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LTC3615/LTC3615-1

3615fb

For more information www.linear.com/LTC3615

Typical applicaTion

DescripTion

Dual 4MHz, 3A Synchronous

Step-Down DC/DC Converter

The LT C

3615/LTC3615-1 are dual 3A synchronous step-

down regulators using a current mode, constant-frequency

architecture. The DC supply current is only 130µA (Burst

Mode operation at no-load) while maintaining the output

voltages, dropping to zero current in shutdown. The 2.25V

to 5.5V input supply range makes the parts ideally suited

for single Li-Ion applications. 100% duty cycle capability

provides low dropout operation, which extends operating

time in battery-operated systems.

The operating frequency is externally programmable up to

4MHz, allowing the use of small surface mount inductors.

0°, 90°, or 180° (LTC3615) or 140°/180° (LTC3615-1) of

phase shift between the two channels can be selected to

minimize input current ripple and output voltage ripple in a

dual 3A or single 6A output configuration. Programmable

slew rate limiting reduces EMI, and external synchroniza

tion can be applied up to 4MHz.

The

internal synchronous switches increase efficiency

and eliminate the need for external catch diodes, saving

external components and board space.

The LTC3615/LTC3615-1 are offered in leadless 24-pin

4mm × 4mm QFN and thermally enhanced 24-pin TSSOP

packages.

Efficiency and Power Loss vs Load Current

FeaTures

applicaTions

High Efficiency: Up to 94%

Dual Outputs with 2 × 3A Output Current Capability

Low Output Ripple Burst Mode

Operation: I

= 130µA

2.25V to 5.5V Input Voltage Range

±1% Output Voltage Accuracy

Output Voltages Down to 0.6V

Programmable Slew Rate at Switch Pins

Low Dropout Operation: 100% Duty Cycle

Shutdown Current ≤1µA

Adjustable Switching Frequency Up to 4MHz

Internal or External Compensation

Selectable Pulse-Skipping/Forced Continuous/

Burst Mode Operation with Adjustable Burst Clamp

Optional Active Voltage Positioning (AVP) with

Internal Compensation

Selectable 0°/90°/180° (LTC3615) or selectable

140°/180° (LTC3615-1) Phase Shift Between Channels

Fixed Internal and Programmable External Soft-Start

Accurate Start-Up Tracking Capability

DDR Memory Mode I

OUT

= ±1.5A

Available in 4mm × 4mm QFN-24 and TSSOP-24 Packages

Point-of-Load Supplies

Distributed Power Supplies

Portable Computer Systems

DDR Memory Termination

Handheld Devices

SW1

FB1

LTC3615

3615 TA01a

SGND

PGND

RUN1

TRACK/SS1

ITH1

PHASE

RUN2

TRACK/SS2

PGOOD2

ITH2

PGOOD1

SRLIM

/SYNC

MODE

IN1

IN2

100µF

SW2

FB2

0.47µH

47µF

OUT2

2.5V/3A

665k

210k

0.47µH

47µF

OUT1

1.8V/3A

422k

210k

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

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

owners. Protected by U.S. Patents, including 5481178, 5994885, 6304066, 6498466, 6580258,

6611131.

OUTPUT CURRENT (A)

EFFICIENCY (%)

POWER LOSS (W)

100

0.001 0.1 1

3615 TA01b

0.001

0.0001

0.1

0.01

= 3.3V

= 4V

= 5V

2.25MHz

OUT

= 2.5V

Downloaded from Arrow.com.

Summary of content (32 pages)

