LT1946
1.2MHz Boost
DC/DC Converter with
1.5A Switch and Soft-Start
FEATURES
■
■
■
■
■
■
■
DESCRIPTIO
1.5A, 36V Internal Switch
1.2MHz Switching Frequency
Integrated Soft-Start Function
Output Voltage Up to 34V
Low V
CESAT
Switch: 300mV at 1.5A (Typ)
8V at 430mA from a 3.3V Input
Small 8-Lead MSOP Package
APPLICATIO S
■
■
■
■
TFT-LCD Bias Supplies
GPS Receivers
DSL Modems
Local Power Supplies
The LT
®
1946 is a fixed frequency step-up DC/DC converter
containing an internal 1.5A, 36V switch. Capable of gener-
ating 8V at 430mA from a 3.3V input, the LT1946 is ideal
for large TFT-LCD panel power supplies. The LT1946
switches at 1.2MHz, 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 1.2mm high.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
TYPICAL APPLICATIO
V
IN
3.3V
V
IN
OFF ON
2.2µF
V
C
49.9k
470pF
100nF
SS
SHDN
LT1946
FB
COMP GND
4.7µH
D1
90
V
OUT
8V
430mA
EFFICIENCY (%)
85
80
SW
28.7k
75
70
65
60
55
20µF
5.23k
C1: 2.2µF, X5R OR X7R, 6.3V
C2: 2
×
10µF, X5R OR X7R, 10V
D1: MICROSEMI UPS120 OR EQUIVALENT
L1: TDK RLF5018T-4R7M1R4
1946 F01
50
0
100
Figure 1. 3.3V to 8V, 430mA Step-Up DC/DC Converter
U
Efficiency
400
300
LOAD CURRENT (mA)
200
500
1946 F01b
U
U
1946fb
1
LT1946
ABSOLUTE
(Note 1)
AXI U
RATI GS
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
V
IN
Voltage ............................................................. 16V
SW Voltage ............................................... – 0.4V to 36V
FB Voltage ............................................................. 2.5V
SHDN Voltage ......................................................... 16V
Current Into FB Pin ..............................................
±1mA
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
9
ORDER PART
NUMBER
LT1946EMS8E
MS8 PART MARKING
LTBNW
V
C
FB
SHDN
GND
1
2
3
4
MS8E PACKAGE
8-LEAD PLASTIC MSOP
EXPOSED PAD (PIN 9) IS GROUND
(MUST BE SOLDERED TO PCB)
T
JMAX
= 125°C,
θ
JA
= 40°C/W,
θ
JC
= 10°C/W
Order Options
Tape and Reel: Add #TR Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF
Lead Free Part Marking:
http://www.linear.com/leadfree/
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
SYMBOL
Minimum Operating Voltage
Maximum Operating Voltage
Feedback Voltage
CONDITIONS
The
●
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 specified. (Note 2)
MIN
TYP
2.45
1.230
1.220
1.250
20
40
300
V
SHDN
= 2.5V, Not Switching
V
SHDN
= 0V, V
IN
= 3V
2.6V
≤
V
IN
≤
16V
●
FB Pin Bias Current
Error Amp Transconductance
Error Amp Voltage Gain
Quiescent Current
Quiescent Current in Shutdown
Reference Line Regulation
Switching Frequency
Switching Frequency in Foldback
Maximum Duty Cycle
Switch Current Limit
Switch V
CESAT
Switch Leakage Current
Soft-Start Charging Current
V
FB
= 1.250V (Note 3)
∆I
= 2µA
V
FB
= 0V
●
(Note 4)
I
SW
= 1A
V
SW
= 5V
V
SS
= 0.5V
2
U
U
W
W W
U
W
TOP VIEW
8
7
6
5
SS
COMP
V
IN
SW
ORDER PART
NUMBER
LT1946EMS8
MS8 PART MARKING
LTUG
MS8 PACKAGE
8-LEAD PLASTIC MSOP
T
JMAX
= 125°C,
θ
JA
= 125°C/W
(4-LAYER BOARD)
MAX
2.6
16
1.270
1.270
120
UNITS
V
V
V
V
nA
µmhos
V/V
●
●
3.2
0
0.01
0.9
0.8
86
1.5
1.2
0.4
90
2.1
240
0.01
2.5
4
●
5
1
0.05
1.4
1.5
mA
µA
%/V
MHz
MHz
MHz
%
3.1
340
1
6
A
mV
µA
µA
1946fb
LT1946
ELECTRICAL CHARACTERISTICS
SYMBOL
SHDN Input Voltage High
SHDN Input Voltage Low
SHDN Pin Bias Current
V
SHDN
= 3V
V
SHDN
= 0V
CONDITIONS
The
●
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 specified. (Note 2)
MIN
2.4
0.5
16
0
32
0.1
TYP
MAX
UNITS
V
V
µA
µA
Note 1:
Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2:
The LT1946E 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 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.
