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LTC3561A

3561af

TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

1A, 4MHz, Synchronous

Step-Down DC/DC Converter

The LTC

3561A is a constant frequency, synchronous

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

applications, it operates from a 2.5V to 5.5V input voltage

range and has a user conﬁ gurable operating frequency up

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

inductors 1mm or less in height. The output voltage is

adjustable from 0.8V to 5.5V. Internal synchronous power

switches provide high efﬁ ciency. The LTC3561A’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).

In shutdown, the device draws <1μA.

Step-Down 2.5V/1A Regulator

■

Uses Tiny Capacitors and Inductor

■

High Frequency Operation: Up to 4MHz

■

Low R

DS(ON)

Internal Switches: 0.15Ω

■

High Efﬁ ciency: Up to 96%

■

Stable with Ceramic Capacitors

■

Current Mode Operation for Excellent Line

and Load Transient Response

■

Short-Circuit Protected

■

Low Dropout Operation: 100% Duty Cycle

■

Low Shutdown Current: I

≤ 1μA

■

Low Quiescent Current: 330μA

■

Output Voltages from 0.8V to 5V

■

: 2.5V to 5.5V

■

Small 8-Lead 3mm × 3mm DFN Package

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Notebook Computers

■

Digital Cameras

■

Cellular Phones

■

Handheld Instruments

■

Board Mounted Power Supplies

Efﬁ ciency and Power Loss vs Output Current

, 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, 6580258, 6498466, 6611131.

OUTPUT CURRENT (mA)

110

EFFICIENCY (%)

POWER LOSS (W)

100 1

0.1

0.01

0.001

0.0001

1000100 10000

3561A TA01b

= 2.7V

= 3.6V

= 4.2V

OUT

= 2.5V

= 1MHz

LTC3561A

SHDN/R

PGNDSGND

2.2μH

OUT

2.5V

2.5V TO 5.5V

249k

22pF

118k

680pF

3561a TA01a

22μF

16.9k

10μF

549k

Summary of content (20 pages)

