Datasheet

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LTC3546

3546fc

For more information www.linear.com/3546

LOAD CURRENT (A)

0.001 0.01

EFFICIENCY (%)

POWER LOSS (W)

100

0.1

0.01

0.0001

0.001

3546 TA01b

100.1

SW1D CONNECTED TO SW2

= 3.6V

OUT

= 2.5V

EFFICIENCY

POWER LOSS

3546 TA01a

LTC3546

PGOOD1

SW1

RUN1

FB1

BMC1

TRACK/SS1

FREQ

TH1

PGOOD2

SW2A

RUN2

FB2

BMC2

TRACK/SS2

PHASE

TH2

22µF

1000pF

10pF10pF

IN1

OUT1

1.2V, 1A

CCA

CCD

IN2

PGND2

EXPOSED PAD

GNDA PGND1

1.22µH

68µF

OUT2

2.5V, 3A

3.6V TO

5.5V

0.56µH

SYNC/MODE

SW2B

SW1D

22µF

100pF

1000pF

13k 13k

30.1k

95.3k

100pF

30.1k

Typical applicaTion

FeaTures

applicaTions

DescripTion

Dual Synchronous, 3A/1A

or 2A/2A Conﬁgurable

Step-Down DC/DC Regulator

The LT C

3546 is a dual, constant-frequency, synchronous

step down DC/DC converter for medium power applications.

The design consists of 2A and 1A primary output switches.

In addition to the 2A/1A capability, a 1A dependant output

switch can be externally connected to either of the primary

outputs to produce 3A/1A dual regulator or 2A/2A dual

regulator conﬁgurations.

Supply operation is from 2.25V to 5.5V. The switch

ing frequency can be set to 2.25MHz, adjustable from

0.75MHz

to 4MHz, or

synchronized to an external clock.

Each output is adjustable from 0.6V to 5V and has output

tracking on power-up. Internal synchronous low R

DS(ON)

power switches provide high efﬁciency without external

Schottky diodes.

User-selectable modes (Burst Mode

operation, pulse

skipping and forced continuous) allow a trade-off between

ripple noise and power efﬁciency. Burst Mode operation

provides high efﬁciency at light loads. Pulse-skipping

mode provides low ripple noise at light loads.

The device is capable of low dropout conﬁgurations and

both channels can operate at 100% duty cycle. In shut

down, the device draws <1µA.

Range: 2.25V to 5.5V

OUT

Range: 0.6V to 5V

Programmable Frequency Operation; 2.25MHz, or

Adjustable Between 0.75MHz to 4MHz

Low R

DS(ON)

Internal Switches

High Efﬁciency: Up to 96%

No Schottky Diodes Required

Short-Circuit Protected

Current Mode Operation for Excellent Line and Load

Transient Response

Low Ripple Burst Mode Operation (30mV

P-P

= 160µA

Ultralow Shutdown Current: I

< 1µA

Low Dropout Operation: 100% Duty Cycle

Power Good Output For Each Channel

Externally or Internally Programmable Burst Level

External or Internal Soft-Start or Supply Tracking

Available in Thermally Enhanced 28-Lead

(4mm × 5mm) QFN and TSSOP Packages

Netbooks/Ultra-Mobile PCs

PC Cards

Wireless and DSL Modems

Point of Load DC/DC Conversion

OUT2

Efﬁciency (Burst Mode Operation)

L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks

of Linear Technology Corporation. All other trademarks are the property of their respective

owners. Protected by U.S. Patents, including 5481178, 6611131, 6304066, 6498466, 6580258.

Summary of content (30 pages)

