LM27952

LM27952 White LED Adaptive 1.5X/1X Switched Capacitor Current Driver May 2005 LM27952 White LED Adaptive 1.5X/1X Switched Capacitor Current Driver General Description The LM27952 is a switched capacitor white-LED driver capable of driving up to 4 LEDs with 30mA through each LED. Its 4 tightly regulated current sinks ensure excellent LED current and brightness matching. LED drive current is programmed by an external sense resistor. The LM27952 operates over an input voltage range from 3.0V to 5.5V and requires only four low-cost ceramic capacitors. The LM27952 provides excellent efficiency without the use of an inductor by operating the charge pump in a gain of 3/2, or in a gain of 1. Maximum efficiency is achieved over the input voltage range by actively selecting the proper gain based on the LED forward voltage requirements. The LM27952 uses constant frequency pre-regulation to minimize conducted noise on the input. It has a fixed 750kHz switching frequency optimized for portable applications. The LM27952 consumes less than 1µA of supply current when shut down. The LM27952 is available in a 14-pin No-Pullback Leadless Leadframe Package: LLP-14. Features n n n n n n n n n n Drives up to 4 LEDs with up to 30mA each Regulated current sources with 0.2%(typ.) matching 3/2x, 1x Gain transition based on LED VF Peak Efficiency Over 85% Input Voltage Range: 3.0V to 5.5V PWM Brightness Control Very Small Solution Size - NO INDUCTOR Fixed 750kHz Switching Frequency < 1µA Shutdown Current 14-pin LLP Package: 4.0mm X 3.0mm X 0.8mm Applications n White LED Display Backlights n White LED Keypad Backlights n General Purpose LED Lighting Typical Application Circuit 20148001 © 2005 National Semiconductor Corporation DS201480 www.national.com LM27952 Connection Diagram LM27952 14-pin No-Pullback Leadless Leadframe Package (LLP-14) 4mm x 3mm x 0.8mm NS Package Number SDA14A 20148002 Pin Description Pin 1 2 3 4 5 6 7 8 9 10 Name C2+ VOUT C1+ D4 D3 D2 D1 ISET EN PWM Flying Capacitor C2 Connection Pre-Regulated Charge Pump Output Flying Capacitor C1 Connection Regulated Current Sink Input. Regulated Current Sink Input. Regulated Current Sink Input. Regulated Current Sink Input. Current Set Input. Placing a resistor (RSET) between this pin and GND sets the LED current for all the LEDs. LED Current = 200 x (1.25V ÷ RSET). Enable Logic Input Pin. Logic Low = Shut Down, Logic High = Enabled. There is a 150kΩ (typ.) resistor connected internally between the EN pin and GND. Current Sink Modulation Logic Input Pin. Logic Low = Off, Logic High = On. Applying a Pulse Width Modulated (PWM) signal to this pin allows the regulated current sinks to be modulated without shutting down the internal Charge Pump and the VOUT node. Input Supply Range: 3.0V to 5.5V. Flying Capacitor C2 Connection. Power Supply Ground Connection. Flying Capacitor C1 Connection. Description 11 12 13 14 VIN C2GND C1- Ordering Information Order Number LM27952SD LM27952SDX Package Description No-Pullback LLP-14 Package Marking XXXXX YYYYY = D005B Supplied as Tape and Reel (Units) 1000 4500 www.national.com 2 LM27952 Absolute Maximum Ratings (Notes 1, 2) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VIN EN, PWM Continuous Power Dissipation (Note 3) Junction Temperature (TJ-MAX-ABS) Storage Temperature Range Lead Temp. (Soldering, 5 sec.) ESD Rating (Note 4) Human Body Model -0.3V to 6.0V -0.3V to (VIN + 0.3V) w/ 6.0V max Internally Limited 150˚C -65˚C to 150˚C 260˚C 2kV (Notes 2, 7) Operating Ratings (Notes 2, 7) Input Voltage VIN LED Voltage Range Junction Temperature Range (TJ) Ambient Temperature Range (TA) (Note 5) 3.0V to 5.5V 2.5V to 3.9V -40˚C to +115˚C -40˚C to +85 ˚C Thermal Information Junction-to-Ambient Thermal Resistance, LLP-14 Package (θJA) (Note 6) 45˚C/W Electrical Characteristics Limits in standard typeface are for TA = 25˚C, and limits in boldface type apply over the full operating junction temperature range (-40˚C to +85 ˚C). Unless otherwise noted, specifications apply to the LM27952 