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LTC3549

3549f

LOAD CURRENT (mA)

EFFICIENCY (%)

100

0.1 10 100 1k

3549 TA02

0.1

0.01

0.001

0.0001

= 1.8V

= 2.5V

= 3.1V

POWER LOST

AT V

= 2.5V

POWER LOSS (W)

250mA Low V

Buck

Regulator in 2mm × 3mm DFN

The LTC

3549 is a high efﬁ ciency, monolithic synchronous

buck regulator using a constant-frequency, current mode

architecture. The output voltage is adjusted via an external

resistor divider.

A ﬁ xed switching frequency of 2.25MHz is supported.

This switching frequency allows the use of small surface

mount inductors and capacitors, including ceramics.

Supply current during Burst Mode operation is only 50µA

dropping to < 1µA in shutdown. The 1.6V to 5.5V input

voltage range makes the LTC3549 ideally suited for single

cell Li-Ion, Li-Metal and 2-cell alkaline, NiCd or NiMH

battery-powered applications. 100% duty cycle capabil-

ity provides low dropout operation, extending battery

life in portable systems. Burst Mode operation can be

user-enabled, increasing efﬁ ciency at light loads, further

extending battery life.

The internal synchronous switch increases efﬁ ciency and

eliminates the need for an external Schottky diode. Internal

soft-start offers controlled output voltage rise time at start-

up without the need for external components.

■

2 AA-Cell Applications

■

Cellular Phones

■

Digital Cameras

■

MP3 Players

■

1.6V to 5.5V Input Voltage Range

■

Internal Soft-Start

■

Low Ripple Burst Mode

Operation

Output Ripple: <20mV

P-P

: 50µA

■

2.25MHz Constant-Frequency Operation

■

High Efﬁ ciency: Up to 93%

■

250mA Output Current (V

= 1.8V, V

OUT

= 1.2V)

■

450mA Peak Inductor Current

■

No Schottky Diode Required

■

Low Dropout Operation: 100% Duty Cycle

■

0.611V Reference Voltage

■

Stable with Ceramic Capacitors

■

Shutdown Mode Draws <1µA Supply Current

■

Current Mode Operation for Excellent Line and Load

Transient Response

■

Overtemperature Protection

■

Available in a Low Proﬁ le (0.75mm) 6-Lead

(2mm x 3mm) DFN Package

High Efﬁ ciency Step-Down Converter

APPLICATIO S

FEATURES

DESCRIPTIO

TYPICAL APPLICATIO

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

Burst Mode is a registered trademark of Linear Technology Corporation. All other

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

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

RUN

MODE

GND

LTC3549

3.3µH*

OUT

1.5V

200k

4.7µF

CER

*TDK VLF3012AT-3R3MR87

1.8V TO

5.5V

137k

3549 TA01

22pF

4.7µF

CER

Burst Mode Efﬁ ciency, V

OUT

= 1.5V

Summary of content (16 pages)