PAGE 1
LTC3546 Dual Synchronous, 3A/1A or 2A/2A Configurable Step-Down DC/DC Regulator Description Features n n n n n n n n n n n n n n n VIN Range: 2.25V to 5.5V VOUT Range: 0.6V to 5V Programmable Frequency Operation; 2.25MHz, or Adjustable Between 0.
PAGE 2
LTC3546 Absolute Maximum Ratings (Note 1) VIN1, VIN1D, VIN2, VCCA, VCCD Voltages.......... –0.3V to 6V SYNC/MODE, SW1, SW1D, SW2A, SW2B, RUN1, RUN2, VFB1, VFB2, PHASE, FREQ, ITH1, ITH2, TRACK/SS1, TRACK/SS2, BMC1, BMC2 Voltages................. –0.3V to (VIN1 or VIN2) + 0.3V Maximum Difference Between Any of VIN1, VIN1D, VIN2, VCCA, VCCD ...................................0.3V PGOOD1, PGOOD2 Voltage........................... –0.3V to 6V Operating Junction Temperature Range (Notes 2, 6, 7)................
PAGE 3
LTC3546 Electrical Characteristics The l denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at TA = 25°C. VIN = VCCA = 3.6V, unless otherwise specified. (Note 2) SYMBOL PARAMETER CONDITIONS MIN VIN1, VIN1D, VIN2, VCCA, VCCD Operating Voltage Range VIN1 = VIN1D = VIN2 = VCCA = VCCD 2.
PAGE 4
LTC3546 Electrical Characteristics The l denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at TA = 25°C. VIN = VCCA = 3.6V, unless otherwise specified. (Note 2) SYMBOL PARAMETER CONDITIONS TYP MAX ISW1D(LKG) Switch Leakage Current SW1D VIN = 6V VITH1 = VITH2 = 0V VRUN1 = VRUN2 = 0V MIN 0.01 1 µA ISW2A/B(LKG) Switch Leakage Current SW2A/B VIN = 6V VITH2 = 0V VRUN2 = 0V 0.
PAGE 5
LTC3546 Typical Performance Characteristics OUT1 Pulse-Skipping Mode Operation OUT1 Burst Mode Operation VOUT 20mV/DIV ILOAD 500mA/DIV 2µs/DIV OUT1 Forced Continuous Mode Operation VOUT 20mV/DIV VOUT 20mV/DIV ILOAD 250mA/DIV ILOAD 250mA/DIV 3546 G02 1µs/DIV 3546 G03 1µs/DIV 3546 G04 SW1D CONNECTED TO SW1 VIN = 3.6V VOUT = 1.8V ILOAD = 300mA SW1D CONNECTED TO SW1 VIN = 3.6V VOUT = 1.2V ILOAD = 100mA SW1D CONNECTED TO SW1 VIN = 3.6V VOUT = 1.
PAGE 6
LTC3546 Typical Performance Characteristics ILOAD 500mA/DIV ILOAD 250mA/DIV OUT1 Efficiency vs Load Current 85 85 75 1 80 75 60 0.001 10 3546 G14 80 75 Burst Mode OPERATION PULSE SKIP FORCED CONTINUOUS 1 75 60 0.001 1 10 96 96 3546 G17 400mA 90 88 86 2A 10 3546 G16 94 92 88 86 84 82 82 3.75 4.25 VIN (V) 4.75 5.25 3546 G18 200mA 90 84 3.25 1 SW1D CONNECTED TO SW2 98 VOUT = 1.8V 94 2.75 0.1 ILOAD (A) OUT1 Efficiency vs VIN 100 80 2.25 0.
PAGE 7
LTC3546 Typical Performance Characteristics OUT2 Efficiency vs VIN 100 98 98 96 96 94 94 600mA 92 EFFICIENCY (%) 90 88 3A 86 OUT2 Efficiency vs VIN VOUT 100mV/DIV 400mA 92 90 88 86 84 82 SW1D CONNECTED TO SW2 VOUT = 2.5V 80 2.25 2.75 3.25 3.75 4.25 VIN (V) 82 SW1D CONNECTED TO SW1 VOUT = 2.5V 80 2.25 2.75 3.25 3.75 4.25 VIN (V) 5.25 3546 G20 OUT1 Load Step 4.75 SW1D CONNECTED TO SW1 VIN = 3.6V VOUT = 1.2V ILOAD = 200mA TO 1.9A 5.
PAGE 8
LTC3546 Typical Performance Characteristics OUT1 RDS(ON) vs VIN 0.24 94 0.115 0.23 93 0.110 0.22 92 0.105 91 0.100 89 88 SW1D CONNECTED TO SW1 87 V = 3.6V IN 86 VOUT = 2.5V ILOAD = 400mA 85 0.5 1.0 1.5 2.0 2.5 3.0 FREQUENCY (MHz) 4.0 3546 G29 0.090 0.16 SW1D CONNECTED TO SW2 TA = 27°C 0.15 2.0 2.5 3.0 3.5 4.0 VIN (V) 0.120 0.050 2.0 2.5 3.0 3.5 4.0 VIN (V) 0.095 0.090 N-CHANNEL IMP 0.080 4.5 5.0 5.5 3546 G32 Frequency Variation vs Temperature 4.0 PEAK CURRENT (A) 3.
PAGE 9