Typical Application Circuit (pg.1) with VIN = 3.6V, V(EN) = 1.8V, V(PWM) = 1.8V, 4 LEDs, VDX = 0.45V, CIN = COUT = 3.3µF, C1 = C2 = 1µF, RSET = 12.5kΩ (Note 8) Symbol IDX Parameter LED Current Regulation Conditions 3.0V ≤ VIN ≤ 5.5V RSET = 12.5kΩ IVOUT = 0mA 3.0V ≤ VIN ≤ 5.5V RSET = 8.32kΩ IVOUT = 0mA 3.0V ≤ VIN ≤ 5.5V RSET = 24.9kΩ IVOUT = 0mA ID-MATCH IQ ISD VSET IDX / ISET VHR LED Current Matching (Note 9) Quiescent Supply Current Shutdown Supply Current ISET Pin Voltage Output Current to Current Set Ratio Current Sink Voltage Headroom Requirement (Note 10) IDX = 95% IDX (nom.) RSET = 8.32kΩ (IDX nom. = 31mA) IDX = 95% IDX (nom.) RSET = 12.5kΩ (IDX nom. = 21mA) fSW VIH VIL Switching Frequency Logic Input High Logic Input Low Input Pins: EN, PWM 3.0V ≤ VIN ≤ 5.5V Input Pins: EN, PWM 3.0V ≤ VIN ≤ 5.5V 525 (-30%) 1.0 0 RSET = 8.32kΩ D(1-4) = OPEN RSET = OPEN 3.0V ≤ VIN ≤ 5.5V V(EN) = 0V 3.0V ≤ VIN ≤ 5.5V Min 19.32 (−8%) Typ 21 Max 22.68 (+8%) Units mA 31 11 0.2 1.3 0.1 1.25 200 360 1 1.7 1 % mA µA V mV 240 750 975 (+30%) VIN 0.4 kHz V 3 www.national.com LM27952 Electrical Characteristics (Notes 2, 7) (Continued) Limits in standard typeface are for TA = 25˚C, and limits in boldface type apply over the full operating junction temperature range (-40˚C to +85 ˚C). Unless otherwise noted, specifications apply to the LM27952 Typical Application Circuit (pg.1) with VIN = 3.6V, V(EN) = 1.8V, V(PWM) = 1.8V, 4 LEDs, VDX = 0.45V, CIN = COUT = 3.3µF, C1 = C2 = 1µF, RSET = 12.5kΩ (Note 8) Symbol IIH Parameter Logic Input High Current Input Pin: PWM V(PWM) = 1.8V Input Pin: EN V(EN) = 1.8V (Note 11) IIL ROUT VGDX tON Logic Input Low Current Charge Pump Output Resistance (Note 12) 1x to 3/2x Gain Transition Voltage Threshold on VDX Startup Time VDX Falling IDX = 90% steady state Input Pins: EN, PWM V(EN, PWM) = 0V Conditions Min Typ 10 12 10 3.3 450 330 Max Units nA µA nA Ω mV µs Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the component may occur. Operating Ratings are conditions under which operation of the device is guaranteed. Operating Ratings do not imply guaranteed performance limits. For guaranteed performance limits and associated test conditions, see the Electrical Characteristics tables. Note 2: All voltages are with respect to the potential at the GND pin. Note 3: Internal thermal shutdown circuitry protects the device from permanent damage. Thermal shutdown engages at TJ=150˚C (typ.) and disengages at TJ = 140˚C (typ.). Note 4: The Human-body model is a 100 pF capacitor discharged through a 1.5kΩ resistor into each pin. Note 5: In applications where high power dissipation and/or poor package thermal resistance is present, the maximum ambient temperature may have to be derated. Maximum ambient temperature (TA-MAX) is dependent on the maximum operation junction temperature (TJ-MAX-OP = 115oC), the maximum power dissipation of the device in the application (PD-MAX), and the junction-to ambient thermal resistance of the part/package in the application (θJA), as given by the following equation: TA-MAX = TJ-MAX-OP - (θJA x PD-MAX). Note 6: Junction-to-ambient thermal resistance (θJA) is taken from a thermal modeling result, performed under the conditions and guidelines set forth in the JEDEC standard JESD51-7. The test board is a 4 layer FR-4 board measuring 102mm x 76mm x 1.6mm with a 2 x 1 array of thermal vias. The ground plane on the board is 50mm x 50mm. Thickness of copper layers are 36µm/18µm /18µm/36µm (1.5oz/1oz/1oz/1.5oz). Ambient temperature in simulation is 22˚C, still air. Power dissipation is 1W. The value of θJA of the LM27952 in LLP-14 could fall in a range as wide as 45oC/W to 150oC/W (if not wider), depending on PWB material, layout, and environmental conditions. In applications where high maximum power dissipation exists (high VIN, high IOUT), special care must be paid to thermal dissipation issues. For more information on these topics, please refer to Application Note 1187: Leadless Leadframe Package (LLP) and the Power Efficiency and Power