LTC3560 2.25MHz, 800mA Synchronous Step-Down Regulator in ThinSOT
FEATURES
■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■
DESCRIPTIO
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. PC Board Layout Checklist 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: 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. 3. Does the (+) plate of CIN connect to VIN as closely as possible? This capacitor provides the AC current to the internal power MOSFETs. 4. Keep the (–) plates of CIN and COUT as close as possible. 5. Keep the switching node, SW, away from the sensitive VFB node. 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
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LTC3560
APPLICATIO S I FOR ATIO
–
VOUT COUT
+
L1
BOLD LINES INDICATE HIGH CURRENT PATHS
Figure 4. LTC3560 Layout Diagram
VOUT
PIN 1 L1 LTC3560 SW
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 ⎟ ( f)(∆IL ) ⎝ VIN ⎠
Substituting VOUT = 2.5V, VIN = 4.2V, ∆IL = 320mA and f = 2.25MHz in equation (3) gives:
L= 2.5V ⎛ 2.5V ⎞ 1− ≅ 1.4µH ⎝ ⎠ 2.25MHz(320mA) ⎜ 4.2V ⎟
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.
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1 RUN SYNC/MODE LTC3560 2 GND VFB VIN CIN VIN 5 R2 3 SW 4 CFWD R1 6
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VIA TO GND
R1 VIA TO VIN R2 CFWD VIN VIA TO VOUT
COUT GND
CIN
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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.
(3)
LTC3560
APPLICATIO S I FOR ATIO
VIN 2.7V TO 4.2V
4 CIN* 10µF CER
VIN LTC3560 RUN
SW
3
1.5µH** 10pF
EFFICIENCY (%)
1 6
SYNC/MODE VFB GND 2
5 976k 309k
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* TDK C2012X5R0J106M **TDK VLF3010AT-1R5N1R2
Figure 6a
VOUT 200mV/DIV AC COUPLED IL 1A/DIV ILOAD 1A/DIV VIN = 3.6V 20µs/DIV VOUT = 2.5V ILOAD = 100mA TO 800mA Burst Mode OPERATION
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Figure 6c
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100 90 80 70
VOUT 2.5V COUT* 10µF CER
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VOUT = 2.5V Burst Mode OPERATION PULSE SKIPPING
60 50 40 30 20 10 0 0.1
VIN = 3.6V VIN = 4.2V 1 10 100 OUTPUT CURRENT (mA) 1000
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Figure 6b
VOUT 200mV/DIV AC COUPLED IL 1A/DIV ILOAD 1A/DIV 20µs/DIV VIN = 3.6V VOUT = 2.5V ILOAD = 100mA TO 800mA PULSE SKIPPING MODE
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Figure 6d
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LTC3560
APPLICATIO S I FOR ATIO
EFFICIENCY (%)
VIN 2.7V TO 4.2V
4 CIN* 10µF CER
VIN LTC3560 RUN
SW
3
1µH** 10pF
1 6
3MHz CLK
SYNC/MODE VFB GND 2
5 301k 301k
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*TDK C2012X5R0J106M **MURATA LQH32CN1R0M33
Figure 7a
VOUT 100mV/DIV AC COUPLED IL 500mA/DIV ILOAD 500mA/DIV
VIN = 3.6V 20µs/DIV VOUT = 1.2V ILOAD = 300A TO 800mA
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Figure 7c
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100 90 80 70 60 50 40 30 20 10 0 1 10 100 LOAD CURRENT (mA) VIN = 2.7V VIN = 3.6V VIN = 4.2V 1000
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VOUT 1.2V COUT* 10µF CER
Figure 7b
VOUT 100mV/DIV AC COUPLED IL 500mA/DIV
ILOAD 500mA/DIV 20µs/DIV VIN = 3.6V VOUT = 1.2V ILOAD = 0mA TO 500mA
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Figure 7d
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LTC3560
PACKAGE DESCRIPTIO
0.62 MAX
0.95 REF
3.85 MAX 2.62 REF
RECOMMENDED SOLDER PAD LAYOUT PER IPC CALCULATOR
0.20 BSC 1.00 MAX DATUM ‘A’
0.30 – 0.50 REF
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
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.
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S6 Package 6-Lead Plastic TSOT-23
(Reference LTC DWG # 05-08-1636)
2.90 BSC (NOTE 4) 1.22 REF 1.4 MIN 2.80 BSC 1.50 – 1.75 (NOTE 4) PIN ONE ID 0.95 BSC 0.30 – 0.45 6 PLCS (NOTE 3) 0.80 – 0.90 0.01 – 0.10 0.09 – 0.20 (NOTE 3) 1.90 BSC
S6 TSOT-23 0302
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LTC3560 RELATED PARTS
PART NUMBER LTC3405/LTC3405A LTC3406/LTC3406B LTC3407/LTC3407-2 LTC3409 LTC3410/LTC3410B LTC3411 LTC3412 LTC3441/LTC3442 LTC3443 LTC3531/LTC3531-3 LTC3531-3.3 LTC3532 LTC3548/LTC3548-1 LTC3548-2 LTC3561 DESCRIPTION 300mA (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converters 600mA (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converters Dual 600mA/800mA (IOUT), 1.5MHz/2.25MHz, Synchronous Step-Down DC/DC Converters 600mA (IOUT), 1.7MHz/2.6MHz, Synchronous Step-Down DC/DC Converter 300mA (IOUT), 2.25MHz, Synchronous Step-Down DC/DC Converters 1.25A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 2.5A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 1.2A (IOUT), 2MHz, Synchronous Buck-Boost DC/DC Converters 200mA (IOUT), 1.5MHz, Synchronous Buck-Boost DC/DC Converters 500mA (IOUT), 2MHz, Synchronous Buck-Boost DC/DC Converter Dual 400mA/800mA (IOUT), 2.25MHz, Synchronous Step-Down DC/DC Converters 1.25A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter COMMENTS 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 20µA, ISD =