LTC3545/LTC3545-1 Triple 800mA Synchronous Step-Down Regulator–2.25MHz FEATURES
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DESCRIPTION
The LTC ®3545/LTC3545-1 are triple, high efficiency, monolithic synchronous buck regulators using a constant frequency, current mode architecture. The regulators operate independently with separate run pins. The 2.25V to 5.5V input voltage range makes the LTC3545/LTC3545-1 well suited for single Li-Ion battery-powered applications. Low ripple pulse skip mode or high efficiency Burst Mode operation is externally selectable. PWM pulse skip mode operation provides very low output ripple voltage while Burst Mode operation increases efficiency at low output loads. Switching frequency is internally set to 2.25MHz, or the switching frequency can be synchronized to an external 1MHz to 3MHz clock. Power good indicators easily allow power on sequencing between the three regulators. The internal synchronous switches increase efficiency and eliminate external Schottky diodes. Low output voltages are supported with the 0.6V feedback reference voltage. The LTC3545-1 replaces the SYNC/MODE function with a third PGOOD pin and forces Burst Mode operation.
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Three 800mA Outputs High Efficiency: Up to 95% 2.25V to 5.5V Input Voltage Range Low 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. Design Example As a design example, consider using the LTC3545/LTC35451 in a portable application with a Li-Ion battery. The battery provides VIN ranging from 2.8V to 4.2V. The demand on one channel at 2.5V is 600mA. Using this channel as an example, first calculate the inductor value for 40% ripple current (240mA in this example) at maximum VIN. Using a form of Equation 1: L1= 2.5V ⎛ 2.5V ⎞ 1– = 1.41µH ⎝ ⎠ (2.25MHz )(240mA ) ⎜ 3.6V ⎟
Use the closest standard value of 1.5μH. For low ripple applications, 10μF is a good choice for the output capacitor. A smaller output capacitor will shorten transient response settling time, but also increase the load transient ripple. A value for C5 = 4.7μF should suffice as the source impedance of a Li-Ion battery is very low. C5 and C1 both provide switching current to the output power switches. They should be placed as close a possible to the chip between VIN/GNDA and PVIN/PGND respectively. PVIN and PGND are the supply and return power paths for both channels 2 and 3, so a value of 10μF for C1 is appropriate. The feedback resistors program the output voltage. Minimizing the current in these resistors will maximize efficiency at very light loads, but totals on the order of 200k are a good compromise between efficiency and immunity to any adverse effects of PCB parasitic capacitance on the feedback pins. Choosing 10μA as the feedback current with 0.6V feedback voltage makes R4 = 60k. A close standard 1% resistor is 60.4k. Using: ⎛ 2.5V ⎞ R3 = ⎜ – 1 • R4 = 191.1k ⎝ 0.6 V ⎟ ⎠ The closest standard 1% resistor is 191k. A 20pF feedforward capacitor is recommended to improve transient response. The component values for the other channels are chosen in a similar fashion. Figure 4 shows the complete schematic for this example, along with the efficiency curve and burst mode ripple at an output current for the 2.5V output.
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LTC3545/LTC3545-1 APPLICATIONS INFORMATION
PC Board Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC3545/LTC3545-1. These items are also illustrated graphically in Figures 3 and 4. Figure 3 shows the power path components and traces. In this figure the feedback networks are not shown since they reside on the bottom side of the board. Check the following in your layout: 1. The power traces consisting of the PGND trace, the SW trace, the PVIN trace, the VIN and GNDA traces, should be kept short direct and wide. 2. Does each of the VFBx pins connect directly to the respective feedback resistors? The resistive dividers must be connected between the (+) plate of the corresponding output filter capacitor (e.g. C2) and GNDA. If the circuit being powered is at such a distance from the part where voltage drops along circuit traces are large, consider a Kelvin connection from the powered circuit back to the resistive dividers. 3. Keep C1 and C5 as close to the part as possible. 4. Keep the switching nodes (SWx) away from the sensitive VFBx nodes. 5. Keep the ground connected plates of the input and output capacitors as close as possible. 6. Care should be taken to provide enough space between unshielded inductors in order to minimize any transformer coupling.
