Datasheet

ManualsBrandsLINEAR TECHNOLOGY ManualsVoltage RegulatorsLTC3614EUDD#PBF

LTC3614

3614fa

TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

4A, 4MHz Monolithic

Synchronous Step-Down

DC/DC Converter

The LTC

3614 is a low quiescent current monolithic syn-

chronous buck regulator using a current mode, constant

frequency architecture. The no-load DC supply current

in sleep mode is only 75µA while maintaining the output

voltage (Burst Mode operation) at no load, dropping to zero

current in shutdown. The 2.25V to 5.5V input supply voltage

range makes the LTC3614 ideally suited for single Li-Ion

as well as ﬁ xed low voltage input applications. 100% duty

cycle capability provides low dropout operation, extending

the operating time in battery-powered systems.

The operating frequency is externally programmable up to

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

For switching-noise-sensitive applications, the LTC3614

can be synchronized to an external clock at up to 4MHz.

Forced continuous mode operation in the LTC3614 reduces

noise and RF interference. Adjustable compensation allows

the transient response to be optimized over a wide range

of loads and output capacitors.

The internal synchronous switch increases efﬁ ciency and

eliminates the need for an external catch diode, saving

external components and board space. The LTC3614 is

offered in a leadless 24-pin 3mm × 5mm thermally en-

hanced QFN package.

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 6580258, 5481178, 5994885, 6304066, 6498466, 6611131.

Efﬁ ciency and Power Loss

vs Load Current

4A Output Current

2.25V to 5.5V Input Voltage Range

Low Output Ripple Burst Mode

Operation: I

= 75µA

±1% Output Voltage Accuracy

Output Voltage Down to 0.6V

High Efﬁ ciency: Up to 95%

Low Dropout Operation: 100% Duty Cycle

Programmable Slew Rate on SW Node Reduces

Noise and EMI

Adjustable Switching Frequency: Up to 4MHz

Optional Active Voltage Positioning (AVP) with

Internal Compensation

Selectable Pulse-Skipping/Forced Continuous/Burst

Mode Operation with Adjustable Burst Clamp

Programmable Soft-Start

Inputs for Start-Up Tracking or External Reference

DDR Memory Mode, I

OUT

= ±3A

Available in a 24-Pin 3mm × 5mm QFN

Thermally Enhanced Package

Point-of-Load Supplies

Distributed Power Supplies

Portable Computer Systems

DDR Memory Termination

Handheld Devices

OUTPUT CURRENT (mA)

EFFICIENCY (%)

POWER LOSS (W)

100

1 100 1000 10000

3614 TA01b

0.1

0.01

= 2.8V

= 3.3V

= 5V

OUT

= 2.5V

RUN

TRACK/SS

RT/SYNC

PGOOD

ITH

SGND

PGND

2.7V TO 5.5V

SRLIM/DDR

LTC3614 SW

330nH

665k

210k

3614 TA01a

10µF

MODE V

47µF

OUT

2.5V

Summary of content (30 pages)