Dissipation section of this datasheet.. Note 7: Min and Max limits are guaranteed by design, test, or statistical analysis. Typical numbers are not guaranteed, but do represent the most likely norm. Note 8: CIN, COUT, C1, C2: Low-ESR Surface-Mount Ceramic Capacitors (MLCCs) used in setting electrical characteristics Note 9: LED Current Matching is based on two calculations: [(IMAX - IAVG) ÷ IAVG] and [(IAVG - IMIN) ÷ IAVG]. IMAX and IMIN are the highest and lowest respective Dx currents, and IAVG is the average Dx current of all four current sinks. The largest number of the two calculations (worst case) is considered the matching figure for the part. The typical specification provided is the most likely norm of the matching figure for all parts. Note 10: Headroom Voltage = VDX to GND. If headroom voltage requirement is not met, LED current regulation will be compromised. Note 11: EN Logic Input High Current (IIH) is due to a 150kΩ (typ.) pull-down resistor connected internally between the EN and GND pins. Note 12: The open loop output resistance (ROUT) models all voltage losses in the charge pump. ROUT can be used to estimate the voltage at the charge pump output VOUT and the maximum current capability of the device under low VIN and high IOUT conditions, beyond what is specified in the electrical specifications table: VOUT = (G x VIN) - (ROUT x IOUT). In the equation, G is the charge pump gain mode, and IOUT is the total output current (sum of all active Dx current sinks and all current drawn from VOUT). Note 13: Turn-on time is measured from when the EN signal is pulled high until the output voltage on VOUT crosses 90% of its final value. www.national.com 4 LM27952 Application Information CIRCUIT DESCRIPTION The LM27952 is an adaptive 1.5x/1x CMOS charge pump, optimized for driving white LEDs used in backlighting smallformat displays. It provides four constant current inputs capable of sinking up to 30mA through each LED. The wellmatched current sinks ensure the current through all the LEDs are virtually identical, providing a uniform brightness across the entire display. Each LED is driven from VOUT and connected to one of the four current sinks. LED drive current is programmed by connecting a resistor, RSET, to the current set pin, ISET. LED brightness is adjusted by applying a Pulse Width Modulated (PWM) signal to the dedicated PWM input pin. CHARGE PUMP The input to the 1.5x/1x charge pump is connected to the VIN pin, and the loosely regulated output of the charge pump is connected to the VOUT pin. The recommended input voltage range of the LM27952 is 3.0V to 5.5V. The device’s looselyregulated charge pump has both open loop and closed loop modes of operation. When the device is in open loop, the voltage at VOUT is equal to the gain times the voltage at the input. When the device is in closed loop, the voltage at VOUT is loosely regulated to 4.5V (typ.). The charge pump gain transitions are actively selected to maintain regulation based on LED forward voltage and load requirements. This allows the charge pump to stay in the most efficient gain (1x) over as much of the input voltage range as possible, reducing the power consumed from the battery. SOFT START The LM27952 contains internal soft-start circuitry to limit input inrush currents when the part is enabled. Soft start is implemented internally with a controlled turn-on of the internal voltage reference. Due to the soft-start circuitry, startup time of the LM27952 is approximately 330µs (typ.). ENABLE AND PWM PINS The LM27952 has 2 logic control pins. Both pins are activehigh logic (HIGH = ON). There is an internal pull-down resistor (150kΩ typ.) connected between the enable pin (EN) and GND. There is no pull-up or pull-down connected to the Pulse Width Modulated (PWM) pin. The EN pin is the master enable pin for the part. When the voltage on this pin is low ( < 0.4V), the part is in shutdown mode. In this mode, all internal circuitry is OFF and the part consumes very little supply current ( < 1µA