VOUT3 L3 C4 (VIA TO FEEDBACK NETWORK)
SW3 VIN C1 C5 GNDA SW1 SW2 C3 C2 L1 L2 VOUT2
3545 F03
PVIN
PGND
(VIA TO FEEDBACK NETWORK)
(VIA TO FEEDBACK NETWORK)
VOUT1
Figure 3. Layout Diagram
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LTC3545/LTC3545-1 TYPICAL APPLICATIONS
L1 1.5μH C6 20pF C5 4.7μF 1 2 3 4 5 6 VIN 2.7V TO 5.5V 7 C1 10μF 10V 8 SW1 PGOOD1 RUN2 PGOOD2 SW2 PGND PVIN SW3 LTC3545 GNDA VIN RUN1 VFB1 VFB2 VFB3 RUN3 SYNC/MODE GND 17 L3 1.5μH C8 20pF R7 165k R8 110k
3545 TA02
R3 191k R4 60.4k
C2 10μF 6.3V
E3 VOUT1 2.5V AT 0.8A E4 GND
R2 511k E2 PGOOD2 E1 PGOOD1
R1 511k
16 15 14 13 12 11 10 9 R6 100k C7 20pF R5 100k L2 1.5μH C3 10μF 6.3V E7 VOUT2 1.2V AT 0.8A E6 GND
C4 10μF 6.3V
E5 VOUT3 1.5V AT 0.8A E8 GND
Overall Efficiency vs Channel 1 Load Current
100 90 OVERALL EFFICIENCY (%) 80 70 60 50 40 30 20 10 0 0.1 TA = 25°C VIN = 3.6V VOUT = 2.5V fOSC = 2.25MHz CHANNEL 2 = 1.2V, ILOAD = 400mA CHANNEL 3 = 1.5V, ILOAD = 400mA 1 10 100 CHANNEL 1 LOAD CURRENT (mA) 1000
3545 TA03
Burst Mode Ripple
VOUT3 AC COUPLED 20mV/DIV IL3 250mA/DIV
SW3 2V/DIV
3545 TA04
TA = 25°C VIN = 3.6V VOUT = 1.5V ILOAD = 50mA fOSC = 2.25MHz
1μs/DIV
Figure 4. LTC3545 Low Ripple Burst Mode Operation
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LTC3545/LTC3545-1 TYPICAL APPLICATIONS
L1 1.5μH C6 20pF C5 10μF R3 100k R4 100k C2 10μF E3 VOUT1 1.2V AT 0.8A E4 GND
R9 511k E9 PGOOD3 E2 PGOOD2
R2 511k E1 PGOOD1
R1 511k
1 2 3 4 5 6
SW1 PGOOD1 RUN2 PGOOD2 SW2 PGND PVIN SW3
LTC3545-1 GNDA VIN RUN1 VFB1 VFB2 VFB3 RUN3 PGOOD3 GND 17
16 15 14 13 12 11 10 9
L2 1.5μH C7 20pF R5 165k R6 110k L3 1.5μH C8 20pF R7 133k R8 66.5k
3545 TA05
C3 10μF
E7 VOUT2 1.5V AT 0.8A E6 GND
VIN 2.5V TO 5.5V
7 C1 4.7μF 8
C4 10μF
E5 VOUT3 1.8V AT 0.8A E8 GND
3-Channel Power Sequencing
RUN1
VOUT1 VOUT2 VOUT3 PGOOD3
TA = 25°C VIN = 3.6V
400μs/DIV
3545 TA06
Figure 5. LTC3545-1 Three PGOODs and Power Sequencing
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LTC3545/LTC3545-1 PACKAGE DESCRIPTION
UD Package 16-Lead Plastic QFN (3mm 3mm)
(Reference LTC DWG # 05-08-1700 Rev A)
Exposed Pad Variation AA
0.70 0.05
3.50
0.05 2.10
1.65 0.05 0.05 (4 SIDES)
PACKAGE OUTLINE 0.25 0.05 0.50 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 0.75 0.05 BOTTOM VIEW—EXPOSED PAD R = 0.115 TYP 15 16 0.40 1 1.65 0.10 (4-SIDES) 2 0.10 PIN 1 NOTCH R = 0.20 TYP OR 0.25 45 CHAMFER
3.00 0.10 (4 SIDES) PIN 1 TOP MARK (NOTE 6)
(UD16 VAR A) QFN 1207 REV A
0.200 REF 0.00 – 0.05 NOTE: 1. DRAWING CONFORMS TO JEDEC PACKAGE OUTLINE MO-220 VARIATION (WEED-4) 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
0.25
0.05
0.50 BSC
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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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LTC3545/LTC3545-1 RELATED PARTS
PART NUMBER LTC3405/LTC3405A LTC3406/LTC3406B LTC3407/LTC3407-2 LTC3409 LTC3410/LTC3410B LTC3411 LTC3412 LTC3419 LTC3441/LTC3442 LTC3443 LTC3531/LTC3531-3 LTC3531-3.3 LTC3532 LTC3544/LTC3544B LTC3547 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 COMMENTS 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 20μA, ISD < 1μA, ThinSOT™ Package 96% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 20μA, ISD < 1μA, ThinSOT Package 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 40μA, ISD < 1μA, 10-Lead MSE, DFN Packages 96% Efficiency, VIN: 1.6V to 5.5V, VOUT(MIN) = 0.6V, IQ = 65μA, ISD < 1μA, DFN Package 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 26μA, ISD < 1μA, SC70 Package 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 60μA, ISD < 1μA, 10-Lead MSE, DFN Packages 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 60μA, ISD < 1μA, 16-Lead TSSOPE Package
Dual 600mA, 2.25MHz, Synchronous Step-Down DC/DC 96% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 35μA, ISD < 1μA, MS10, 3mm × 3mm DFN Package 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 300mA, 2 × 200mA, 100mA, 2.25MHz Quad Synchronous Step-Down DC/DC Converter Dual 300mA IOUT, 2.25MHz, Synchronous Step-Down 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 95% Efficiency, VIN: 2.4V to 5.5V, VOUT(MIN): 2.4V to 5.25V, IQ = 50μA, ISD < 1μA, DFN Package 95% Efficiency, VIN: 1.8V to 5.5V, VOUT(MIN): 2V to 5V, IQ = 16μA, ISD < 1μA, ThinSOT, DFN Packages 95% Efficiency, VIN: 2.4V to 5.5V, VOUT(MIN): 2.4V to 5.25V, IQ = 35μA, ISD < 1μA, 10-Lead MSE, DFN Packages 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 60μA, ISD < 1μA, 3mm × 3mm QFN Package 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 40μA, ISD < 1μA, 8-Lead DFN Package 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 40μA, ISD < 1μA, 10-Lead MSE, DFN Packages 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 240μA, ISD < 1μA, DFN Package
ThinSOT is a Trademark of Linear Technology Corporation.
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20 Linear Technology Corporation
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