PAGE 1
LTC3615/LTC3615-1 Dual 4MHz, 3A Synchronous Step-Down DC/DC Converter Description Features n n n n n n n n n n n n n n n n n n High Efficiency: Up to 94% Dual Outputs with 2 × 3A Output Current Capability Low Output Ripple Burst Mode® Operation: IQ = 130µA 2.25V to 5.5V Input Voltage Range ±1% Output Voltage Accuracy Output Voltages Down to 0.
PAGE 2
LTC3615/LTC3615-1 Absolute Maximum Ratings (Notes 1, 11) PVIN1, PVIN2 Voltages..................... –0.3V to SVIN + 0.3V SVIN Voltage................................................. –0.3V to 6V SW1 Voltage ..............................–0.3V to (PVIN1 + 0.3V) SW2 Voltage...............................–0.3V to (PVIN2 + 0.3V) PGOOD1, PGOOD2 Voltages......................... –0.3V to 6V All Other Pins............................... –0.3V to (SVIN + 0.3V) Operating Junction Temperature Range (Notes 2, 11)..
PAGE 3
LTC3615/LTC3615-1 Electrical Characteristics The l denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at TA = 25°C (Note 2), SVIN = PVINx = 3.3V, RT = 178k, RSRLIM = 40.2k, unless otherwise specified. SYMBOL PARAMETER VIN Operating Voltage Range VUVLO Undervoltage Lockout Threshold CONDITIONS SVIN Ramping Down MIN l 2.25 l 1.7 TYP 5.
PAGE 4
LTC3615/LTC3615-1 Electrical Characteristics The l denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at TA = 25°C (Note 2), SVIN = PVINx = 3.3V, RT = 178k, RSRLIM = 40.2k, unless otherwise specified. SYMBOL PARAMETER CONDITIONS jSW1–SW2 Output Phase Shift Between SW1 and SW2 (LTC3615) VPHASE < 0.15 • SVIN 0 Deg 0.35 • SVIN < VPHASE < 0.65 • SVIN VPHASE > 0.
PAGE 5
LTC3615/LTC3615-1 Typical Performance Characteristics 100 100 VOUT = 1.8V 100 VOUT = 1.2V 90 80 80 70 70 70 60 50 40 30 40 30 0 0.001 3615 G01 Efficiency vs Load Current (VMODE = 0.55 • SVIN) 100 90 100 90 80 70 70 EFFICIENCY (%) 80 60 50 40 30 20 0 0.001 60 50 40 30 0 0.001 80 75 70 65 IOUT = 3A IOUT = 2A IOUT = 1A IOUT = 0.3A IOUT = 0.2A 55 50 2.25 10 0.01 0.1 1 OUTPUT CURRENT (A) 3615 G04 2.75 3615 G05 3.25 3.75 4.25 4.75 INPUT VOLTAGE (V) 5.
PAGE 6
LTC3615/LTC3615-1 Typical Performance Characteristics Forced Continuous Mode Operation (FCM) Pulse-Skipping Mode Operation VOUT 20mV/DIV IL 200mA/DIV VOUT = 1.8V IOUT = 100mA VMODE = 1.5V 1µs/DIV VIN = 3.3V, RT /SYNC = SVIN, unless otherwise noted. Burst Mode Operation VOUT 20mV/DIV VOUT 20mV/DIV IL 500mA/DIV IL 500mA/DIV VOUT = 1.8V IOUT = 75mA VMODE = 3.3V 3615 G09 Load Step Transient in FCM External Compensation 20µs/DIV 3615 G10 VOUT = 1.
PAGE 7
LTC3615/LTC3615-1 Typical Performance Characteristics Reference Voltage vs Temperature Switch On-Resistance vs Input Voltage 0.10 0.606 0.09 0.604 0.08 MAIN SWITCH 0.07 0.602 RDS(ON) (Ω) REFERENCE VOLTAGE (V) VIN = 3.3V, RT /SYNC = SVIN, unless otherwise noted. 0.600 0.598 0.06 0.05 SYNCHRONOUS SWITCH 0.04 0.03 0.02 0.596 0.01 0.594 –60 –25 80 10 45 TEMPERATURE (°C) 115 0 2.25 150 Switch On-Resistance vs Temperature 4.0 100 90 2.4 fOSC (MHz) SWITCH LEAKAGE (µA) 2.
PAGE 8
LTC3615/LTC3615-1 Typical Performance Characteristics No Load Supply Current vs Input Voltage Switch Leakage vs Temperature 180 180 4.0 MODE = 0V 160 RUNx = ITHx = SVIN 3.5 140 SYNCHRONOUS SWITCH 2.0 1.5 1.0 0.5 –25 10 45 80 TEMPERATURE (°C) 120 100 80 60 40 20 MAIN SWITCH 0 –60 115 150 0 2.25 3.25 3.75 4.25 VIN (V) 4.75 5.25 120 100 80 60 40 20 MODE = 0V RUNx = ITHx = SVIN 0 10 –60 45 80 –25 TEMPERATURE (°C) 3615 G26 Slew Rate of Rising Edge at SW1/2 vs SRLIM Resistor VIN = 3.
PAGE 9
