LTC3406-1.2

LTC3406-1.2 1.5MHz, 600mA Synchronous Step-Down Regulator in ThinSOT FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO High Efficiency: Up to 90% Very Low Quiescent Current: Only 20µA 600mA Output Current at VIN = 3V 2.5V to 5.5V Input Voltage Range 1.5MHz Constant Frequency Operation No Schottky Diode Required Shutdown Mode Draws < 1µA Supply Current Current Mode Operation for Excellent Line and Load Transient Response Overtemperature Protected Low Profile (1mm) ThinSOTTM Package The LTC ®3406-1.2 is a high efficiency monolithic synchronous buck regulator using a constant frequency, current mode architecture. Supply current during operation with only 20µA drops 1µF) supply bypass capacitors. The discharged bypass capacitors are effectively put in parallel with COUT, causing a rapid drop in VOUT. No regulator can deliver enough current to prevent this problem if the load switch resistance is low and it is driven quickly. The only solution is to limit the rise time of the switch drive so that the load rise time is limited to approximately (25 • CLOAD). Thus, a 10µF capacitor charging to 3.3V would require a 250µs rise time, limiting the charging current to about 130mA. PC Board Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC3406-1.2. These items are also illustrated graphically in Figures 3 and 4. Check the following in your layout: 1. The power traces, consisting of the GND trace, the SW trace and the VIN trace should be kept short, direct and wide. 2. Does the (+) plate of CIN connect to VIN as closely as possible? This capacitor provides the AC current to the internal power MOSFETs. 3. Keep the (–) plates of CIN and COUT as close as possible. 1 RUN LTC3406-1.2 5 2 – VOUT COUT GND VOUT SW VIN CIN + 3 L1 4 VIN 340612 F03 BOLD LINES INDICATE HIGH CURRENT PATHS Figure 3. LTC3406-1.2 Layout Diagram 10 U Design Example As a design example, assume the LTC3406-1.2 is used in a single lithium-ion battery-powered cellular phone application. The VIN will be operating from a maximum of 4.2V down to about 2.7V. The load current requirement is a maximum of 0.6A but most of the time it will be in standby mode, requiring only 2mA. Efficiency at both low and high load currents is important. With this information we can calculate L using equation (1), W UU L= ⎛ 1.2V ⎞ 1 1.2V⎜ 1 − ⎠ ( f)(∆IL ) ⎝ VIN ⎟ (3) Substituting VIN = 4.2V, ∆IL = 240mA and f = 1.5MHz in equation (3) gives: L= 1.2V ⎛ 1.2V ⎞ ⎜1 − ⎟ = 2.38 µH 1.5MHz(240mA) ⎝ 4.2V ⎠ A 2.2µH inductor works well for this application. For best efficiency choose a 720mA or greater inductor with less than 0.2Ω series resistance. CIN will require an RMS current rating of at least 0.3A ≅ ILOAD(MAX)/2 at temperature and COUT will require an ESR of less than 0.25Ω. In most cases, a ceramic capacitor will satisfy this requirement. VIA TO VOUT VIA TO VIN VIN PIN 1 VOUT L1 SW LTC3406-1.2 COUT GND CIN 340612 F04 Figure 4. LTC3406-1.2 Suggested Layout 340612f LTC3406-1.2 TYPICAL APPLICATIO S Single Li-Ion 1.2V/600mA Regulator for Lowest Profile, ≤1mm High VIN 2.7V TO 4.2V 4 CIN** 4.7µF CER VIN RUN VOUT GND 2 *MURATA GRM219R60JI06KE19B **AVX06036D475MAT † FDK MIPW3226D2R2M 5 340612 TA02 LTC3406-1.2 Efficiency 100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.1 1 10 LOAD (mA) VIN = 2.7V VIN = 3.6V VIN = 4.2V 100 1000 340612 TA03 PACKAGE DESCRIPTIO 0.62 MAX 3.85 MAX 2.62 REF RECOMMENDED SOLDER PAD LAYOUT PER IPC CALCULATOR 0.20 BSC DATUM ‘A’ 0.30 – 0.50 REF NOTE: 1. DIMENSIONS ARE IN MILLIMETERS 2. DRAWING NOT TO SCALE 3. DIMENSIONS ARE INCLUSIVE OF PLATING 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 U SW 3 2.2µH† COUT1* 10µF CER VOUT 1.2V LTC3406-1.2 1 Load Step VOUT 100mV/DIV AC COUPLED ILOAD 500mA/DIV IL 500mA/DIV 20µs/DIV VIN = 3.6V ILOAD = 20mA TO 600mA 340612 TA04 S5 Package 5-Lead Plastic TSOT-23 (Reference LTC DWG # 05-08-1635) 0.95 REF 2.90 BSC (NOTE 4) 1.22 REF 1.4 MIN 2.80 BSC 1.50 – 1.75 (NOTE 4) PIN ONE 0.30 – 0.45 TYP 5 PLCS (NOTE 3) 0.95 BSC 0.80 – 0.90 0.09 – 0.20 (NOTE 3) 1.00 MAX 0.01 – 0.10 1.90 BSC S5 TSOT-23 0302 4. DIMENSIONS ARE EXCLUSIVE OF MOLD FLASH AND METAL BURR 5. MOLD FLASH SHALL NOT EXCEED 0.254mm 6. JEDEC PACKAGE REFERENCE IS MO-193 340612f 11 LTC3406-1.2 RELATED PARTS PART NUMBER LT1616 LT1676 LTC1701/LT1701B LT1776 LTC1877 LTC1878 LTC1879 LTC3403 LTC3404 LTC3405/LTC3405A LTC3406 LTC3411 LTC3412 LTC3440 DESCRIPTION 500mA (IOUT), 1.4MHz, High Efficiency Step-Down DC/DC Converter 450mA (IOUT), 100kHz, High Efficiency Step-Down DC/DC Converter 750mA (IOUT), 1MHz, High Efficiency Step-Down DC/DC Converter 500mA (IOUT), 200kHz, High Efficiency Step-Down DC/DC Converter 600mA (IOUT), 550kHz, Synchronous Step-Down DC/DC Converter 600mA (IOUT), 550kHz, Synchronous Step-Down DC/DC Converter 1.2A (IOUT), 550kHz, Synchronous Step-Down DC/DC Converter 600mA (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converter with Bypass Transistor 600mA (IOUT), 1.4MHz, Synchronous Step-Down DC/DC Converter 300mA (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converter 600mA (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converter 1.25A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 2.5A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 600mA (IOUT), 2MHz, Synchronous Buck-Boost DC/DC Converter COMMENTS 90% Efficiency, VIN = 3.6V to 25V, VOUT = 1.25V, IQ = 1.9mA, ISD =