LTC3546 Pin Functions (UFD/FE) BMC2 (Pin 1/Pin 4): Burst Mode Clamp for Channel 2. Connecting this pin to an external voltage between 0V and 0.6V sets the Burst Mode clamp level. If this pin is pulled to VCCA, an internal Burst Mode clamp level is used. This pin should be tied to GND when Burst Mode operation is not selected. SW2B (Pin 11/Pin 14): Half of the switch node connection to the inductor for Channel 2 SW2A and SW2B must be externally tied together. This pin swings from VIN2 to PGND2.
PAGE 10
LTC3546 Pin Functions (UFD/FE) VFB1 (Pin 20/Pin 23): Feedback voltage from external resistive divider from Channel 1 output. Nominal voltage for this pin is 0.6V. When using external synchronization this pin compensates the internal PLL. Typical compensation components are a 200k resistor in series with a 100pF capacitor. TRACK/SS1 (Pin 21/Pin 24): Tracking input for Channel 1 output or optional external soft-start input. VOUT1 will track an external voltage at this pin.
PAGE 11
LTC3546 Functional Diagram ITH1 BMC1 1A REGULATOR ITH LIMIT + + – VFB1 – – VB 0.552V VIN1 + SLOPE COMPENSATION + SW1 ANTI-SHOOT THRU – + + LOGIC 0.63V – – RUN1 – VCCA 1.15µA TRACK/SS1 INTERNAL SOFT-START PGND1 + PGOOD1 MUX VOLTAGE REFERENCE PHASE SYNC/MODE OSCILLATOR FREQ VIN1D VCCA 1A DEPENDENT SWITCHES GNDA DEPENDENT SWITCH AUTO DETECT AND CONTROL SW1D ANTI-SHOOT THRU MUX VCCD PGND1D GNDD PGOOD2 VCCA 1.
PAGE 12
LTC3546 Operation The LTC3546 uses a constant-frequency, current mode architecture. Both channels share the same clock frequency. The PHASE pin sets whether the channels are running in-phase, or 180° out-of-phase. The operating frequency is determined by connecting the FREQ pin to VIN for 2.25MHz operation or by connecting a resistor from FREQ to GNDA for frequencies between 0.75MHz to 4MHz. A 143k resistor to GNDA will set the frequency to 1.5MHz.
PAGE 13
LTC3546 Operation The output capacitor and the inductor supply the power to the load until ITH exceeds 0.31V, turning on the switch and the main control loop which starts another cycle. continuous operation, an overvoltage comparator monitors the VFB pin and decreases the current limit whenever an overvoltage condition is detected (VFB > 0.63V). The Burst Mode peak inductor current can be set externally via the BMC pin. When this pin is set somewhere between 0V to 0.
PAGE 14
LTC3546 Applications Information A general LTC3546 application circuit is shown in Figure 7. External component selection is driven by the load requirement, and begins with the selection of the inductors L1, and L2. Once L1 and L2 are chosen, CIN, COUT1, and COUT2 can be selected. Assuming a worst-case minimum on-time of 150ns, this can be calculated as: Operating Frequency The minimum frequency is limited by leakage and noise coupling due to the large resistance of RT.
PAGE 15
LTC3546 Applications Information Burst Mode Operation Considerations Inductor Core Selection There are two factors that determine the load current at which the LTC3546 enters Burst Mode operation: the inductor value and the BMC pin voltage. The transition from low current operation begins when the peak inductor current falls below a level set by the burst clamp. Lower inductor values result in higher ripple current which causes Burst Mode operation to occur at lower load currents.
PAGE 16