LTC3615/LTC3615-1 Pin Functions (FE/UF) PHASE (Pin 1/Pin 4): Phase Shift Selection. If pin is tied to SGND, the phase between SW1 and SW2 will be 0° (LTC3615) or 140° (LTC3615-1). With the PHASE pin tied to half of the SVIN voltage, 90° (LTC3615) or 140° (LTC3615-1) of phase shift will be selected. Tying PHASE to SVIN will select 180° (LTC3615 and LTC3615-1). PGOOD2 (Pin 13/Pin 16): Power Good Output for Channel 2. See PGOOD1. VFB2 (Pin 2/Pin 5): Voltage Feedback Input Pin for Chan nel 2. See VFB1. 2.
PAGE 10
LTC3615/LTC3615-1 Pin Functions (FE/UF) behavior for channel 1 is programmable with the TRACK/ SS1 pin: 1. Internal soft-start with a fixed timing can be programmed by tying TRACK/SS1 to SVIN. 2. External soft-start can be programmed with the timing set by a capacitor to ground and a resistor to SVIN. 3. Tracking the start-up behavior of another supply is programmable (see the Applications Information section). 4. The pin can be used as external reference input.
PAGE 11
LTC3615/LTC3615-1 Functional Block Diagram ITH1 PGOOD1 PGOOD WINDOWCOMPARATOR DELAY + ERROR AMPLIFIER + IDEAL DIODE TRACK/SS1 PMOS CURRENT SENSE – PMOS CURRENT COMPARATOR CONTROLLER LOGIC SOFT-START RUN1 PVIN1 GATE DRIVER SW1 CLK1 OR RUN2 RT /SYNC PHASE PLL OSCILLATOR AND PHASE SELECTOR SVIN SGND + SLOPE COMPENSATION – + MODE BURST COMPARATOR MODE – VREF INTERNAL/ EXTERNAL COMPENSATION ITH-VOLTAGE LIMIT – FB1 CHANNEL 1 + – CLK2 UNDERVOLTAGE LOCKOUT SHUTDOWN NMOS CURRENT S
PAGE 12
LTC3615/LTC3615-1 Operation Main Control Loop MODE SELECTION The LTC3615 is a dual monolithic step-down DC/DC converter featuring current-mode, constant-frequency operation. Both channels are identical and share common clock and reference circuits to improve channel-to-channel matching.
PAGE 13
LTC3615/LTC3615-1 Operation Burst Mode Operation—External Clamp Forced Continuous Mode Operation Connecting the MODE pin to a voltage in the range of 0.5V to 0.8V enables Burst Mode operation with external clamp. During this mode of operation, the minimum voltage on the ITH pin is externally set by the voltage on the MODE pin. It is recommended to use Burst Mode operation with the internal clamp for ambient temperatures above 85°C.
PAGE 14
LTC3615/LTC3615-1 Operation Low Supply Operation The LTC3615 is designed to operate down to an input supply voltage of 2.25V. An important consideration at low input supply voltages is that the RDS(ON) of the P-channel and N-channel power switches increases by 50% compared to 5V. The user should calculate the power dissipation when the LTC3615 is used at 100% duty cycle with low input voltages to ensure that thermal limits are not exceeded.
PAGE 15
LTC3615/LTC3615-1 Applications Information Operating Frequency Selection of the operating frequency is a trade-off between efficiency and component size. High frequency operation allows the use of smaller inductor and capacitor values. Operation at lower frequencies improves efficiency by reducing internal gate charge losses but requires larger inductance values and/or capacitance to maintain low output ripple voltage.
PAGE 16
LTC3615/LTC3615-1 Applications Information VIN LTC3615 SVIN RT/SYNC VIN fSW 2.25MHz 0.4V ROSC LTC3615 SVIN RT/SYNC SGND VIN fSW ∝1/ROSC 1.2V 0.3V LTC3615 SVIN RT/SYNC SGND VIN fSW 1/TP TP 15pF 1.2V 0.3V RT TP LTC3615 SVIN RT/SYNC SGND fSW 1/TP 3615 F04 Figure 4. Setting the Switching Frequency of periods to settle until the frequency at SW matches the frequency and phase of RT/SYNC.
PAGE 17
