Datasheet

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LTC3561

3561f

The LTC

3561 is a constant-frequency, synchronous,

step-down DC/DC converter. Intended for medium power

applications, it operates from a 2.63V to 5.5V input voltage

range and has a user configurable operating frequency up

to 4MHz, allowing the use of tiny, low cost capacitors and

inductors 2mm or less in height. The output voltage is

adjustable from 0.8V to 5V. Internal synchronous 0.11Ω

power switches with 1.4A peak current ratings provide

high efficiency. The LTC3561’s current mode architecture

and external compensation allow the transient response

to be optimized over a wide range of loads and output

capacitors.

To further maximize battery life, the P-channel MOSFET is

turned on continuously in dropout (100% duty cycle). The

no-load quiescent current is only 240µA. In shutdown, the

device draws <1µA.

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Wireless LAN Power

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Notebook Computers

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Digital Cameras

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Cellular Phones

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Board Mounted Power Supplies

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Uses Tiny Capacitors and Inductor

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High Frequency Operation: Up to 4MHz

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High Switch Current: 1.4A

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Low R

DS(ON)

Internal Switches: 0.110Ω

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High Efficiency: Up to 95%

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: 2.63V to 5.5V

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Stable with Ceramic Capacitors

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Current Mode Operation for Excellent Line

and Load Transient Response

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Short-Circuit Protected

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Low Dropout Operation: 100% Duty Cycle

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Low Shutdown Current: I

≤ 1µA

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Low Quiescent Current: 240µA

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Output Voltages from 0.8V to 5V

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Low Noise Pulse-Skipping Operation

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Small 8-Pin DFN Package

1A, 4MHz, Synchronous

Step-Down DC/DC Converter

Efficiency and Power Loss vs Load CurrentStep-Down 2.5V/1A Regulator

DESCRIPTIO

FEATURES

APPLICATIO S

TYPICAL APPLICATIO

LOAD CURRENT (mA)

EFFICIENCY (%)

POWER LOSS (mW)

100

1000

100

100 1000

3561 TA01

POWER LOSS

EFFICIENCY

= 3.3V

OUT

= 2.5V

= 1MHz

, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.

All other trademarks are the property of their respective owners. Protected by U.S. Patents,

including 5481178, 6127815, 6304066, 6498466, 6580258, 6611131.

LTC3561

SHDN/R

PGNDSGND

2.2µH

OUT

2.5V/1A

2.63V TO 5.5V

887k

412k

1000pF

3561 F01

22µF

13k

22µF

324k

NOTE: IN DROPOUT, THE OUTPUT TRACKS

THE INPUT VOLTAGE.

Summary of content (16 pages)