typ.). When the voltage on the EN pin is high ( > 1.0V), the part will activate the charge pump and regulate the output voltage to its nominal value. The PWM pin serves as a dedicated logic input for LED brightness control. When the voltage on this pin is low ( < 0.4V), the current sinks will be turned off and no current will flow through the LEDs. When the voltage on this pin is high ( > 1.0V), the currents sinks will turn on and regulate to the current level set by the resistor connected to the ISET pin. SETTING LED CURRENTS The current through the four LEDs connected to D1-4 can be set to a desired level simply by connecting an appropriately sized resistor (RSET) between the ISET pin of the LM27952 and GND. The LED currents are proportional to the current that flows out of the ISET pin and are a factor of 200 times 5 greater than the ISET current. The feedback loop of an internal amplifier sets the voltage of the ISET pin to 1.25V (typ.). The statements above are simplified in the equations below: IDx = 200 x(VSET / RSET) RSET = 200 x (1.25V / IDx) ADJUSTING LED BRIGHTNESS (PWM control) Perceived LED brightness can be adjusted using a PWM control signal on the LM27952 PWM logic input pin, turning the current sources ON and OFF at a rate faster than perceptible by the eye. When this is done, the total brightness perceived is proportional to the duty cycle (D) of the PWM signal (D = the percentage of time that the LED is on in every PWM cycle). A simple example: if the LEDs are driven at 15mA each with a PWM signal that has a 50% duty cycle, perceived LED brightness will be about half as bright as compared to when the LEDs are driven continuously with 15mA. The minimum recommended PWM frequency is 100Hz. Frequencies below this may be visibly noticeable as flicker or blinking. The maximum recommended PWM frequency is 1kHz. Frequencies above this may cause interference with internal current driver circuitry and/or noise in the audible range. Due to the regulation control loop, the maximum frequency and minimum duty cycle applied to the PWM pin should be chosen such that the minimum ON time is no less than 30µs in duration. If a PWM signal is applied to the EN pin instead, the maximum frequency and minimum duty cycle should be chosen to accommodate both the LM27952 startup time (330µs typ.) and the 30µs control loop delay. The preferred method to adjust brightness is to keep the master EN voltage ON continuously and apply a PWM signal to the dedicated PWM input pin. The benefit of this type of connection can be best understood with a contrary example. When a PWM signal is connected to the master enable (EN) pin, the charge pump repeatedly turns on and off. Every time the charge pump turns on, there is an inrush of current as the capacitances, both internal and external, are recharged. This inrush current results in a current spike and a voltage dip at the input of the part. By only applying the PWM signal to PWM logic input pin, the charge pump continuously stays on, resulting in much lower input noise. In cases where a PWM signal must be connected to the EN pin, measures can be taken to reduce the magnitude of the charge-pump turn-on transient response. More input capacitance, series resistors and/or ferrite beads may provide benefits. If the current spikes and voltage dips can be tolerated, connecting the PWM signal to the EN pin does provide a benefit of lower supply current consumption. When the PWM signal to the EN pin is low, the LM27952 will be shutdown and input current will only be a few micro-amps. This results in a lower time-averaged input current than the prior suggestion, where EN is kept on continuously. MAXIMUM OUTPUT CURRENT, MAXIMUM LED VOLTAGE, MINIMUM INPUT VOLTAGE The LM27952 can drive 4 LEDs at 30mA each from an input voltage as low as 3.0V, so long as the LEDs have a forward voltage of 3.5V or less (room temperature). The statement above is a simple example of the LED drive capabilities of the LM27952. The statement contains key application