PAGE 1
LTC3614 4A, 4MHz Monolithic Synchronous Step-Down DC/DC Converter DESCRIPTION FEATURES n n n n n n n n n n n n n n n 4A Output Current 2.25V to 5.5V Input Voltage Range Low Output Ripple Burst Mode® Operation: IQ = 75μA ±1% Output Voltage Accuracy Output Voltage Down to 0.
PAGE 2
LTC3614 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) MODE VFB ITH TRACK/SS TOP VIEW 24 23 22 21 SRLIM/DDR 1 20 PGOOD RT/SYNC 2 19 RUN SGND 3 18 SVIN PVIN 4 17 PVIN 25 PGND SW 5 16 SW 15 SW SW 7 14 SW SW 8 13 SW NC 10 11 12 PVIN 9 PVIN SW 6 NC PVIN, SVIN Voltages...................................... –0.3V to 6V SW Voltage ..................................–0.3V to (PVIN + 0.3V) ITH, RT/SYNC Voltages ............... –0.3V to (SVIN + 0.3V) SRLIM, TRACK/SS Voltages .......
PAGE 3
LTC3614 ELECTRICAL CHARACTERISTICS The l denotes the speciﬁcations which apply over the full operating junction temperature range, otherwise speciﬁcations are at TA = 25°C. VIN = 3.3V, RT/SYNC = SVIN unless otherwise speciﬁed (Notes 1, 2, 11). SYMBOL PARAMETER VIN Operating Voltage Range VUVLO Undervoltage Lockout Threshold VFB Feedback Voltage Internal Reference Feedback Voltage External Reference (Note 7) CONDITIONS MIN l 2.25 SVIN Ramping Down SVIN Ramping Up l l 1.
PAGE 4
LTC3614 ELECTRICAL CHARACTERISTICS The l denotes the speciﬁcations which apply over the full operating junction temperature range, otherwise speciﬁcations are at TA = 25°C. VIN = 3.3V, RT/SYNC = SVIN unless otherwise speciﬁed (Notes 1, 2, 11).
PAGE 5
LTC3614 TYPICAL PERFORMANCE CHARACTERISTICS VIN = 3.3V, RT/SYNC = SVIN unless otherwise noted. Efﬁciency vs Frequency Burst Mode Operation (VMODE = 0V), IOUT = 2A 100 VOUT = 1.8V 90 EFFICIENCY (%) EFFICIENCY (%) 80 70 60 50 IOUT = 6mA IOUT = 600mA IOUT = 2A 40 30 2.5 3 3.5 4 4.5 INPUT VOLTAGE (V) 5 5.5 95 94 93 92 91 90 89 88 87 86 85 84 83 82 0.5 Load Regulation (VOUT = 1.8V) 1.5 VIN = 3.3V VOUT = 1.8V FORCED CONTINUOUS MODE PULSE-SKIPPING MODE INTERNAL Burst Mode OPERATION 1.3 1.
PAGE 6
LTC3614 TYPICAL PERFORMANCE CHARACTERISTICS VIN = 3.3V, RT/SYNC = SVIN unless otherwise noted. Load Step Transient in Forced Continuous Mode without AVP Mode Load Step Transient in Forced Continuous Mode Sourcing and Sinking Current Load Step Transient in Forced Continuous Mode with AVP Mode VOUT 100mV/DIV VOUT 100mV/DIV VOUT 200mV/DIV ILOAD 2A/DIV ILOAD 2A/DIV ILOAD 2A/DIV 100μs/DIV VOUT = 1.8V ILOAD = 100mA TO 4A, VMODE = 1.5V COMPENSATION FIGURE 1 3614 G13 100μs/DIV VOUT = 1.
PAGE 7
LTC3614 TYPICAL PERFORMANCE CHARACTERISTICS VIN = 3.3V, RT/SYNC = SVIN unless otherwise noted. Switch On-Resistance vs Temperature Frequency vs Resistor on RT/SYNC Pin 0.040 MAIN SWITCH 0.035 FREQUENCY (kHz) RDS(ON) (Ω) 0.8 4000 0.6 3500 0.030 SYNCHRONOUS SWITCH 0.025 0.020 0.015 3000 2500 2000 1500 0.010 1000 0.005 500 0 –50 –30 –10 10 30 50 70 90 110 130 TEMPERATURE (°C) 0 7000 200 400 600 800 1000 1200 1400 RESISTOR ON RT/SYNC PIN (kΩ) 0 –0.5 –1.0 –1.5 –2.
PAGE 8
LTC3614 TYPICAL PERFORMANCE CHARACTERISTICS VIN = 3.3V, RT/SYNC = SVIN unless otherwise noted. Output Voltage During Sinking vs Input Voltage (VOUT = 1.8V, 0.47μH Inductor) Start-Up from Shutdown with Prebiased Output (Overvoltage) (Forced Continuous Mode) 1.88 PGOOD 5V/DIV 1.86 1.84 VOUT (V) VOUT 500mV/DIV 1.82 –3A, 2MHz, 120°C 1.80 –3A, 2MHz, 25°C 1.78 IL 5A/DIV 1.76 50μs/DIV PREBIASED VOUT = 2.2V VOUT = 1.2V, IOUT = 0A VMODE = 1.5V 3614 G31 1.74 2.25 2.75 3.25 4 4.5 INPUT VOLTAGE (V) 5.
PAGE 9
LTC3614 PIN FUNCTIONS RUN (Pin 19): Enable Pin. Forcing this pin to ground shuts down the LTC3614. In shutdown, all functions are disabled and the chip draws <1μA of supply current. VFB (Pin 22): Voltage Feedback Input Pin. Senses the feedback voltage from the external resistive divider across the output. PGOOD (Pin 20): Power Good. This open-drain output is pulled down to SGND on start-up and while the FB voltage is outside the power good voltage window.
PAGE 10
