AnalogicTech product. No circuit patent licenses, copyrights, mask work rights, or other intellectual property rights are implied. AnalogicTech reserves the right to make changes to their products or specifications or to discontinue any product or service without notice. Customers are advised to obtain the latest version of relevant information to verify, before placing orders, that information being relied on is current and complete. All products are sold subject to the terms and conditions of sale supplied at the time of order acknowledgement, including those pertaining to warranty, patent infringement, and limitation of liability. AnalogicTech warrants performance of its semiconductor products to the specifications applicable at the time of sale in accordance with AnalogicTech’s standard warranty. Testing and other quality control techniques are utilized to the extent AnalogicTech deems necessary to support this warranty. Specific testing of all parameters of each device is not necessarily performed. AnalogicTech and the AnalogicTech logo are trademarks of Advanced Analogic Technologies Incorporated. All other brand and product names appearing in this document are registered trademarks or trademarks of their respective holders. Advanced Analogic Technologies, Inc. 830 E. Arques Avenue, Sunnyvale, CA 94085 Phone (408) 737- 4600 Fax (408) 737- 4611 1235.2007.02.1.1 21