PAGE 1
LTC3561A 1A, 4MHz, Synchronous Step-Down DC/DC Converter FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTION The LTC®3561A is a constant frequency, synchronous step-down DC/DC converter. Intended for medium power applications, it operates from a 2.5V 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 1mm or less in height. The output voltage is adjustable from 0.8V to 5.5V.
PAGE 2
LTC3561A ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) PVIN, SVIN Voltages ..................................... –0.3V to 6V VFB, ITH, SHDN/RT Voltages ..........–0.3V to (VIN + 0.3V) SW Voltage ..................................–0.3V to (VIN + 0.3V) Operating Junction Temperature Range (Notes 2, 5, 8) ........................................ –40°C to 125°C Storage Temperature Range................... –65°C to 125°C Lead Temperature (Soldering, 10 sec) ..................
PAGE 3
LTC3561A ELECTRICAL CHARACTERISTICS The ● denotes the speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C. VIN = 3.6V, RT = 125k unless otherwise speciﬁed. (Note 2) SYMBOL PARAMETER IS fOSC Input DC Supply Current (Note 4) Active Mode Shutdown Shutdown Threshold High Active Oscillator Resistor Oscillator Frequency ILIM Peak Switch Current Limit RT = 125k (Note 7) VFB = 0.
PAGE 4
LTC3561A TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, VIN = 3.6V, fO = 1MHz, unless otherwise noted. Efﬁciency vs Input Voltage 100 VOUT = 1.8V IOUT = 100mA EFFICIENCY (%) EFFICIENCY (%) IOUT = 1A 85 80 75 VOUT = 1.8V 90 90 IOUT = 10mA 70 65 80 70 70 60 50 40 30 20 55 10 2.5 3.0 4.0 4.5 3.5 INPUT VOLTAGE (V) 5.0 100 1000 10 OUTPUT CURRENT (mA) 1 VIN = 2.7V VIN = 3.6V VIN = 4.2V 10 0 10000 100 1000 10 OUTPUT CURRENT (mA) 1 10000 3561A G03 Load Regulation 0.4 VOUT = 1.
PAGE 5
LTC3561A TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, VIN = 3.6V, fO = 1MHz, unless otherwise noted. 0.30 0.20 0.25 RDS(ON) (Ω) RDS(ON) (Ω) MAIN SWITCH 0.15 SYNCHRONOUS SWITCH 300 DYNAMIC SUPPLY CURRENT (μA) 0.25 0.10 Dynamic Supply Current vs Input Voltage RDS(ON) vs Temperature RDS(ON) vs Input Voltage 0.20 MAIN SWITCH 0.15 SYNCHRONOUS SWITCH 0.10 0.05 0.05 4.5 4.0 3.5 INPUT VOLTAGE (V) 3.0 5.0 0.0 –50 5.
PAGE 6
LTC3561A TYPICAL PERFORMANCE CHARACTERISTICS TA = 25°C, VIN = 3.6V, fO = 1MHz, unless otherwise noted. Start-Up from Shutdown Load Step Load Step SHDN/RT 2V/DIV VOUT 100mV/DIV AC COUPLED VOUT 100mV/DIV AC COUPLED VOUT 1V/DIV IL 1A/DIV IL 1A/DIV ILOAD 1A/DIV ILOAD 1A/DIV IL 1A/DIV VIN = 3.6V VOUT = 1.8V ILOAD = 1A 200μs/DIV 3561A G21 3561A G23 40μs/DIV VIN = 3.6V VOUT = 1.8V ILOAD = 0A to 1A Load Step 40μs/DIV VIN = 3.6V VOUT = 1.
PAGE 7
LTC3561A PIN FUNCTIONS SHDN/RT (Pin 1): Combination Shutdown and Timing Resistor Pin. The oscillator frequency is programmed by connecting a resistor from this pin to ground. Forcing this pin to SVIN causes the device to be shut down. In shutdown all functions are disabled. SVIN (Pin 6): The Signal Power Pin. All active circuitry is powered from this pin. Must be closely decoupled to SGND. SVIN must be greater than or equal to PVIN.
PAGE 8
LTC3561A BLOCK DIAGRAM 0.
PAGE 9
LTC3561A OPERATION The LTC3561A uses a constant frequency, current mode architecture. The operating frequency is determined by the value of the RT resistor. VFB decrease slightly. This decrease in VFB causes the error ampliﬁer to increase the ITH voltage until the average inductor current matches the new load current. The output voltage is set by an external divider returned to the VFB pin. An error ampliﬁer compares the divided output voltage with the reference voltage of 0.
PAGE 10
LTC3561A APPLICATIONS INFORMATION A general LTC3561A 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. 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.
PAGE 11
