LTC3452

LTC3452 Synchronous Buck-Boost MAIN/CAMERA White LED Driver FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO High Efficiency: ≥85% Over Entire Li-Ion Battery Range Wide VIN Range: 2.7V to 5.5V Independent MAIN/CAMERA Current Control Up to 425mA Continuous Output Current Internal Soft-Start Open/Shorted LED Protection PWM Brightness Control LED Current Matching Typically 1.2V). For ENL voltage VIN(MIN)2 • ( VOUT – VIN(MIN) ) • 100% f • IOUT(MAX ) • %Ripple • VOUT 2 and in buck mode is: L> where: VOUT • VIN(MAX ) – VOUT • 100% f • IOUT(MAX ) • %Ripple • VIN(MAX ) ( ) f = operating frequency, Hz %Ripple = allowable inductor current ripple, % VIN(MIN) = minimum input voltage, V VIN(MAX) = maximum input voltage, V VOUT = output voltage, V IOUT(MAX) = maximum output load current For high efficiency, choose an inductor with a high frequency core material, such as ferrite, to reduce core loses. The inductor should have low ESR (equivalent series resistance) to reduce the I2R losses, and must be able to handle the peak inductor current without saturating. Molded chokes or chip inductors usually do not have enough core to support peak inductor currents >1A. To minimize radiated noise, use a toroid, pot core or shielded bobbin inductor. For the white LED application, a 4.7µH inductor value is recommended. See Table 2 for a list of component suppliers. Table 2. Inductor Vendor Information SUPPLIER Coilcraft Cooper/Coiltronics Murata Sumida Vishay-Dale WEB SITE www.coilcraft.com www.cooperet.com www.murata.com www.japanlink.com/sumida www.vishay.com U Input Capacitor Selection Since the VIN pin is the supply voltage for the IC it is recommended to place at least a 2.2µF, low ESR bypass capacitor to ground. See Table 3 for a list of component suppliers. Table 3. Capacitor Vendor Information SUPPLIER AVX Sanyo Taiyo Yuden TDK WEB SITE www.avxcorp.com www.sanyovideo.com www.t-yuden.com www.component.tdk.com W UU Output Capacitor Selection The bulk value of the capacitor is set to reduce the ripple due to charge into the capacitor each cycle. The steady state ripple due to charge is given by: IOUT(MAX ) • VOUT – VIN(MIN) • 100 COUT • VOUT 2 • f %Ripple _ Boost = ( ) % %Ripple _ Buck = 8 • VIN(MAX ) • f 2 • L • COUT ( VIN(MAX) – VOUT ) • 100 % where COUT = output filter capacitor, F The output capacitance is usually many times larger in order to handle the transient response of the converter. For a rule of thumb, the ratio of the operating frequency to the unity-gain bandwidth of the converter is the amount the output capacitance will have to increase from the above calculations in order to maintain the desired transient response. The other component of ripple is due to the ESR (equivalent series resistance) of the output capacitor. Low ESR capacitors should be used to minimize output voltage ripple. For surface mount applications, Taiyo Yuden, TDK, AVX ceramic capacitors, AVX TPS series tantalum capacitors or Sanyo POSCAP are recommended. For the white LED application, a 4.7µF capacitor value is recommended. See Table 3 for a list of component suppliers. 3452f 11 LTC3452 APPLICATIO S I FOR ATIO Optional Schottky Diodes Schottky diodes across the synchronous switches B and D are not required, but provide a lower drop during the break-before-make time (typically 20ns) of the NMOS to PMOS transition, improving efficiency. Use a Schottky diode such as an MBRM120T3 or equivalent. Do not use ordinary rectifier diodes, since the slow recovery times will compromise efficiency. Closing the Feedback Loop The LTC3452 incorporates voltage mode PWM control. The control to output gain varies with operation region (Buck, Boost, Buck/Boost), but is usually no greater than 15. The output filter exhibits a double pole response given by: fFILTER _ POLE = 1 Hz 2 • π • L • COUT where COUT is the output filter capacitor. The output filter zero is given by: fFILTER _ ZERO = 1 2 • π • RESR • COUT Hz where RESR is the capacitor equivalent series resistance. A troublesome feature in Boost mode is the right-half plane zero (RHP), and is given by: fRHPZ VIN = Hz 2 • π • IOUT • L • VOUT 2 The loop gain is typically rolled off before the RHP zero frequency. A simple Type I compensation network can be incorporated to stabilize the loop but at a cost of reduced bandwidth and slower transient response. To ensure proper phase margin, the loop is required to be crossed over a decade before the LC double pole. 12 U The unity-gain frequency of the error amplifier with the Type I compensation is given by: fUG = gm 2 • π • CVC W UU where gm is the error amp transconductance (typically 1/5.2k) and CVC is the external capacitor to GND at the VC pin. For the white LED application, a 0.1µF or greater capacitor value is recommended. Paralleling LED Outputs for Higher Current Two or more