LINEAR TECHNOLOGY LT1946A Technical data

LT1946A

LOAD CURRENT (mA)

0

100 200 300 400 500

EFFICIENCY (%)

1946A TA01

90

85

80

75

70

65

60

55

50

2.7MHz Boost DC/DC

Converter with 1.5A Switch

and Soft-Start

FEATURES

■

1.5A, 36V Internal Switch

■

2.7MHz Switching Frequency

■

Integrated Soft-Start Function

■

Adjustable Output from VIN to 35V

■

Low V

■

12V at 430mA from a 5V Input

■

Small Thermally Enhanced 8-Lead MSOP Package

Switch: 300mV at 1.5A (Typical)

CESAT

U

APPLICATIO S

■

TFT-LCD Bias Supplies

■

GPS Receivers

■

DSL Modems

■

Local Power Supply

U

DESCRIPTIO

The LT®1946A is a fixed frequency step-up DC/DC con­verter containing an internal 1.5A, 36V switch. Capable of
generating 12V at 430mA from a 5V input, the LT1946A is
ideal for powering large TFT-LCD panels. The LT1946A
switches at 2.7MHz, allowing the use of tiny, low profile
inductors and low value ceramic capacitors. Loop com­pensation can be either internal or external, giving the user
flexibility in setting loop compensation and allowing opti­mized transient response with low ESR ceramic output
capacitors. Soft-start is controlled with an external capaci­tor which determines the input current ramp rate during
start up. The 8-lead MSOP package and high switching
frequency ensure a low profile overall solution less than

1.1mm high.

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

TYPICAL APPLICATIO

U

L1

V

IN

5V

OFF ON

C1

2.2µF

R

C

27.4k
C

C

270pF

C1: 2.2µF, X5R or X7R, 6.3V
C2: 2.2µF, X5R or X7R, 16V
D1: MICROSEMI UPS120 OR EQUIVALENT
L1: SUMIDA CR43-2R2
* EXPOSED PAD MUST ALSO BE GROUNDED

C

100nF

SS

2.2µH

65

V

IN

3

SHDN

LT1946A

1

V

C

GND*SS

Figure 1. 5V to 12V, 430mA Step-Up DC/DC Converter

SW

COMP

48

D1

R1
182k

2

FB

7

R2
21k

V
12V
430mA

C2

2.2µF

1946A TA01

OUT

Efficiency

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LT1946A

PACKAGE/ORDER I FOR ATIO

UU

W

WWWU

ABSOLUTE AXI U RATI GS

(Note 1)

VIN Voltage .............................................................. 16V

SW Voltage ................................................–0.4V to 36V

FB Voltage .............................................................. 2.5V

Current into FB Pin ............................................... ±1mA

SHDN Voltage .......................................................... 16V

Maximum Junction Temperature .......................... 125°C

Operating Temperature

Range (Note 2) ....................................... – 40°C to 85°C

Storage Temperature Range ................. –65°C to 150°C

Lead Temperature (Soldering, 10 sec)..................300°C

ORDER PART

TOP VIEW

V

1

C

2

FB

3

SHDN

4

GND

MS8E PACKAGE

8-LEAD PLASTIC MSOP

EXPOSED PAD IS GROUND

(MUST BE SOLDERED TO PCB)

T

= 125°C, θJA = 40°C/W,

JMAX

= 10°C/W

θ

JC

Consult LTC Marketing for parts specified with wider operating temperature ranges.

8
7
6
5

SS
COMP
V

IN

SW

NUMBER

LT1946AEMS8E

MS8E PART

MARKING

LTYZ

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifications are at TA = 25°C. VIN = 3V, V

PARAMETER CONDITIONS MIN TYP MAX UNITS

Minimum Operating Voltage 2.45 2.6 V
Maximum Operating Voltage 16 V
Feedback Voltage 1.23 1.25 1.27 V

FB Pin Bias Current VFB = 1.25V (Note 3) ● 20 120 nA
Error Amp Transconductance ∆I = 2µA40µmhos
Error Amp Voltage Gain 300 V/V
Quiescent Current V
Quiescent Current in Shutdown V
Reference Line Regulation 2.6V ≤ VIN ≤ 16V 0.01 0.05 %/V
Switching Frequency 2.4 2.7 3 MHz

Switching Frequency in Foldback VFB = 0V 0.85 MHz
Maximum Duty Cycle ● 73 80 %
Switch Current Limit (Note 4) ● 1.5 2.1 3.1 A
Switch V

CESAT

Switch Leakage Current VSW = 5V 0.01 1 µA
Soft-Start Charging Current VSS = 0.5V 2.5 4 6 µA
SHDN Input Voltage High 2.4 V
SHDN Input Voltage Low 0.5 V
SHDN Pin Bias Current V

Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.