LTC3561A APPLICATIONS INFORMATION size requirements and any radiated ﬁeld/EMI requirements than on what the LTC3561A requires to operate. Table 1 shows some typical surface mount inductors that work well in LTC3561A applications. Table 1. Representative Surface Mount Inductors MANUFACTURER PART NUMBER MAX DC VALUE CURRENT Toko A914BYW-1R2M=P3: D52LC 1.2μH A960AW-1R2M=P3: D518LC Coilcraft Sumida Taiyo Yuden DCR HEIGHT 2.15A 44mΩ 2mm 1.2μH 1.8A 46mΩ 1.8mm DB3015C-1068AS-1R0N 1.0μH 2.
PAGE 12
LTC3561A APPLICATIONS INFORMATION is adequate for ﬁltering. The output ripple (ΔVOUT) is determined by: ⎛ 1 ⎞ ΔVOUT ≈ ΔIL ⎜ESR + ⎟ 8fO COUT ⎠ ⎝ where fO = operating frequency, COUT = output capacitance and ΔIL = ripple current in the inductor. The output ripple is highest at maximum input voltage since ΔIL increases with input voltage. With ΔIL = 0.
PAGE 13
LTC3561A APPLICATIONS INFORMATION components and the output capacitor size. Typically, 3 to 4 cycles are required to respond to a load step, but only in the ﬁrst cycle does the output drop linearly. The output droop, VDROOP, is usually about 2 to 3 times the linear drop of the ﬁrst cycle. Thus, a good place to start is with the output capacitor value of approximately: COUT ≈ 2.5 ΔIOUT fO • VDROOP More capacitance may be required depending on the duty cycle and load step requirements.
PAGE 14
LTC3561A APPLICATIONS INFORMATION Checking Transient Response The OPTI-LOOP® compensation allows the transient response to be optimized for a wide range of loads and output capacitors. The availability of the ITH pin not only allows optimization of the control loop behavior but also provides a DC coupled and AC ﬁltered closed loop response test point. The DC step, rise time and settling time at this test point truly reﬂects the closed loop response.
PAGE 15
LTC3561A APPLICATIONS INFORMATION does decrease due to the decreasing voltage across the inductor. Applications that require large load step capability near dropout should use a different topology such as SEPIC, Zeta or single inductor, positive buck/boost. In some applications, a more severe transient can be caused by switching in loads with large (>1μF) input capacitors. The discharged input capacitors are effectively put in parallel with COUT, causing a rapid drop in VOUT.
PAGE 16
LTC3561A APPLICATIONS INFORMATION Thermal Considerations In a majority of applications, the LTC3561A does not dissipate much heat due to its high efﬁciency. However, in applications where the LTC3561A 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 17
LTC3561A APPLICATIONS INFORMATION 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. 1. Does the capacitor CIN connect to the power VIN (Pin 5) and power GND (Pin 4) as close as possible? This capacitor provides the AC current to the internal power MOSFETs and their drivers. 2.
PAGE 18
LTC3561A TYPICAL APPLICATION General Purpose Buck Regulator Using Ceramic Capacitors VIN 2.5V TO 5.5V C1 22μF PVIN SVIN PGND L1 2.2μH LTC3561A VOUT 1.2V/1.5V/1.8V AT 1A SW R2 249k VFB ITH SHDN/RT 1.8V SGND 1.5V 1.2V PGND C2 22μF C4 22pF R3 16.
PAGE 19
LTC3561A PACKAGE DESCRIPTION DD Package 8-Lead Plastic DFN (3mm × 3mm) (Reference LTC DWG # 05-08-1698) 0.675 ±0.05 3.5 ±0.05 1.65 ±0.05 2.15 ±0.05 (2 SIDES) PACKAGE OUTLINE 0.25 ± 0.05 0.50 BSC 2.38 ±0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 3.00 ±0.10 (4 SIDES) R = 0.115 TYP 5 0.38 ± 0.10 8 1.65 ± 0.10 (2 SIDES) PIN 1 TOP MARK (NOTE 6) (DD) DFN 1203 0.200 REF 0.75 ±0.05 4 0.25 ± 0.05 1 0.50 BSC 0.00 – 0.05 2.38 ±0.10 (2 SIDES) BOTTOM VIEW—EXPOSED PAD NOTE: 1.
PAGE 20
LTC3561A TYPICAL APPLICATIONS 1mm Height, 2MHz, Li-Ion to 1.8V Converter VIN 2.5V TO 4.2V L1 0.9μH PVIN C1 10μF SVIN SW C4 22pF LTC3561A ITH R3 16.9k SGND PGND C3 470pF C2 10μF s2 VOUT 1.8V AT 1A VFB SHDN/RT R4 178k R2 R1 200k 249k 3561A TA04a C1, C2: TAIYO YUDEN JMK107BJ106MA L1: FDK MIPW3226DORGM Efﬁciency vs Output Current 100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 VOUT 100mV/DIV AC COUPLED VOUT 100mV/DIV AC COUPLED IL 1A/DIV IL 1A/DIV ILOAD 1A/DIV ILOAD 1A/DIV VIN = 2.7V VIN = 3.