PAGE 1
LTC3549 250mA Low VIN Buck Regulator in 2mm × 3mm DFN U DESCRIPTIO FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ 1.6V to 5.5V Input Voltage Range Internal Soft-Start Low Ripple Burst Mode® Operation Output Ripple: <20mVP-P IQ: 50µA 2.25MHz Constant-Frequency Operation High Efﬁciency: Up to 93% 250mA Output Current (VIN = 1.8V, VOUT = 1.2V) 450mA Peak Inductor Current No Schottky Diode Required Low Dropout Operation: 100% Duty Cycle 0.
PAGE 2
LTC3549 U W W W ABSOLUTE AXI U RATI GS U W U PACKAGE/ORDER I FOR ATIO (Note 1) VFB RUN MODE TOP VIEW Input Supply Voltage ................................... –0.3V to 6V RUN, VFB, MODE Voltages .............–0.3V to (VIN + 0.3V) SW Voltage < 100ns Pulse .............–0.3V to (VIN + 0.3V) Operating Temperature Range (Note 2) ... –40°C to 85°C Junction Temperature (Note 3) ........................... 125°C Storage Temperature Range..................
PAGE 3
LTC3549 ELECTRICAL CHARACTERISTICS The ● denotes the speciﬁcations which apply over the full operating temperature range, otherwise speciﬁcations are at TA = 25°C, VIN = 2.2V. SYMBOL PARAMETER CONDITIONS MIN IS Input DC Bias Current Active Mode Sleep Mode Shutdown (Note 5) VOUT = 90%, ILOAD = 0A VOUT = 103%, ILOAD = 0A VRUN = 0V, VIN = 5.5V ● MAX UNITS 300 50 0.1 475 95 5 µA µA µA 2.25 2.
PAGE 4
LTC3549 U W TYPICAL PERFOR A CE CHARACTERISTICS (From Typical Application on the front page, except for the resistive divider resistor values) Efﬁciency/Power Loss vs Load Current, VOUT = 1.5V, Burst Mode 100 Efﬁciency/Power Loss vs Load Current, VOUT = 1.5V, Pulse Skip 100 1 40 30 EFFICIENCY (%) 50 0.01 40 30 0.001 20 60 20 10 10 1 10 100 LOAD CURRENT (mA) EFFICIENCY VIN = 1.8V VIN = 2.5V VIN = 3.1V 0 0.1 0.0001 1000 3549 G01a 80 80 70 70 EFFICIENCY (%) 90 60 50 40 20 10 10 3.5 4.
PAGE 5
LTC3549 U W TYPICAL PERFOR A CE CHARACTERISTICS (From Typical Application on the front page, except for the resistive divider resistor values) Oscillator Frequency vs Temperature Oscillator Frequency Shift vs Input Voltage VIN = 2.7V VIN = 1.6V 2.25 2.20 VIN = 4.2V 2.15 2.10 –25 0 25 50 75 TEMPERATURE (°C) 100 2.0 1.204 1.5 OUTPUT VOLTAGE (V) OSCILLATOR FREQUENCY SHIFT (%) 1.0 0.5 0 3549 G08 DYNAMIC SUPPLY CURRENT—PULSE SKIP (µA) RDS(ON) (Ω) MAIN SWITCH 0.4 SYNCHRONOUS SWITCH 0.65 0.
PAGE 6
LTC3549 U W TYPICAL PERFOR A CE CHARACTERISTICS (From Typical Application on the front page, except for the resistive divider resistor values) Start-Up from Shutdown Burst Mode Operation Burst Mode Operation ILOAD = 20mA VOUT 20mV/DIV AC COUPLED VOUT 500mV/DIV VSWITCH 2V/DIV VRUN 1V/DIV IL 100mA/DIV IL 50mA/DIV 3549 G19 3549 G16 VIN = 3.6V VOUT = 1.2V 1kΩ Load 200µs/DIV VIN = 2.5V VOUT = 1.
PAGE 7
LTC3549 U U U PI FU CTIO S VIN (Pin 1): Main Supply Pin. Must be closely decoupled to GND, Pins 2 and 7, with a 4.7µF or greater ceramic capacitor. GND (Pin 2): N/C. Ground this pin. SW (Pin 3): Switch Node Connection to Inductor. This pin connects to the drains of the internal main and synchronous power MOSFET switches. MODE (Pin 4): Mode Select Input. To select pulse-skipping mode, force this pin above 1.1V. Forcing this pin below 0.3V selects Burst Mode operation. Do not leave MODE ﬂoating.
PAGE 8