LTC3615/LTC3615-1 Applications Information when the peak inductor current falls below a level set by the burst clamp. Lower inductor values result in higher ripple current which causes this to occur at lower DC load currents. This causes a dip in efficiency in the upper range of low current operation. In Burst Mode operation, lower inductance values will cause the burst frequency to increase. Inductor Core Selection Once the value for L is known, the type of inductor must be selected.
PAGE 18
LTC3615/LTC3615-1 Applications Information Output Capacitor COUT Selection The selection of COUT is typically driven by the required ESR to minimize voltage ripple and load step transients (low-ESR ceramic capacitors are discussed in the next section). Typically, once the ESR requirement is satisfied, the capacitance is adequate for filtering.
PAGE 19
LTC3615/LTC3615-1 Applications Information Output Voltage Programming Pulse-Skipping Mode The output voltages are set by external resistive dividers. For example, VOUT2 can be set according to the following equation: Pulse-skipping mode, which is a compromise between low output voltage ripple and efficiency, can be implemented by connecting the MODE pin to SVIN. This sets IBURST to 0A. In this condition, the peak inductor current is limited by the minimum on-time of the current comparator.
PAGE 20
LTC3615/LTC3615-1 Applications Information Applications section uses faster compensation to improve load step response. A second, more severe transient is caused by switching in loads with large (>1µF) supply bypass capacitors. The discharged bypass capacitors are effectively put in parallel with COUT, causing a rapid drop in VOUT. No regulator can alter its delivery of current quickly enough to prevent this sudden step change in output voltage if the load switch resistance is low and it is driven quickly.
PAGE 21
LTC3615/LTC3615-1 Applications Information The LTC3615 allows the user to control the slew rate of the switching node SW by using the SRLIM pin. Tying this pin to ground selects the fastest slew rate. The slowest slew rate is selected when the pin is open. Connecting a resistor (between 10k to 100k) from SRLIM pin to ground adjusts the slew rate between the maximum and minimum values.
PAGE 22
LTC3615/LTC3615-1 Applications Information Through the TRACK/SS pin, the output voltage can be set up to either coincidental or ratiometric tracking, as shown in Figures 8 and 9. Output Voltage Tracking Input If SRLIM is low, once VTRACK/SS reaches or exceeds 0.6V the run state is entered, and the MODE selection, power good and current foldback circuits are enabled.
PAGE 23
LTC3615/LTC3615-1 Applications Information External Reference Input (DDR Mode) If SRLIM is tied to SVIN, the TRACK/SS pin can be used as an external reference input between 0.3V and 0.5V, if desired (see Figure 13). In DDR mode, the maximum slew rate is selected. If VTRACK/ SS is within 0.3V and 0.5V, the PGOOD function is enabled. If VTRACK/SS is less than 0.3V, the output current foldback is disabled and the PGOOD pin is always pulled down. VFB PIN 0.6V VOLTAGE 0V 0.6V TRACK/SS PIN VOLTAGE 0.
PAGE 24
LTC3615/LTC3615-1 Applications Information DDR Application The LTC3615 can be used in DDR memory power supply applications by tying the SRLIM pin to SVIN. In DDR mode, the maximum slew rate is selected. The output can both source and sink current. Current sinking is typically limited to 1.5A, for 1MHz frequency and 1µH inductance, but can be lower at higher frequencies and low output voltages. If higher ripple current can be tolerated, smaller inductor values can increase the sink current limit.
