Datasheet

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LTC1911-1.5/LTC1911-1.8

1911f

APPLICATIO S

FEATURES

DESCRIPTIO

TYPICAL APPLICATIO

Low Noise, High Efficiency,

Inductorless Step-Down

DC/DC Converter

The LTC

1911 is a switched capacitor step-down DC/DC

converter that produces a 1.5V or 1.8V regulated output

from a 2.7V to 5.5V input. The part uses switched capaci-

tor fractional conversion to achieve high efficiency over the

entire input range. No inductors are required. Internal cir-

cuitry controls the step-down conversion ratio to optimize

efficiency as the input voltage and load conditions vary.*

Typical efficiency is over 25% higher than that of a linear

regulator.

A unique constant frequency architecture provides a low

noise regulated output as well as lower input noise than

conventional charge pump regulators. High frequency

operation (f

OSC

= 1.5MHz) simplifies output filtering to

further reduce conducted noise. To optimize efficiency,

the part enters Burst Mode

operation under light load

conditions.

Low operating current (180µA with no load, 10µA in

shutdown) and low external parts count (two 1µF flying

capacitors and two 10µF bypass capacitors) make the

LTC1911 ideally suited for space constrained battery-

powered applications. The part is short-circuit and

overtemperature protected, and is available in an 8-pin

MSOP package.

■

Low Noise Constant Frequency Operation

■

2.7V to 5.5V Input Voltage Range

■

No Inductors

■

Typical Efficiency 25% Higher Than LDOs

■

Shutdown Disconnects Load from V

■

Output Voltage: 1.8V ±4% or 1.5V ±4%

■

Output Current: 250mA

■

Low Operating Current: I

= 180µA Typ

■

Low Shutdown Current: I

= 10µA Typ

■

Oscillator Frequency: 1.5MHz

■

Soft-Start Limits Inrush Current at Turn On

■

Short-Circuit and Overtemperature Protected

■

Available in an 8-Pin MSOP Package

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

■

Handheld Computers

■

Cellular Phones

■

Smart Card Readers

■

Portable Electronic Equipment

■

Handheld Medical Instruments

■

Low Power DSP Supplies

–

GND

SS/SHDN

OUT

–

LTC1911-1.8

1µF*

1-CELL Li-Ion

3-CELL NiMH

10µF*

*CERAMIC CAPACITOR

10µF*

1911 TA01

OUT

= 1.8V

OUT

= 250mA

1µF*

2.7V TO 5.5V INPUT

Single Cell Li-Ion to 1.8V DC/DC Converter

Efficiency

Burst Mode is a registered trademark of Linear Technology Corporation.

*U.S. Patent #6,438,005

INPUT VOLTAGE (V)

EFFICIENCY (%)

3456

1911 G05

IDEAL LDO

250mA100mA

OUT

= 1.8V

Summary of content (12 pages)

