LT1946A
2.7MHz Boost DC/DC
Converter with 1.5A Switch
and Soft-Start
FEATURES
s
s
s
s
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DESCRIPTIO
1.5A, 36V Internal Switch
2.7MHz Switching Frequency
Integrated Soft-Start Function
Adjustable Output from V
IN
to 35V
Low V
CESAT
Switch: 300mV at 1.5A (Typical)
12V at 430mA from a 5V Input
Small Thermally Enhanced 8-Lead MSOP Package
APPLICATIO S
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s
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TFT-LCD Bias Supplies
GPS Receivers
DSL Modems
Local Power Supply
The LT
®
1946A is a fixed frequency step-up DC/DC con-
verter 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 com-
pensation can be either internal or external, giving the user
flexibility in setting loop compensation and allowing opti-
mized transient response with low ESR ceramic output
capacitors. Soft-start is controlled with an external capaci-
tor 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
V
IN
5V
3
1
L1
2.2µH
6
V
IN
SHDN
V
C
SS
C
SS
100nF
8
OFF ON
C1
2.2µF
R
C
27.4k
C
C
270pF
5
SW
D1
V
OUT
12V
430mA
R1
182k
EFFICIENCY (%)
90
85
80
2
LT1946A FB
7
COMP
GND*
4
R2
21k
C2
2.2µF
75
70
65
60
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
1946A TA01
55
50
0
100
200
300
400
LOAD CURRENT (mA)
500
1946A TA01
Figure 1. 5V to 12V, 430mA Step-Up DC/DC Converter
U
Efficiency
sn1946a 1946afs
U
U
1
LT1946A
ABSOLUTE
(Note 1)
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
V
C
FB
SHDN
GND
1
2
3
4
8
7
6
5
SS
COMP
V
IN
SW
V
IN
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
NUMBER
LT1946AEMS8E
MS8E PART
MARKING
LTYZ
MS8E PACKAGE
8-LEAD PLASTIC MSOP
EXPOSED PAD IS GROUND
(MUST BE SOLDERED TO PCB)
T
JMAX
= 125°C,
θ
JA
= 40°C/W,
θ
JC
= 10°C/W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 3V, V
SHDN
= V
IN
unless otherwise noted. (Note 2)
PARAMETER
Minimum Operating Voltage
Maximum Operating Voltage
Feedback Voltage
q
ELECTRICAL CHARACTERISTICS
CONDITIONS
MIN
TYP
2.45
MAX
2.6
16
1.27
1.27
120
UNITS
V
V
V
V
nA
µmhos
V/V
1.23
1.22
1.25
20
40
300
3.6
0
0.01
FB Pin Bias Current
Error Amp Transconductance
Error Amp Voltage Gain
Quiescent Current
Quiescent Current in Shutdown
Reference Line Regulation
Switching Frequency
V
FB
= 1.25V (Note 3)
∆I
= 2µA
V
SHDN
= 2.5V, Not Switching
V
SHDN
= 0V, V
IN
= 3V
2.6V
≤
V
IN
≤
16V
q
5
1
0.05
3
3.1
q
2.4
2.3
73
1.5
2.7
0.85
Switching Frequency in Foldback
Maximum Duty Cycle
Switch Current Limit
Switch V
CESAT
Switch Leakage Current
Soft-Start Charging Current
SHDN Input Voltage High
SHDN Input Voltage Low
SHDN Pin Bias Current
V
FB
= 0V
q
80
2.1
240
0.01
3.1
340
1
6
0.5
(Note 4)
I
SW
= 1A
V
SW
= 5V
V
SS
= 0.5V
q
2.5
2.4
4
V
SHDN
= 3V
V
SHDN
= 0V
16
0
32
0.1
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
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.
sn1946a 1946afs
2
U
mA
µA
%/V
MHz
MHz
MHz
%
A
mV
µA
µA
V
V
µA
µA
W
U
U
W W
W
LT1946A
TYPICAL PERFOR A CE CHARACTERISTICS
Feedback Pin Voltage
1.28
1.27
3000
2700
OSCILLATOR FREQUENCY (kHz)
FEEDBACK VOLTAGE (V)
1.26
1.25
1.24
1.23
1.22
1.21
1.20
–50
1800
1500
1200
900
600
300
T
A
= 25°C
–25
0
25
50
75
TEMPERATURE (°C)
100
125
0
0
0.2
0.4
0.6
0.8
FEEDBACK VOLTAGE (V)
1
1.2
1946A G02
CURRENT LIMIT (A)
Switch Saturation Voltage
0.35
0.30
QUIESCENT CURRENT (mA)
0.25
4.0
3.8
3.6
3.4
3.2
3.0
2.8
2.6
2.4
0
0.2
0.4 0.6 0.8 1 1.2
SWITCH CURRENT (A)
1.4
1.6
V
CESAT
(V)
0.20
0.15
0.10
0.05
0
Transient Response for
Figure 1 Circuit
V
OUT
100mV/DIV
AC COUPLED
V
OUT
2V/DIV
I
LI
0.5A/DIV
I
LOAD
250mA
150mA
50µs/DIV
1946A G07
U W
Oscillator Frequency
Current Limit
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
–50
2400
2100
T
A
= –30°C
T
A
= 100°C
–25
0
25
50
75
TEMPERATURE (°C)
100
125
1946A G01
1946A G03
Quiescent Current
V
OUT
100mV/DIV
AC COUPLED
V
SW
10V/DIV
0V
Switching Waveforms for
Figure 1 Circuit
I
LI
0.5A/DIV
100ns/DIV
1946A G06
2.2
–50
–25
0
25
50
75
TEMPERATURE (°C)
100
125
1946A G04
1946A G05
Start-Up Waveforms for
Figure 1 Circuit
I
IN
200mA/DIV
0A
V
SHDN
5V
0V
R
LOAD
= 250Ω
1ms/DIV
1946A G08
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3
LT1946A
PI FU CTIO S
V
C
(Pin 1):
Error Amplifier Output Pin. Tie external com-
pensation network to this pin or use the internal compen-
sation network by shorting the V
C
pin to the COMP pin.
External compensation consists of placing a resistor and
capacitor in series from V
C
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
OUT
according to V
OUT
= 1.25 • (1+R1/R2).
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.
SW (Pin 5):
Switch Pin. This is the collector of the internal
NPN power switch. Minimize the metal trace area con-
nected to this pin to minimize EMI.
V
IN
(Pin 6):
Input Supply Pin. Must be locally bypassed.
COMP (Pin 7):
Internal Compensation Pin. Provides an
internal compensation network. Tie directly to the V
C
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
U
U
U
sn1946a 1946afs
LT1946A
BLOCK DIAGRA
V
IN
6
V
OUT
R1 (EXTERNAL)
FB
R2 (EXTERNAL)
0.5V
SHUTDOWN
SHDN
3
W
SS
8
V
C
1
COMP
7
4µA
120k
90pF
–
1.25V
REFERENCE
COMPARATOR
DRIVER
A2
R
S
Q
5 SW
Q1
+
A1
+
Σ
+
0.01Ω
–
–
RAMP
GENERATOR
+
A3
4 GND
÷
3
2.7MHz
OSCILLATOR
EXPOSED
PAD
–
2
FB
1946A F02
Figure 2. Block Diagram
sn1946a 1946afs
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