PAGE 25
LTC3615/LTC3615-1 Applications Information 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. If the junction temperature reaches approximately 170°C, all four power switches will be turned off and the SW node will become high impedance. To prevent the LTC3615 from exceeding the maximum junction temperature, the user will need to do some thermal analysis.
PAGE 26
LTC3615/LTC3615-1 Applications Information Decoupling the PVIN with two 47µF capacitors is adequate for most applications. Finally, it is possible to define the soft-start up time choosing the proper value for the capacitor and the resistor connected to TRACK/SS pin. If one sets minimum TSS = 5ms and a resistor of 4.7M, the following equation can be solved with the maximum SVIN = 5.5V: CSS = 5ms = 9.2nF ⎛ 5.5V ⎞ 4.7M • In ⎜ ⎟ ⎝ 5.5V – 0.6V ⎠ The standard value of 10nF and 4.
PAGE 27
LTC3615/LTC3615-1 Typical Applications External Compensation, Forced Continuous Operation, In-Phase Switching, Slew Rate Limit, Common PGOOD Output VIN 3.3V 47µF 47µF 1µF SVIN RUN (2×) PVIN1 (2×) PVIN2 (2×) SW1 RUN1 0.47µH R1 412k TRACK/SS1 RC1 43k CC1 220pF R6 226k PGOOD1 ITH1 RT 10pF 178k R5 40.2k R7 174k 100k RT /SYNC SRLIM (2×) SW2 MODE FB2 0.47µH R3 665k 47µF VOUT2 2.
PAGE 28
LTC3615/LTC3615-1 Typical Applications Master and Slave for Coincident Tracking Outputs Using a 2MHz External Clock RF1 24Ω CF1 1µF VIN 3.3V C1 47µF C2 47µF 4.7M SVIN (2×) PVIN1 (2×) PVIN2 (2×) SW1 RUN1 L1 0.47µH R1 715k TRACK/SS1 CSYNC 15pF PGOOD1 2MHz CLOCK RT 200k R5 100k RC1 15k 10nF MODE R9 226k CO11 47µF CO12 22µF R4 453k (2×) SW2 L2 0.47µH PHASE TRACK/SS2 R7 100k ITH2 CC4 10pF R5 294k FB2 C7 22pF CO21 47µF CO22 22µF VOUT2 1.
PAGE 29
LTC3615/LTC3615-1 Package Description FE Package 24-Lead Plastic TSSOP (4.4mm) (Reference LTCFEDWG # 05-08-1771 Rev B) Package 24-Lead Plastic TSSOP (4.4mm) Exposed Pad Variation AA (Reference LTC DWG # 05-08-1771 Rev B) Exposed Pad Variation AA 7.70 – 7.90* (.303 – .311) 3.25 (.128) 3.25 (.128) 24 23 22 21 20 19 18 17 16 15 14 13 6.60 ±0.10 2.74 (.108) 4.50 ±0.10 6.40 2.74 (.252) (.108) BSC SEE NOTE 4 0.45 ±0.05 1.05 ±0.10 0.65 BSC 1 2 3 4 5 6 7 8 9 10 11 12 RECOMMENDED SOLDER PAD LAYOUT 4.
PAGE 30
LTC3615/LTC3615-1 Package Description UF Package 24-Lead Plastic QFN (4mm × 4mm) Package (Reference LTCUF DWG # 05-08-1697 Rev B) 24-Lead Plastic QFN (4mm × 4mm) (Reference LTC DWG # 05-08-1697 Rev B) 0.70 ±0.05 4.50 ±0.05 2.45 ±0.05 3.10 ±0.05 (4 SIDES) PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 4.00 ±0.10 (4 SIDES) BOTTOM VIEW—EXPOSED PAD R = 0.115 TYP 0.75 ±0.05 PIN 1 NOTCH R = 0.20 TYP OR 0.35 × 45° CHAMFER 23 24 PIN 1 TOP MARK (NOTE 6) 0.40 ±0.10 1 2 2.
PAGE 31
LTC3615/LTC3615-1 Revision History REV DATE DESCRIPTION A 7/10 LTC3615-1 added. Reflected throughout the data sheet. PAGE NUMBER B 6/13 Clarified temperature maximum ratings. 2 Clarified the Ordering Information section. 2 Clarified the Feedback Voltage specification in the Electrical Characteristics section. 3 Clarified the temperature specifications on Notes 2 and 11. 4 1 to 32 Clarified Typical Performance Characteristics graphs.
PAGE 32
LTC3615/LTC3615-1 Typical Application VIN 3.3V 47µF 1µF (2×) (2×) PVIN1 PVIN2 SVIN RUN1 (2×) SW1 TRACK/SS1 VSW1 L1 0.47µH R1 102k PGOOD1 ITH1 LTC3615 RT /SYNC 20pF RC 7.5k CC 2000pF R9 174k R8 226k FB1 L2 0.47µH SRLIM (2×) SW2 MODE FB2 VOUT 1.2V/6A 47µF 2V/DIV, 1A/DIV R2 102k VSW2 IL1 IL2 IL1 + IL2 MODE = FCM 200ns/DIV 3615 F16 Figure 16.