LTC3546 Applications Information the maximum RMS current. The RMS current calculation is different if the part is used in-phase or out-of-phase.
PAGE 17
LTC3546 Applications Information density, but it has a larger ESR and it is critical that the capacitors are surge tested for use in switching power supplies. An excellent choice is the AVX TPS series of surface tantalums, available in case heights ranging from 2mm to 4mm. Aluminum electrolytic capacitors have a significantly larger ESR, and are often used in extremely cost-sensitive applications provided that consideration is given to ripple current ratings and long term reliability.
PAGE 18
LTC3546 Applications Information Keeping the current small (<20µA) in these resistors maximizes efficiency, but making the current too small may allow stray capacitance to cause noise problems and reduce the phase margin of the error amp loop. To improve the frequency response, a feedforward capacitor CFF may also be used. Typical values used here are 10pf to 100pf. Great care should be taken to route the VFB node away from noise sources, such as the inductor or an SW line.
PAGE 19
LTC3546 Applications Information The LTC3546 can also track an external voltage during startup by using an external voltage divider to the TRACK/SS pin, and also insuring that the ramp rate on the TRACK/SS pin is slower than the internal 1.2ms ramp rate. OUTPUT VOLTAGE VX As indicated in Figure 3, a resistor divider from an external voltage can be connected to the TRACK/SS pin to allow the start-up of VOUT to ratiometrically track an external voltage VX. VOUT TIME For VTRACKSS < 0.6V Figure 4.
PAGE 20
LTC3546 Applications Information ulators. This mode provides the best low current efficiency at the cost of a higher output voltage ripple. When SYNC/ MODE is connected to ground, pulse-skipping operation is selected for both regulators. This mode provides a lower output voltage and current ripple at the cost of low current efficiency. Applying VIN/2 results in forced continuous mode for both regulators. This mode creates a fixed output ripple and is capable of sinking some current (about 1/2 • ΔIL).
PAGE 21
LTC3546 Applications Information overall supply performance. For a detailed explanation of optimizing the compensation components, including a review of control loop theory, refer to Linear Technology Application Note 76. Although a buck regulator is capable of providing the full output current in dropout, it should be noted that as the input voltage VIN drops toward VOUT, the load step capability does decrease due to the decreasing voltage across the inductor.
PAGE 22
LTC3546 Applications Information tion to exceed the maximum junction temperature of the part. In a majority of applications, the LTC3546 does not dissipate much heat due to its high efficiency. However, in applications where the LTC3546 is running at high ambient temperature with low supply voltage and high duty cycles, such as in dropout, the heat dissipated may exceed the maximum junction temperature of the part.
PAGE 23
LTC3546 Applications Information operating frequency of 1.5MHz and assumes roughly a 300mA/A load step. 22µF 280mA • 0.8A • = 20.5µF 300mA A • 0.8A A 22µF 875mA COUT1 = • 2.5A • = 64.2µF 300mA A • 2.5A A COUT1 = 2. All, or part, of CIN should connect from Pin 9 to Pin 14 on the same side of the PC board as the chip and as close to the chip as possible, where the SW traces will go directly under the capacitor. CIN provides the AC current to the internal power MOSFETs and their drivers.