LTC3549 U OPERATIO Main Control Loop The LTC3549 uses a constant-frequency, current mode step-down architecture. Both the main (P-channel MOSFET) and synchronous (N-channel MOSFET) switches are internal. During normal operation, the internal top power MOSFET is turned on each cycle when the oscillator sets the RS latch, and turned off when the current comparator, ICOMP, resets the RS latch. The peak inductor current at which ICOMP resets the RS latch is controlled by the output of error ampliﬁer EA.
PAGE 9
LTC3549 OPERATION be deﬁned by the combination of the current needed to charge the output capacitance and the current provided to the load as the output voltage ramps up. The start-up waveform, shown in the Typical Performance Characteristics, shows the output voltage start-up from 0V to 1.2V with a 1kΩ load and VIN = 3.6V. U W U U APPLICATIO S I FOR ATIO The basic LTC3549 application circuit is shown on the ﬁrst page of this data sheet.
PAGE 10
LTC3549 U W U U APPLICATIO S I FOR ATIO tions do not offer much relief. Note that the capacitor manufacturer’s ripple current ratings are often based on 2000 hours of life. This makes it advisable to further derate the capacitor, or choose a capacitor rated at a higher temperature than required. Always consult the manufacturer if there is any question. The selection of COUT is driven by the required effective series resistance (ESR).
PAGE 11
LTC3549 U W U U APPLICATIO S I FOR ATIO 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 12
LTC3549 U W U U APPLICATIO S I FOR ATIO Thermal Considerations Checking Transient Response In most applications the LTC3549 does not dissipate much heat due to its high efﬁciency. But, in applications where the LTC3549 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 13
LTC3549 U W U U APPLICATIO S I FOR ATIO Board Layout Considerations When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC3549. These items are also illustrated graphically in the layout diagram of Figure 3. Check the following in your layout. 1. Does the capacitor CIN connect to the power VIN (Pin 1) and GND (Pins 2, 7) as close as possible? This capacitor provides the AC current to the internal power MOSFETs and their drivers. 2.
PAGE 14
LTC3549 U U W U APPLICATIO S I FOR ATIO Design Example Substituting VOUT = 1.5V, VIN = 3.1V, ΔIL = 100mA and As a design example, assume the LTC3549 is used in a 2-alkaline cell battery-powered application. The VIN will be operating from a maximum of 3.1V down to about 1.8V. The load current requirement is a maximum of 250mA but most of the time it will be in standby mode, requiring only 2mA. Efﬁciency at both low and high load currents is important. Output voltage is 1.5V.
PAGE 15
LTC3549 U PACKAGE DESCRIPTIO DCB Package 6-Lead Plastic DFN (2mm × 3mm) (Reference LTC DWG # 05-08-1715) 0.70 ±0.05 3.55 ±0.05 1.65 ±0.05 (2 SIDES) 2.15 ±0.05 PACKAGE OUTLINE 0.25 ± 0.05 0.50 BSC 1.35 ±0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS R = 0.115 TYP R = 0.05 TYP 2.00 ±0.10 (2 SIDES) 3.00 ±0.10 (2 SIDES) 0.40 ± 0.10 4 6 1.65 ± 0.10 (2 SIDES) PIN 1 NOTCH R0.20 OR 0.25 × 45° CHAMFER PIN 1 BAR TOP MARK (SEE NOTE 6) 3 0.200 REF 0.75 ±0.05 1 (DCB6) DFN 0405 0.25 ± 0.05 0.
PAGE 16
LTC3549 U TYPICAL APPLICATIO L1, 4.7µH* VIN 2.5V TO 4.2V RUN CIN 4.7µF CERAMIC SW LTC3549 CL, 22pF VIN VFB MODE GND *TDK VLF3012AT-4R7MR74 VOUT 1.8V COUT 4.7µF CERAMIC R1, 267k R2 137k 3549 TA03a Efﬁciency vs Load Current Load Step Response 100 90 80 VIN = 2.5V VIN = 3.6V VIN = 4.2V 70 EFFICIENCY (%) VOUT 100mV/DIV AC COUPLED 60 ILOAD 200mA/DIV 50 40 IL 200mA/DIV 30 20 3549 TA03c 20µs/DIV VIN = 3.6V VOUT = 1.8V ILOAD = 100mA to 250mA 10 VOUT = 1.8V 0 1 0.