LED output pins can be connected together in parallel to achieve higher output current in fewer than 7 LEDs. For a very high power LED such as a LumiLED, all 7 outputs can be connected in parallel for maximum total output current, as shown in the back page application of this data sheet. Maximum LED Current As described in the Operation section, the maximum output LED currents are equal to: ⎛ 0.8 V ⎞ IMAXL = 256⎜ ⎟ ⎝ RISETL ⎠ and ⎛ 0.8 V ⎞ IMAXH = 768⎜ ⎟ ⎝ RISETH ⎠ Since the maximum LED current for the low power bank is 25mA, this sets a minimum limit on RISETL of: ⎛ 0.8 V ⎞ RMINL = 256 ⎜ = 8192Ω ⎝ 25mA ⎟ ⎠ Similarly, for the high power bank: ⎛ 0.8 V ⎞ = 4096Ω RMINH = 768 ⎜ ⎠ ⎝ 150mA ⎟ In addition, since the maximum continuous output current for the buck-boost is limited to 425mA, this may impose higher resistor value minimums if all outputs are used. 3452f LTC3452 APPLICATIO S I FOR ATIO Although the LTC3452 can safely provide this current continuously, the external LED(s) may not be rated for this high a level of continuous current. Higher current levels in a single LED are generally reserved for pulsed applications, such as LED camera flash. This is accomplished by programming a high current with one or both of the RISET resistors and pulsing the appropriate enable pin or pins as shown in the back page application. VIN VOUT ENL ISETL RSET ≥ RMINL VOLTAGE DAC VDAC LEDL1 LTC3452 LEDL5 0.8V – VDAC ILED = 256 RSET CURRENT DAC (3a) VIN VOUT VIN ENL ISETL RMINL RPOT LEDL1 LTC3452 LEDL5 0.8V ILED = 256 RMINL + RPOT RSET 100 (3c) Figure 3. Additional Brightness Control Methods: (3a) Using Voltage DAC, (3b) Using Current DAC, (3c) Using Potentiometer, (3d) Using PWM Input U Varying LED Brightness Linearly Continuously variable LED brightness control can be achieved by interfacing directly to one or both of the ISET pins. Figure 3 shows four such methods employing a voltage DAC, a current DAC, a simple potentiometer or a PWM input applied to the ISETL pin for controlling the low power bank LED currents. These four techniques can be similarly applied to the ISETH pin for controlling the high power bank LED currents. VIN VOUT ENL ISETL IDAC ≤ 0.8V RMINL LEDL1 LTC3452 LEDL5 ILED = 256 • IDAC W UU (3b) VOUT ENL ISETL RSET ≥ RMINL VPWM 1µF LEDL1 LTC3452 LEDL5 ILED = 256 = 256 DVCC fPWM ≥ 10kHz 0.8V – VPWM RSET 0.8V – (DC% • VDVCC) RSET (3d) 3452 F03 3452f 13 LTC3452 APPLICATIO S I FOR ATIO Unused Outputs If fewer than 7 LED pins are to be used, unused LEDx pins should be connected to VOUT. The LTC3452 senses which current source outputs are not being used and shuts off the corresponding output currents to save power. A small trickle current (10µA: low power bank, 30µA: high power bank) is still applied to unused outputs to detect if a white LED is later switched in and also to distinguish unused outputs from used outputs during start-up. LED Failure Modes If an individual LED fails as a short circuit, the current source biasing it is shut off to save power. This is the same operation as described previously (if the output were 14 U initially designated unused at power-up by connecting its LEDx pin to VOUT). Efficiency is not materially affected. If an individual LED fails as an open circuit, the control loop will initially attempt to regulate off of its current source feedback signal, since it will appear to be the one requiring the largest forward voltage drop to run at its programmed current. This will drive VOUT higher. As the open circuited LED will never accept its programmed current, VOUT must be voltage-limited by means of a secondary control loop. The LTC3452 limits VOUT to 4.5V in this failure mode. The other LEDs will still remain biased at the correct programmed current but the overall circuit efficiency will decrease. 3452f W UU LTC3452 PACKAGE DESCRIPTIO 4.50 ± 0.05 3.10 ± 0.05 2.45 ± 0.05 (4 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS BOTTOM VIEW—EXPOSED PAD 4.00 ± 0.10 (4 SIDES) PIN 1 TOP MARK (NOTE 6) 2.45 ± 0.10 (4-SIDES) 0.75 ± 0.05 R = 0.115 TYP 19 20 0.38 ± 0.10 1 2 PIN 1 NOTCH R = 0.30 TYP NOTE: 1. DRAWING IS PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WGGD-1)—TO BE APPROVED 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. U UF Package 20-Lead Plastic QFN (4mm × 4mm) (Reference LTC DWG # 05-08-1710) 0.70 ± 0.05 PACKAGE OUTLINE 0.25 ± 0.05 0.50 BSC (UF20) QFN 10-04 0.200 REF 0.00 – 0.05 0.25 ± 0.05 0.50 BSC 3452f 15 LTC3452 TYPICAL APPLICATIO VIN 3V TO 5.5V 4 × 20mA White LED Display + 2 × 150mA Camera Light Driver L 4.7µH 2.2µF VIN ENH ENH ISETH 4.02k LEDH2 LEDL1, 20mA VC 0.1µF ENL ENL ISETL 10.2k GND GND PGND EXPOSED PAD 3452 TA02a RELATED PARTS PART NUMBER LT1618 DESCRIPTION Constant Current, Constant Voltage 1.4MHz, High Efficiency Boost Regulator COMMENTS VIN: 1.6V to 18V, VOUT(MAX) = 34V, IQ = 1.8mA, ISD =