PAGE 1
LTC1911-1.5/LTC1911-1.8 Low Noise, High Efficiency, Inductorless Step-Down DC/DC Converter U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO The LTC®1911 is a switched capacitor step-down DC/DC converter that produces a 1.5V or 1.8V regulated output from a 2.7V to 5.5V input. The part uses switched capacitor fractional conversion to achieve high efficiency over the entire input range. No inductors are required.
PAGE 2
LTC1911-1.5/LTC1911-1.8 W W W AXI U U ABSOLUTE RATI GS U U W PACKAGE/ORDER I FOR ATIO (Note 1) VIN to GND ...................................................– 0.3V to 6V SS/SHDN to GND ........................ – 0.3V to (VIN + 0.3V) VOUT Short-Circuit Duration ............................ Indefinite Operating Temperature Range (Note 2) .. – 40°C to 85°C Storage Temperature Range ................. – 40°C to 150°C Lead Temperature (Soldering, 10 sec)..................
PAGE 3
LTC1911-1.5/LTC1911-1.8 U W TYPICAL PERFOR A CE CHARACTERISTICS Input Operating Current vs Input Voltage Input Shutdown Current vs Input Voltage 210 15 TA = 25°C 180 170 TA = –40°C TA = 25°C 11 TA = –40°C 9 150 4 5 6 5 2 3 INPUT VOLTAGE (V) 1.80 1.75 5 6 2 4 3 5 1911 G03 1911 G02 LTC1911-1.8 Efficiency vs Output Current LTC1911-1.5 Efficiency vs Input Voltage (Falling Input Voltage) 1.55 90 100 IOUT = 250mA TA = –40°C TA = 25°C 1.53 TA = 85°C 90 80 100mA 1.
PAGE 4
LTC1911-1.5/LTC1911-1.8 U W TYPICAL PERFOR A CE CHARACTERISTICS Start-Up Time vs Soft-Start Capacitor 1.60 25 1 TA = –40°C 20 15 COUT = 10µF 10 0 100 TA = 85°C 1.40 0 100 150 200 250 50 OUTPUT LOAD CURRENT (mA) 300 2.5 3.0 3.5 4.0 VIN (V) 4.5 LTC1911-1.8 Output Voltage Ripple VOUT 50mV/DIV 2-TO-1 MODE VIN = 5V VOUT 50mV/DIV 3-TO-2 MODE VIN = 3.6V VOUT 50mV/DIV 1-TO-1 MODE VIN = 2.7V Output Current Transient Response 5.
PAGE 5
LTC1911-1.5/LTC1911-1.8 W W SI PLIFIED BLOCK DIAGRA 1 RA VIN CIN 300k C1+ + 7 C1 C1– – 50k STEP-DOWN CHARGE PUMP MODE CONTROL C2 + + 5 2 150k C2 – – SHDN RSENSE 3 6 + + ADJ OFFSET VOUT C2 COUT AMP1 – – COMP1 VREF – + BURST THRESHOLD 60k COMP2 + OVERTEMP DETECT – SHORT-CIRCUIT THRESHOLD – 1.5MHz OSCILLATOR AMP2 + VIN – 2µA 600mV + 8 SS/SHDN 600mV VREF RAMP + 1.
PAGE 6
LTC1911-1.5/LTC1911-1.8 U W U U APPLICATIO S I FOR ATIO General Operation Step-Down Charge Transfer Operation The LTC1911 uses a switch capacitor-based DC/DC conversion to provide the efficiency advantages associated with inductor-based circuits as well as the cost and simplicity advantages of a linear regulator. The LTC1911’s unique constant frequency architecture provides a low noise regulated output as well as lower input noise than conventional switch-capacitor charge pump regulators.
PAGE 7
LTC1911-1.5/LTC1911-1.8 U W U U APPLICATIO S I FOR ATIO transferred to the parallel combination of C1 and C2 is transferred to the VOUT. In this manner, charge is again transferred from the flying capacitors to the output on both phases of the clock. As in 2-to-1 mode, charge current is sourced from GND on phase two of the clock resulting in increased power efficiency. IOUT in 3-to-2 mode equals approximately (3/2)IIN.
PAGE 8
LTC1911-1.5/LTC1911-1.8 U W U U APPLICATIO S I FOR ATIO 4.7nF capacitor on SS/SHDN results in a 3ms ramp time from 0.6V to 1.9V on the pin. If COUT is 10µF, the 3ms VREF ramp time results in an average COUT charge current of only 6mA (see Figure 2). VOUT 6 COUT LTC1911 8 ON OFF VCTRL SS/SHDN CSS (2a) VCTRL 2V/DIV RLOAD Low Current Burst Mode Operation To improve efficiency at low output currents, a Burst Mode function was included in the design of the LTC1911.
PAGE 9
LTC1911-1.5/LTC1911-1.8 U W U U APPLICATIO S I FOR ATIO To reduce output noise and ripple, it is suggested that a low ESR (≤ 0.1Ω) ceramic capacitor (10µF or greater) be used for COUT. Tantalum and Aluminum capacitors are not recommended because of their high ESR (equivalent series resistance). less than 1µF but the increasing input noise will feed through to the output causing degraded performance. For best performance a 1µF or greater capacitor is suggested for CIN.
PAGE 10
LTC1911-1.5/LTC1911-1.8 U W U U APPLICATIO S I FOR ATIO is needed to ensure that minimum capacitance values are met over operating temperature and bias voltage. Additional output filtering can be achieved by placing a second output capacitor, connected to the ground plane, about 2cm or more from the LTC1911 output capacitor (C4). The inductance of the trace running to the second output capacitor will significantly attenuate the high speed switching transients of the LTC1911.
PAGE 11
LTC1911-1.5/LTC1911-1.8 U PACKAGE DESCRIPTIO MS8 Package 8-Lead Plastic MSOP (Reference LTC DWG # 05-08-1660) 0.889 ± 0.127 (.035 ± .005) 5.23 (.206) MIN 3.2 – 3.45 (.126 – .136) 0.42 ± 0.04 (.0165 ± .0015) TYP 3.00 ± 0.102 (.118 ± .004) (NOTE 3) 0.65 (.0256) BSC 8 7 6 5 0.52 (.206) REF RECOMMENDED SOLDER PAD LAYOUT 0.254 (.010) 3.00 ± 0.102 (.118 ± .004) NOTE 4 4.88 ± 0.1 (.192 ± .004) DETAIL “A” 0° – 6° TYP GAUGE PLANE 0.53 ± 0.015 (.021 ± .006) DETAIL “A” 1 2 3 4 1.10 (.043) MAX 0.
PAGE 12
LTC1911-1.5/LTC1911-1.8 U TYPICAL APPLICATIO DC/DC Converter with Shutdown and Soft-Start LTC1911-1.5 2.7V TO 5.5V INPUT 1-CELL Li-Ion OR 3-CELL NiMH 10µF* 1µF* 1 2 3 4 VIN SS/SHDN C2+ C1+ C2– VOUT GND 8 7 6 10nF 1µF* 5 C1– VOUT = 1.5V IOUT = 250mA 2N7002 ON OFF 10µF* *CERAMIC CAPACITOR 1911 TA03 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LTC1514 50mA, 650kHz, Step-Up/Down Charge Pump with Low Battery Comparator VIN = 2.