PAGE 1
LTC3561 1A, 4MHz, Synchronous Step-Down DC/DC Converter U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO The LTC®3561 is a constant-frequency, synchronous, step-down DC/DC converter. Intended for medium power applications, it operates from a 2.63V to 5.5V input voltage range and has a user configurable operating frequency up to 4MHz, allowing the use of tiny, low cost capacitors and inductors 2mm or less in height. The output voltage is adjustable from 0.8V to 5V. Internal synchronous 0.
PAGE 2
LTC3561 U W W W ABSOLUTE AXI U RATI GS U W U PACKAGE/ORDER I FOR ATIO (Note 1) TOP VIEW PVIN, SVIN Voltages .....................................– 0.3V to 6V VFB, SHDN/RT Voltages ................ – 0.3V to (VIN + 0.3V) ITH Voltage ................................................– 0.3V to 1.4V SW Voltage ................................... – 0.3V to (VIN + 0.3V) Operating Ambient Temperature Range (Note 2) ..................................................
PAGE 3
LTC3561 ELECTRICAL CHARACTERISTICS Note 8: This IC includes overtemperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed 125°C when overtemperature protection is active. Continuous operation above the specified maximum operating junction temperature may impair device reliability.
PAGE 4
LTC3561 U W TYPICAL PERFOR A CE CHARACTERISTICS Frequency Variation vs Temperature Efficiency vs Frequency 100 10 EFFICIENCY (%) 0 –2 –4 TA = 25°C 115 95 RDS(ON) (mΩ) REFERENCE VARIATION (%) 6 2 120 VIN = 3.3V VOUT = 2.5V IOUT = 500mA TA = 25°C 8 4 RDS(ON) vs VIN 90 –6 110 SYNCHRONOUS SWITCH 105 MAIN SWITCH 100 95 –8 –10 –50 85 –25 0 25 50 75 TEMPERATURE (°C) 100 125 0 1 2 3 FREQUENCY (MHz) 3561 G10 4 3561 G11 90 2.5 3 3.5 4 4.5 VIN (V) 5 5.
PAGE 5
LTC3561 W BLOCK DIAGRA 0.8V SVIN SGND ITH PVIN 6 2 8 5 PMOS CURRENT COMPARATOR VOLTAGE REFERENCE ITH LIMIT + + VFB 7 – ERROR AMPLIFIER – – + VB SLOPE COMPENSATION OSCILLATOR 3 SW + LOGIC NMOS COMPARATOR – + REVERSE COMPARATOR – 4 PGND 1 SHDN/RT 3561 BD U OPERATIO The LTC3561 uses a constant frequency, current mode architecture. The operating frequency is determined by the value of the RT resistor. The output voltage is set by an external divider returned to the VFB pin.
PAGE 6
LTC3561 U U W U APPLICATIO S I FOR ATIO A general LTC3561 application circuit is shown in Figure 4. External component selection is driven by the load requirement, and begins with the selection of the inductor L1. Once L1 is chosen, CIN and COUT can be selected. fO(MAX) ≈ 6.67 • (VOUT / VIN(MAX)) (MHz) The minimum frequency is limited by leakage and noise coupling due to the large resistance of RT.
PAGE 7
LTC3561 U U W U APPLICATIO S I FOR ATIO Table 1. Representative Surface Mount Inductors MANUFACTURER PART NUMBER Output Capacitor (COUT) Selection MAX DC VALUE CURRENT DCR HEIGHT Toko A914BYW-2R2M-D52LC 2.2µH 2.05A 49mΩ 2mm Coilcraft D01608C-222 2.2µH Coilcraft LP01704-222M 2.2µH 2.3A 70mΩ 3mm 2.4A 120mΩ 1mm Sumida CDRH2D18/HP-2R2 2.2µH 1.6A 48mΩ Taiyo Yuden N05DB2R2M 2.2µH 2.9A 32mΩ 2.8mm Murata LQN6C2R2M04 2.2µH 3.2A 24mΩ Cooper SD3112-2R2 2.2µH 1.1A 140mΩ 1.
PAGE 8
LTC3561 U W U U APPLICATIO S I FOR ATIO piezoelectric effects. In addition, the high Q of ceramic capacitors along with trace inductance can lead to significant ringing. Other capacitor types include the Panasonic specialty polymer (SP) capacitors. In most cases, 0.1µF to 1µF of ceramic capacitors should also be placed close to the LTC3561 in parallel with the main capacitors for high frequency decoupling.