LTC3614 FUNCTIONAL BLOCK DIAGRAM SVIN ITH PVIN PVIN PVIN PVIN ITH SENSE COMPARATOR + BANDGAP AND BIAS RUN RT/SYNC SGND INTERNAL COMPENSATION OSCILLATOR CURRENT SENSE – SVIN – 0.3V R – PMOS CURRENT COMPARATOR ITH LIMIT + 0.3V – FOLDBACK AMPLIFIER – SLOPE COMPENSATION + 0.6V + VFB + ERROR AMPLIFIER – BURST COMPARATOR SW SLEEP – + DRIVER + SW SW MODE TRACK/SS SW SOFT-START SW 0.555V + SW SW – LOGIC REVERSE COMPARATOR + 0.
PAGE 11
LTC3614 OPERATION Main Control Loop Mode Selection The LTC3614 is a monolithic, constant frequency, current mode step-down DC/DC converter. During normal operation, the internal top power switch (P-channel MOSFET) is turned on at the beginning of each clock cycle. Current in the inductor increases until the current comparator trips and turns off the top power switch. The peak inductor current at which the current comparator trips is controlled by the voltage on the ITH pin.
PAGE 12
LTC3614 OPERATION Burst Mode Operation—External Clamp Dropout Operation Connecting the MODE pin to a voltage in the range of 0.45V 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 internal burst clamp for temperatures above 85°C ambient.
PAGE 13
LTC3614 OPERATION If the output voltage drops below 50% of its nominal value, the clamp voltage at ITH pin is lowered causing the maximum peak inductor current to decrease gradually with the output voltage. When the output voltage reaches 0V the clamp voltage at the ITH pin drops to 40% of the clamp voltage during normal operation. The short-circuit peak inductor current is determined by the minimum on-time of the LTC3614, the input voltage and the inductor value.
PAGE 14
LTC3614 APPLICATIONS INFORMATION Frequency Synchronization Inductor Selection The LTC3614’s internal oscillator can be synchronized to an external frequency by applying a square wave clock signal to the RT/SYNC pin. During synchronization, the top switch turn-on is locked to the falling edge of the external frequency source. The synchronization frequency range is 300kHz to 4MHz. During synchronization all operation modes can be selected.
PAGE 15
LTC3614 APPLICATIONS INFORMATION Ferrite designs have very low core losses and are preferred at high switching frequencies, so design goals can concentrate on copper loss and preventing saturation. Ferrite core material saturates “hard,” meaning that inductance collapses abruptly when the peak design current is exceeded. This results in an abrupt increase in inductor ripple current and consequently output voltage ripple.
PAGE 16
LTC3614 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 17
LTC3614 APPLICATIONS INFORMATION Output Voltage Programming The resistive divider allows pin VFB to sense a fraction of the output voltage as shown in Figure 1. Pulse-skipping mode, which is a compromise between low output voltage ripple and efﬁciency, can be implemented by connecting MODE 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 18
LTC3614 APPLICATIONS INFORMATION The ﬁrst circuit in the Typical Applications section uses faster compensation to improve 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 .
PAGE 19
LTC3614 APPLICATIONS INFORMATION DDR Mode The LTC3614 can both source and sink current if the MODE pin is conﬁgured to forced continuous mode. Current sinking is typically limited to 3A for 1MHz frequency and a 0.47μH inductor, 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. See the Typical Performance Characteristics curves for more information.
PAGE 20
LTC3614 APPLICATIONS INFORMATION During either internal or external soft-start, the MODE pin is ignored and soft-start will always be in pulse-skipping mode. In addition, the PGOOD pin is kept low and foldback of the switching frequency is disabled. Particular attention should be used with very high switching frequencies. Using the slowest slew rate (SRLIM open) can reduce the minimum duty cycle capability.
PAGE 21