Note 2: The LT1946AE is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over the –40°C to 85°C operating

= 2.5V, Not Switching 3.6 5 mA

SHDN

= 0V, VIN = 3V 0 1 µA

SHDN

ISW = 1A 240 340 mV

= 3V 16 32 µA

SHDN

= 0V 0 0.1 µA

V

SHDN

The ● denotes the specifications which apply over the full operating

= VIN unless otherwise noted. (Note 2)

SHDN

● 1.22 1.27 V

● 2.3 3.1 MHz

temperature range are assured by design, characterization and correlation
with statistical process controls.

Note 3: Current flows out of the FB pin.
Note 4: Current limit guaranteed by design and/or correlation to static test.

Current limit is independent of duty cycle and is guaranteed by design.

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TYPICAL PERFOR A CE CHARACTERISTICS

Feedback Pin Voltage Oscillator Frequency Current Limit

1.28

1.27

1.26

1.25

1.24

1.23

FEEDBACK VOLTAGE (V)

1.22

1.21

1.20
–50 –25 0 25 50 75 100 125

TEMPERATURE (°C)

1946A G01

Switch Saturation Voltage Quiescent Current

0.35

0.30

0.25

0.20

(V)

CESAT

0.15

V

0.10

0.05

0

0 0.2

0.4 0.6 0.8 1 1.2 1.4 1.6
SWITCH CURRENT (A)

1946A G04

3000
2700
2400

TA = –30°C TA = 100°C

2100
1800
1500
1200

900
600

OSCILLATOR FREQUENCY (kHz)

300

0

0 0.2

4.0

3.8

3.6

3.4

3.2

3.0

2.8

QUIESCENT CURRENT (mA)

2.6

2.4

2.2
–50 –25 0 25 50 75 100 125

0.4 0.6 0.8 1 1.2

FEEDBACK VOLTAGE (V)

TEMPERATURE (°C)

TA = 25°C

1946A G02

1946A G05

2.6

2.4

2.2

2.0

1.8

1.6

1.4

1.2

1.0

CURRENT LIMIT (A)

0.8

0.6

0.4

0.2
0

–50 –25

V

OUT

100mV/DIV

AC COUPLED

V

SW

10V/DIV

0V

0.5A/DIV

LT1946A

0 25 50 75 100 125

TEMPERATURE (°C)

Switching Waveforms for
Figure 1 Circuit

I

LI

100ns/DIV 1946A G06

1946A G03

V

100mV/DIV

AC COUPLED

0.5A/DIV

250mA

I

LOAD

150mA

Transient Response for
Figure 1 Circuit

OUT

I

LI

50µs/DIV 1946A G07

V

OUT

2V/DIV

200mA/DIV

V

SHDN

Start-Up Waveforms for
Figure 1 Circuit

I

IN

0A

5V
0V

R

= 250Ω 1ms/DIV 1946A G08

LOAD

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LT1946A

U

UU

PI FU CTIO S

VC (Pin 1): Error Amplifier Output Pin. Tie external com­pensation network to this pin or use the internal compen­sation network by shorting the VC pin to the COMP pin.
External compensation consists of placing a resistor and
capacitor in series from VC to GND. Typical capacitor
range is from 90pF to 270pF. Typical resistor range is from
25k to 120k.

FB (Pin 2): Feedback Pin. Reference voltage is 1.25V.
Connect resistive divider tap here. Minimize trace area at
FB. Set V

SHDN (Pin 3): Shutdown Pin. Tie to 2.4V or more to enable
device. Ground to shut down. Do not float this pin.

GND (Pin 4, Exposed Pad): Ground. Tie both Pin 4 and
the exposed pad directly to local ground plane. The

ground metal to the exposed pad should be wide for better
heat dissipation. Multiple vias (local ground plane ↔
ground backplane) placed close to the exposed pad can
further aid in reducing thermal resistance.

according to V

OUT

= 1.25 • (1+R1/R2).

OUT

SW (Pin 5): Switch Pin. This is the collector of the internal
NPN power switch. Minimize the metal trace area con­nected to this pin to minimize EMI.

VIN (Pin 6): Input Supply Pin. Must be locally bypassed.
COMP (Pin 7): Internal Compensation Pin. Provides an

internal compensation network. Tie directly to the VC pin
for internal compensation. Tie to GND if not used.

SS (Pin 8): Soft-Start Pin. Place a soft-start capacitor
here. Upon start-up, 4µA of current charges the capacitor
to 1.5V. Use a larger capacitor for slower start-up. Leave
floating if not in use.

4

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