PAGE 9
LTC3561 U U W U APPLICATIO S I FOR ATIO reduces surge currents from VIN by gradually increasing the peak inductor current. Power supply sequencing can also be accomplished using this pin. The LTC3561 has an internal digital soft-start which steps up a clamp on ITH over 1024 clock cycles, as can be seen in Figure 3. The soft-start time can be increased by ramping the voltage on ITH during start-up as shown in Figure 2(c).
PAGE 10
LTC3561 U U W U APPLICATIO S I FOR ATIO The output voltage settling behavior is related to the stability of the closed-loop system and will demonstrate the actual 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. produce the most improvement.
PAGE 11
LTC3561 U W U U APPLICATIO S I FOR ATIO looking into the SW pin is a function of both top and bottom MOSFET RDS(ON) and the duty cycle (DC) as follows: RSW = (RDS(ON)TOP)(DC) + (RDS(ON)BOT)(1 – DC) The RDS(ON) for both the top and bottom MOSFETs can be obtained from the Typical Performance Characteristics curves.
PAGE 12
LTC3561 U U W U APPLICATIO S I FOR ATIO For cost reasons, a ceramic capacitor will be used. COUT selection is then based on load step droop instead of ESR requirements. For a 5% output droop: C OUT ≈ 2.5 1A = 20µF 1MHz • (5%• 2.5V) The closest standard value is 22µF. Since the output impedance of a Li-Ion battery is very low, CIN is typically 10µF. In noisy environments, decoupling SVIN from PVIN with an R6/C8 filter of 1Ω/0.1µF may help, but is typically not needed.
PAGE 13
LTC3561 U TYPICAL APPLICATIO S Efficiency vs Load Current C1 10µF L1 2.2µH PVIN SW SVIN R2 887K LTC3561 ITH VFB 3.3V 2.5V 1.8V PGND C4 22pF R3 13k R4 324k C3 1000pF 90 80 SHDN/RT SGND 100 VOUT 1.8V/2.5V/3.3V AT 1A R1A 280k R1B 412k EFFICIENCY (%) VIN 2.63V TO 5.5V C2 22µF R1C 698k 70 60 50 40 30 20 VIN = 3.3V VOUT = 2.
PAGE 14
LTC3561 U TYPICAL APPLICATIO S All Ceramic 2-Cell to 3.3V and 1.8V Converters VIN = 2V TO 3V L1 4.7µH D1 VOUT 3.3V 120mA/1A C5 10µF LTC3402 VIN +2 CELLS SHDN 1M VOUT PVIN SVIN MODE/SYNC FB PGOOD C1 10µF SW ITH 604k VC C2 44µF (2 × 22µF) 1000pF RT 49.9k 0 = FIXED FREQ 1 = Burst Mode OPERATION GND 10pF 887k SGND PGND 324k 1000pF C1: TAIYO YUDEN JMK212BJ106MG C2: TAIYO YUDEN JMK325BJ226MM C5, C6: TAIYO YUDEN JMK325BJ226MM SW L2 2.
PAGE 15
LTC3561 U PACKAGE DESCRIPTIO DD Package 8-Lead Plastic DFN (3mm × 3mm) (Reference LTC DWG # 05-08-1698) R = 0.115 TYP 5 0.38 ± 0.10 8 0.675 ±0.05 3.5 ±0.05 1.65 ±0.05 2.15 ±0.05 (2 SIDES) 3.00 ±0.10 (4 SIDES) PACKAGE OUTLINE 1.65 ± 0.10 (2 SIDES) PIN 1 TOP MARK (NOTE 6) (DD8) DFN 1203 0.25 ± 0.05 0.200 REF 0.50 BSC 2.38 ±0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 0.75 ±0.05 0.00 – 0.05 4 0.25 ± 0.05 1 0.50 BSC 2.38 ±0.10 (2 SIDES) BOTTOM VIEW—EXPOSED PAD NOTE: 1.
PAGE 16
LTC3561 U TYPICAL APPLICATIO Efficiency vs Load Current 1mm Height, 2MHz, Li-Ion to 1.8V Converter 100 PVIN SW C1 10µF LTC3561 ITH R3 10k C7 47pF L1 1µH SVIN SGND PGND C4 10pF VOUT 1.8V AT 1A C2 2 x 10µF VFB R1 200k SHDN/RT R2 294k 80 3.6V 70 2.7V 60 4.2V R4 154k C3 1000pF VOUT = 1.8V 90 EFFICIENCY (%) VIN 2.63V TO 4.2V 50 3561 TA04 40 1 C1, C2: AVX 08056D106M L1: TDK VLF3010ATIR5NIRZ 10 100 1000 ILOAD (mA) 3561 TA05 RELATED PARTS PART NUMBER LTC1879 DESCRIPTION 1.