LTC3614 APPLICATIONS INFORMATION To implement the coincident tracking behavior in Figure 6a, connect an extra resistive divider to the output of the master channel and connect its midpoint to the TRACK/SS pin for the slave channel. The ratio of this divider should be selected to be the same as that of the slave channel’s feedback divider (Figure 7a). In this tracking mode, the master channel’s output must be set higher than slave channel’s output.
PAGE 22
LTC3614 APPLICATIONS INFORMATION VFB PIN 0.6V VOLTAGE 0V 0.6V TRACK/SS PIN VOLTAGE 0.2V 0V RUN PIN VOLTAGE SVIN PIN VOLTAGE VIN 0V VIN 0V TIME SHUTDOWN SOFT-START STATE STATE tSS > 1ms RUN STATE REDUCED SWITCHING FREQUENCY DOWN TRACKING STATE RUN STATE 3614 F08 UP TRACKING STATE Figure 8. DDR Pin Not Tied to SVIN 0.45V VFB PIN 0.3V VOLTAGE 0V EXTERNAL VOLTAGE REFERENCE 0.45V 0.45V TRACK/SS 0.3V PIN VOLTAGE 0.
PAGE 23
LTC3614 APPLICATIONS INFORMATION Efﬁciency Considerations The efﬁciency of a switching regulator is equal to the output power divided by the input power times 100%. It is often useful to analyze individual losses to determine what is limiting the efﬁciency and which change would produce the most improvement. Efﬁciency can be expressed as: Efﬁciency = 100% – (L1 + L2 + L3 + ...) where L1, L2, etc. are the individual losses as a percentage of input power.
PAGE 24
LTC3614 APPLICATIONS INFORMATION Thermal Considerations In most applications, the LTC3614 does not dissipate much heat due to its high efﬁciency. However, in applications where the LTC3614 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. If the junction temperature reaches approximately 160°C, both power switches will be turned off and the SW node will become high impedance.
PAGE 25
LTC3614 APPLICATIONS INFORMATION Assuming worst-case conditions of VIN = 2VOUT, CIN should be selected for a maximum current rating of: IRMS = 4A • ⎛ 3.6V ⎞ 1.8V • ⎜ – 1⎟ = 2ARMS 3.6V ⎝ 1.8V ⎠ Decoupling PVIN with four 10μF to 22μF capacitors is adequate for most applications. If we set R2 = 196k, the value of R1 can now be determined by solving the following equation. ⎛ 1.8V ⎞ R1 = 196k • ⎜ − 1⎟ ⎝ 0.6V ⎠ A value of 392k will be selected for R1.
PAGE 26
LTC3614 TYPICAL APPLICATIONS General Purpose Buck Regulator with Fast Compensation and Improved Step Response, 2.25MHz VIN 2.25V TO 5.5V 10μF s4 RF 24Ω CF 1μF RSS 4.7M CSS 10nF R4 100k RC 43k CC 220pF PGOOD CC1 10pF R5A 1M SVIN PVIN RUN TRACK/SS SRLIM/DDR RT/SYNC LTC3614 SW PGOOD SGND ITH PGND MODE VFB R2 196k R5B 1M L1 0.33μH CO2 100μF VOUT 1.
PAGE 27
LTC3614 TYPICAL APPLICATIONS Master and Slave for Coincident Tracking Outputs Using a 1MHz External Clock VIN 2.25V TO 5.5V 22μF s4 4.7M 10nF RF1 24Ω CF1 1μF 1MHz CLOCK R5 100k RC1 15k CC1 470pF PGOOD CC2 10pF 1M SVIN PVIN RUN TRACK/SS SRLIM/DDR RT/SYNC LTC3614 SW PGOOD SGND ITH PGND MODE VFB R2 357k 1M L1 0.68μH CHANNEL 1 MASTER CO12 100μF R1 715k VOUT1 1.
PAGE 28
LTC3614 PACKAGE DESCRIPTION UDD Package 24-Lead Plastic QFN (3mm × 5mm) (Reference LTC DWG # 05-08-1833) 0.70 ±0.05 3.50 ± 0.05 2.10 ± 0.05 3.65 ± 0.05 1.50 REF 1.65 ± 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 0.75 ± 0.05 3.00 ± 0.10 1.50 REF 23 R = 0.05 TYP PIN 1 NOTCH R = 0.20 OR 0.25 s 45° CHAMFER 24 0.40 ± 0.10 PIN 1 TOP MARK (NOTE 6) 1 2 3.65 ± 0.10 5.00 ± 0.10 3.
PAGE 29
LTC3614 REVISION HISTORY REV DATE DESCRIPTION PAGE NUMBER A 11/10 Load Regulation ITH Voltage updated to the Electrical Characteristics table. 3, 11, 12 Note 2 updated to the Electrical Characteristics section. 4 Text updated to the Soft-Start section in the Applications Information section. 19 Related Parts table updated. 30 3614fa Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use.
PAGE 30
LTC3614 TYPICAL APPLICATION DDR Termination With Ratiometric Tracking of VDD, 1MHz VIN 3.3V VDD 1.8V VDD C1 22μF s4 R6 562k R7 187k Ratiometric Start-Up R3 100k R8 365k PGOOD SVIN RUN TRACK/SS RT/SYNC CC 2.2nF R5 1M L1 0.33μH PGOOD CC1 10pF ITH MODE L1: COILCRAFT DO3316T VTT 500mV/DIV SRLIM/DDR LTC3614 RC 6k R4 1M PVIN SW C4 100μF SGND PGND VTT 0.