PAGE 24
LTC3546 Applications Information VIN 2.25V TO 5.5V VOUT2 1.8V, 2A COUT2 47µF CIN 22µF R5 100k VIN1 VCCA VCCD VIN2 R6 100k SYNC/MODE L2 0.56µH PGOOD2 CFF2 100pF VIN SW2A SW1 SW2B SW1D RUN2 VFB2 R3 R4 60.4k 30.1k VIN LTC3546 BMC2 VOUT1 1.2V, 2A CFF1 100pF VIN VFB1 COUT1 47µF R1 R2 30.1k 30.1k VIN TRACK/SS1 FREQ PHASE ITH2 PGND2 RUN1 BMC1 TRACK/SS2 RITH2 13k L1 0.56µH PGOOD1 ITH1 GNDA PGND1 CITH2 1000pF RITH1 13k CITH1 1000pF EXPOSED PAD RFREQ 143k 3546 F07 Figure 7.
PAGE 25
LTC3546 Applications Information 5V VIN C5 22µF, X5R VCC R7 100k C1 22µF, X5R ×2 PGOOD1 PGOOD2 SYNC/MODE R8 100k VOUT2 2.5V/2A RUN1 RUN2 L1 1.0µH C6 10pF R1 316k R6 5.1k C4 470pF SW2A SW1 SW2B SW1D VFB2 R3 100k R9 90.9k L2 1.0µH LTC3546 FREQ ITH2 TRACK/SS2 C9 0.1µF BMC2 GND C7 33pF VFB1 R4 100k PHASE ITH1 TRACK/SS1 BMC1 R2 200k C8 0.1µF VOUT1 1.8V/2A C2 22µF, X5R ×2 R5 8.2k C3 330pF 3546 F08 fSW = 2.25MHz Figure 8. A 2.
PAGE 26
LTC3546 Applications Information VIN 2.25V TO 5.5V VOUT2 2.5V, 3A COUT2 33µF x2 CIN 22µF R5 100k R6 100k VIN1 VCCA VCCD VIN2 SYNC/MODE L2 0.56µH PGOOD2 SW2A CFF2 100pF L1 1.2µH PGOOD1 SW1 SW2B SW1D VIN RUN2 VFB2 R3 R4 95.3k 30.1k VIN LTC3546 BMC2 COUT1 22µF R1 R2 30.1k 59k VIN FREQ ITH2 PGND2 VIN VFB1 TRACK/SS1 PHASE RITH2 13k RUN1 BMC1 TRACK/SS2 VOUT1 1.8V, 1A CFF1 100pF ITH1 GNDA RITH1 13k CITH1 1000pF PGND1 CITH2 1000pF RFREQ 143k EXPOSED PAD 3546 F09 Figure 9.
PAGE 27
LTC3546 Package Description UFD Package 28-Lead Plastic QFN (4mm × 5mm) (Reference LTC DWG # 05-08-1712 Rev B) 0.70 ±0.05 4.50 ± 0.05 3.10 ± 0.05 2.50 REF 2.65 ± 0.05 3.65 ± 0.05 PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC 3.50 REF 4.10 ± 0.05 5.50 ± 0.05 RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 4.00 ± 0.10 (2 SIDES) R = 0.05 TYP 0.75 ± 0.05 PIN 1 NOTCH R = 0.20 OR 0.35 × 45° CHAMFER 2.50 REF R = 0.115 TYP 27 28 0.40 ± 0.
PAGE 28
LTC3546 Package Description FE Package 28-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1663) Exposed Pad Variation EB 9.60 – 9.80* (.378 – .386) 4.75 (.187) 4.75 (.187) 28 2726 25 24 23 22 21 20 19 18 1716 15 6.60 ±0.10 2.74 (.108) 4.50 ±0.10 SEE NOTE 4 0.45 ±0.05 EXPOSED PAD HEAT SINK ON BOTTOM OF PACKAGE 6.40 2.74 (.252) (.108) BSC 1.05 ±0.10 0.65 BSC RECOMMENDED SOLDER PAD LAYOUT 4.30 – 4.50* (.169 – .177) 0.09 – 0.20 (.0035 – .0079) 0.50 – 0.75 (.020 – .030) NOTE: 1.
PAGE 29
LTC3546 Revision History (Revision history begins at Rev C) REV DATE DESCRIPTION C 03/13 Modified Typical App Circuit VIN Range PAGE NUMBER 1 Clarified Electrical Characteristics Notes for Temp Ranges 3, 4 Clarified BMC, BMC1 and GNDD pin operation 9, 10 3546fc Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use.
PAGE 30
LTC3546 Related Parts PART NUMBER DESCRIPTION COMMENTS LTC3406A/ LTC3406B 600mA, 1.5MHz, Synchronous Step-Down DC/DC Converters 96% Efficiency, VIN(MIN): 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 20µA, ISD < 1µA, ThinSOT™ Package LTC3407A-2 Dual 800mA/800mA 2.25MHz, Synchronous StepDown DC/DC Converter 95% Efficiency, VIN(MIN): 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 40µA, ISD < 1µA, MS10E, 3mm × 3mm DFN-10 Packages LTC3409 600mA, 1.7MHz/2.