parameters required to validate an LED-drive design using the LM27952: LED current (ILED), number of active LEDs (N), LED forward voltage (VLED), and minimum input voltage (VIN-MIN). www.national.com LM27952 Application Information (Continued) The equation below can be used to estimate the total output current capability of the LM27952: ILED_MAX = ((1.5 x VIN) - VLED) / ((N x ROUT) + kHR) (eq. 1) ILED_MAX = ((1.5 x VIN ) - VLED) / ((N x 3.3Ω) + 12mV/mA) ROUT – Output resistance. This parameter models the internal losses of the charge pump that result in voltage droop at the pump output VOUT. Since the magnitude of the voltage droop is proportional to the total output current of the charge pump, the loss parameter is modeled as a resistance. The output resistance of the LM27952 is typically 3.3Ω (VIN = 3.0V, TA = 25˚C). In equation form: (eq. 2) VVOUT = 1.5 x VIN – N x ILED x ROUT kHR – Headroom constant. This parameter models the minimum voltage required across the current sinks for proper regulation. This minimum voltage is proportional to the programmed LED current, so the constant has units of mV/mA. The typical kHR of the LM27952 is 12mV/mA. In equation form: (eq. 3) (VVOUT – VLED) > kHR x ILED The "ILED-MAX" equation (eq. 1) is obtained from combining the ROUT equation (eq. 2) with the kHR equation (eq. 3) and solving for ILED. Maximum LED current is highly dependent on minimum input voltage and LED forward voltage. Output current capability can be increased by raising the minimum input voltage of the application, or by selecting LEDs with a lower forward voltage. Excessive power dissipation may also limit output current capability of an application. CAPACITOR SELECTION The LM27952 requires 4 external capacitors for proper operation. Surface-mount multi-layer ceramic capacitors are recommended. These capacitors are small, inexpensive and have very low equivalent series resistance (ESR < 20mΩ typ.). Tantalum capacitors, OS-CON capacitors, and aluminum electrolytic capacitors are not recommended for use with the LM27952 due to their high ESR, as compared to ceramic capacitors. For most applications, ceramic capacitors with X7R or X5R temperature characteristic are preferred for use with the LM27952. These capacitors have tight capacitance tolerance (as good as ± 10%) and hold their value over temperature (X7R: ± 15% over -55˚C to 125˚C; X5R: ± 15% over -55˚C to 85˚C). Capacitors with Y5V or Z5U temperature characteristic are generally not recommended for use with the LM27952. Capacitors with these temperature characteristics typically have wide capacitance tolerance (+80%, -20%) and vary significantly over temperature (Y5V: +22%, -82% over -30˚C to +85˚C range; Z5U: +22%, -56% over +10˚C to +85˚C range). Under some conditions, a nominal 1µF Y5V or Z5U capacitor could have a capacitance of only 0.1µF. Such detrimental deviation is likely to cause Y5V and Z5U capacitors to fail to meet the minimum capacitance requirements of the LM27952. The voltage rating of the output capacitor should be 10V or more. All other capacitors should have a voltage rating at or above the maximum input voltage of the application. PARALLEL DX OUTPUTS FOR INCREASED CURRENT DRIVE Outputs D1-4 may be connected together to drive a one or two LEDs at higher currents. In such a configuration, all four parallel current sinks of equal value drive the single LED. www.national.com 6 The LED current programmed should be chosen so that the current through each of the outputs is programmed to 25% of the total desired LED current. For example, if 60mA is the desired drive current for the single LED, RSET should be selected such that the current through each of the current sink inputs is 15mA. Similarly, if two LEDs are to be driven by pairing up the D1-4 inputs (i.e D1-2, D3-4), RSET should be selected such that the current through each current sink input is 50% of the desired LED current. Connecting the outputs in parallel does not affect internal operation of the LM27952 and has no impact on