LTC3560ES6#TRPBF

LTC3560 2.25MHz, 800mA Synchronous Step-Down Regulator in ThinSOT DESCRIPTION FEATURES n n n n n n n n n n n n n n High Efficiency: Up to 95% Low Output Ripple (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. where TA is the ambient temperature. PC Board Layout Checklist As an example, consider the LTC3560 in dropout at an input voltage of 2.7V, a load current of 800mA and an ambient temperature of 70°C. From the typical performance graph of switch resistance, the RDS(ON) of the P-channel switch at 70°C is approximately 0.31Ω. Therefore, power dissipated by the part is: When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC3560. These items are also illustrated graphically in Figures 4 and 5. Check the following in your layout: PD = ILOAD2 • RDS(ON) = 198mW For the SOT-23 package, the θJA is 250°C/ W. Thus, the junction temperature of the regulator is: TJ = 70°C + (0.198)(250) = 120°C 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 VFB pin connect directly to the feedback resistors? The resistive divider R1/R2 must be connected between the (+) plate of COUT and ground. which is below the maximum junction temperature of 125°C. 3. Does the (+) plate of CIN connect to VIN as closely as possible? This capacitor provides the AC current to the internal power MOSFETs. Note that at higher supply voltages, the junction temperature is lower due to reduced switch resistance (RDS(ON)). 4. Keep the (–) plates of CIN and COUT as close as possible. Checking Transient Response 5. Keep the switching node, SW, away from the sensitive VFB node. The regulator loop response can be checked by looking at the load transient response. Switching regulators take several cycles to respond to a step in load current. When a load step occurs, VOUT immediately shifts by an amount equal to (ΔILOAD • ESR), where ESR is the effective series resistance of COUT. ΔILOAD also begins to charge or discharge COUT, which generates a feedback error signal. The regulator loop then acts to return VOUT to its Design Example As a design example, assume the LTC3560 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 0.8A but most of the time it will be in 3560fb 11 LTC3560 APPLICATIONS INFORMATION 1 RUN SYNC/MODE 6 LTC3560 2 VFB GND – 5 COUT VOUT R2 3 + L1 VIN SW R1 4 CFWD CIN VIN 3560 F04 BOLD LINES INDICATE HIGH CURRENT PATHS Figure 4. LTC3560 Layout Diagram VIA TO GND R1 VOUT VIN VIA TO VIN VIA TO VOUT R2 PIN 1 L1 CFWD LTC3560 SW COUT CIN GND 3560 F05 Figure 5. LTC3560 Suggested Layout standby mode, requiring only 2mA. Efficiency at both low and high load currents is important. Output voltage is 2.5V. With this information we can calculate L using equation (1), L=  V  1 VOUT  1
OUT  VIN  ( f )( IL )  (3) Substituting VOUT = 2.5V, VIN = 4.2V, ΔIL = 320mA and f = 2.25MHz in equation (3) gives: L= 2.5V  2.5V  1
 1.4μH 2.25MHz(320mA)  4.2V  CIN will require an RMS current rating of at least 0.4A ≅ ILOAD(MAX)/2 at temperature and COUT will require an ESR of less than 0.1Ω. In most cases, a ceramic capacitor will satisfy this requirement. For the feedback resistors, choose R1 = 309k. R2 can then be calculated from equation (2) to be: V  R2 =  OUT
1 R1= 978.5k; use 976k  0.6  Figure 6 shows the complete circuit along with its efficiency curve. A 1.5μH inductor works well for this application. For best efficiency choose a 960mA or greater inductor with less than 0.2Ω series resistance. 3560fb 12 LTC3560 APPLICATIONS INFORMATION 100 90 VOUT = 2.5V VIN 2.7V TO 4.2V 4 CIN* 10μF CER VIN SW 3 10pF LTC3560 1 6 1.5μH** COUT* 10μF CER RUN SYNC/MODE VFB 5 976k GND 2 3560 F06a VOUT 2.5V EFFICIENCY (%) 80 Burst Mode OPERATION 70 PULSE SKIPPING 60 50 40 30 20 309k VIN = 3.6V VIN = 4.2V 10 0 0.1 * TDK C2012X5R0J106M **TDK VLF3010AT-1R5N1R2 1 10 100 OUTPUT CURRENT (mA) 1000 3560 F06b Figure 6a Figure 6b VOUT 200mV/DIV AC COUPLED VOUT 200mV/DIV AC COUPLED IL 1A/DIV IL 1A/DIV ILOAD 1A/DIV ILOAD 1A/DIV VIN = 3.6V 20μs/DIV VOUT = 2.5V ILOAD = 100mA TO 800mA Burst Mode OPERATION Figure 6c 3560 F06c 20μs/DIV VIN = 3.6V VOUT = 2.5V ILOAD = 100mA TO 800mA PULSE SKIPPING MODE 3560 F06d Figure 6d 3560fb 13 LTC3560 APPLICATIONS INFORMATION 100 90 4 CIN* 10μF CER 3MHz CLK VIN SW 1μH** 3 10pF LTC3560 1 6 COUT* 10μF CER RUN SYNC/MODE VFB 5 GND 2 3560 F07a VOUT 1.2V 301k EFFICIENCY (%) 80 VIN 2.7V TO 4.2V 70 60 50 40 30 301k 20 VIN = 2.7V VIN = 3.6V VIN = 4.2V 10 *TDK C2012X5R0J106M **MURATA LQH32CN1R0M33 0 1 10 100 LOAD CURRENT (mA) 1000 3560 F07b Figure 7a Figure 7b VOUT 100mV/DIV AC COUPLED VOUT 100mV/DIV AC COUPLED IL 500mA/DIV IL 500mA/DIV ILOAD 500mA/DIV ILOAD 500mA/DIV VIN = 3.6V 20μs/DIV VOUT = 1.2V ILOAD = 300A TO 800mA Figure 7c 3560 F07c 20μs/DIV VIN = 3.6V VOUT = 1.2V ILOAD = 0mA TO 500mA 3560 F07d Figure 7d 3560fb 14 LTC3560 PACKAGE DESCRIPTION S6 Package 6-Lead Plastic TSOT-23 (Reference LTC DWG # 05-08-1636) 0.62 MAX 2.90 BSC (NOTE 4) 0.95 REF 1.22 REF 3.85 MAX 2.62 REF 1.4 MIN 2.80 BSC 1.50 – 1.75 (NOTE 4) PIN ONE ID RECOMMENDED SOLDER PAD LAYOUT PER IPC CALCULATOR 0.30 – 0.45 6 PLCS (NOTE 3) 0.95 BSC 0.80 – 0.90 0.20 BSC 0.01 – 0.10 1.00 MAX DATUM ‘A’ 0.30 – 0.50 REF 0.09 – 0.20 (NOTE 3) 1.90 BSC S6 TSOT-23 0302 REV B NOTE: 1. DIMENSIONS ARE IN MILLIMETERS 2. DRAWING NOT TO SCALE 3. DIMENSIONS ARE INCLUSIVE OF PLATING 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 3560fb 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. 15 LTC3560 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LTC3405/LTC3405A 300mA (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converters 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 20μA, ISD =