the Electrical Characteristics and limits previously presented. The available diode output current, maximum diode voltage, and all other specifications provided in the Electrical Characteristics table apply to this parallel output configuration, just as they do to the standard 4-LED application circuit. POWER EFFICIENCY Efficiency of LED drivers is commonly taken to be the ratio of power consumed by the LEDs (PLED) to the power drawn at the input of the part (PIN). With a 1.5x/1x charge pump, the input current is equal to the charge pump gain times the output current (total LED current). For a simple approximation, the current consumed by internal circuitry can be neglected and the efficiency of the LM27952 can be predicted as follows: PLED = N x VLED x ILED PIN = VIN x IIN PIN = VIN x (Gain x N x ILED + IQ) E = (PLED ÷ PIN) Neglecting IQ will result in a slightly higher efficiency prediction, but this impact will be no more than a few percentage points when several LEDs are driven at full power. It is also worth noting that efficiency as defined here is in part dependent on LED voltage. Variation in LED voltage does not affect power consumed by the circuit and typically does not relate to the brightness of the LED. For an advanced analysis, it is recommended that power consumed by the circuit (VIN x IIN) be evaluated rather than power efficiency. THERMAL PROTECTION Internal thermal protection circuitry disables the LM27952 when the junction temperature exceeds 150˚C (typ.). This feature protects the device from being damaged by high die temperatures that might otherwise result from excessive power dissipation. The device will recover and operate normally when the junction temperature falls below 140˚C (typ.). It is important that the board layout provide good thermal conduction to keep the junction temperature within the specified operating ratings. POWER DISSIPATION The power dissipation (PDISSIPATION) and junction temperature (TJ) can be approximated with the equations below. PIN is the power generated by the 1.5x/1x charge pump, PLED is the power consumed by the LEDs, TAis the ambient temperature, and θJA is the junction-to-ambient thermal resistance for the LLP-14 package. VIN is the input voltage to the LM27952, VLED is the nominal LED forward voltage, and ILED is the programmed LED current. PDISSIPATION = PIN - PLED = [Gain x VIN x (4 x ILED)] − (VLED x 4 x ILED) TJ = TA + (PDISSIPATION x θJA) LM27952 Application Information (Continued) The junction temperature rating takes precedence over the ambient temperature rating. The LM27952 may be operated outside the ambient temperature rating, so long as the junction temperature of the device does not exceed the maximum operating rating of 115˚C. The maximum ambient temperature rating must be derated in applications where high power dissipation and/or poor thermal resistance causes the junction temperature to exceed 115˚C. exposed DAP (die attach pad) at the center of the package measuring 3.0mm x 1.6mm. The main advantage of this exposed DAP is to offer lower thermal resistance when it is soldered to the thermal land on the PCB. For PCB layout, National highly recommends a 1:1 ratio between the package and the PCB thermal land. To further enhance thermal conductivity, the PCB thermal land may include vias to a ground plane. For more detailed instructions on mounting LLP packages, please refer to National Semiconductor Application Note AN-1187. PCB Layout Considerations The LLP is a leadframe based Chip Scale Package (CSP) with very good thermal properties. This package has an 7 www.national.com LM27952 White LED Adaptive 1.5X/1X Switched Capacitor Current Driver Physical Dimensions inches (millimeters) unless otherwise noted 14-Pin LLP NS Package Number SDA14A National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications. For the most current product information visit us at www.national.com. LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. 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