TYPICAL PERFOR A CE CHARACTERISTICS
Feedback Pin Voltage
1.28
OSCILLATOR FREQUENCY (kHz)
1.27
FEEDBACK VOLTAGE (V)
1.26
1.25
1.24
1.23
1.22
1.21
1.20
– 50 – 25
0
50
75
25
TEMPERATURE (°C)
100
125
T
A
= –30°C
800
600
400
200
0
0
0.2
T
A
= 100°C
T
A
= 25°C
CURRENT LIMIT (A)
Switch Saturation Voltage
0.35
0.30
0.25
V
CESAT
(V)
QUIESCENT CURRENT (mA)
0.20
0.15
0.10
0.05
0
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
SWITCH CURRENT (A)
1946 G04
U W
1946 G01
Oscillator Frequency
1400
1200
1000
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
0.8
0.6
1.0
0.4
FEEDBACK VOLTAGE (V)
1.2
1946 G02
–25
75
0
50
25
TEMPERATURE (°C)
100
125
1946 G03
Quiescent Current
3.8
3.6
3.4
3.2
3.0
2.8
2.6
2.4
2.2
2.0
–50
–25
0
25
50
75
TEMPERATURE (°C)
100
125
Switching Waveforms
for Figure 1 Circuit
V
OUT
20mV/DIV
AC COUPLED
V
SW
5V/DIV
0V
I
LI
0.5A/DIV
AC COUPLED
0.5µs/DIV
1946 G06
1946 G05
1946fb
3
LT1946
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.
FB (Pin 2):
Feedback Pin. Reference voltage is 1.250V.
Connect resistive divider tap here. Minimize trace area at
FB. Set V
OUT
according to V
OUT
= 1.250(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):
Ground. Tie directly to local ground plane.
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.
Exposed Pad (MS8E, Pin 9):
Ground. Must be soldered to
PCB.
BLOCK DIAGRA
V
IN
6
1.250V
REFERENCE
+
A1
Σ
–
RAMP
GENERATOR
4
V
OUT
R1 (EXTERNAL)
FB
R2 (EXTERNAL)
0.5V
–
+
A3
÷3
1.2MHz
OSCILLATOR
–
SHUTDOWN
3
SHDN
2
FB
Figure 2. Block Diagram
4
+
–
W
U
U
U
SS
8
V
C
1
COMP
7
120k
4µA
90pF
SW
5
COMPARATOR
DRIVER
A2
R
S
Q
Q1
+
0.01Ω
9
GND
(MSE8 ONLY)
1946 BD
GND
1946fb
LT1946
OPERATIO
The LT1946 uses a constant frequency, current mode
control scheme to provide excellent line and load regula-
tion. Please refer to Figure 2 for the following description
of the part’s operation. At the start of the oscillator cycle,
the SR latch is set, turning on the power switch Q1. The
switch current flows through the internal current sense
resistor generating a voltage. This voltage is added to a
stabilizing ramp and the resulting sum is fed into the
positive terminal of the PWM comparator A2. When this
voltage exceeds the level at the negative input of A2, the SR
latch is reset, turning off the power switch. The level at the
negative input of A2 (V
C
pin) is set by the error amplifier
(A1) and is simply an amplified version of the difference
between the feedback voltage and the reference voltage of
1.250V. In this manner, the error amplifier sets the correct
peak current level to keep the output in regulation.
Two functions are provided to enable a very clean start-up
for the LT1946. Frequency foldback is used to reduce the
oscillator frequency by a factor of 3 when the FB pin is
APPLICATIO S I FOR ATIO
Inductor Selection
Several inductors that work well with the LT1946 are listed
in Table 1. This table is not exclusive; there are many other
manufacturers and inductors that can be used. Consult
each manufacturer for more detailed information and for
their entire selection of related parts, as many different
sizes and shapes are available. Ferrite core inductors
should be used to obtain the best efficiency, as core losses
at 1.2MHz are much lower for ferrite cores than for the
cheaper powdered-iron ones. Choose an inductor that can
handle at least 1.5A without saturating, and ensure that the
inductor has a low DCR (copper wire resistance) to mini-
mize I
2
R power losses. A 4.7µH to 10µH inductor will be
the best choice for most LT1946 designs. Note that in
some applications, the current handling requirements of
the inductor can be lower, such as in the SEPIC topology
where each inductor only carries one-half of the total
switch current.
U
W
U U
U
below a nominal value of 0.5V. This is accomplished via
comparator A3. This feature reduces the minimum duty
cycle that the part can achieve thus allowing better control
of the switch current during start-up. When the FB pin
voltage exceeds 0.5V, the oscillator returns to the normal
frequency of 1.2MHz. A soft-start function is also provided
by the LT1946. When the part is brought out of shutdown,
4µA of current is sourced out of the SS pin. By connecting
an external capacitor to the SS pin, the rate of voltage rise
on the pin can be set. Typical values for the soft-start
capacitor range from 10nF to 200nF. The SS pin directly
limits the rate of rise on the V
C
pin, which in turn limits the
peak switch current. Current limit is not shown in Figure 2.
The switch current is constantly monitored and not al-
lowed to exceed the nominal value of 2.1A. If the switch
current reaches 2.1A, the SR latch is reset regardless of
the output of comparator A2. This current limit helps
protect the power switch as well as the external compo-
nents connected to the LT1946.
The inductors shown in Table 1 were chosen for small size.
For better efficiency, use similar valued inductors with a
larger volume.
Table 1. Recommended Inductors
L
(µH)
4.1
5.4
5.3
6.2
8.2
4.7
5.6
6.8
4.7
MAX
DCR
(mΩ)
57
76
38
45
53
50
59
62
45
SIZE
L
×
W
×
H
(mm)
5.7
×
5.7
×
2
5.7
×
5.7
×
3
PART
CDRH5D18-4R1
CDRH5D18-5R4
CDRH5D28-5R3
CDRH5D28-6R2
CDRH5D28-8R2
ELL6SH-4R7M
ELL6SH-5R6M
ELL6SH-6R8M
RLF5018T-
4R7M1R4
VENDOR
Sumida
(847) 956-0666
www.sumida.com
6.4
×
6
×
3
Panasonic
(408) 945-5660
www.panasonic.com
TDK
(847) 803-6100
www.tdk.com
5.6
×
5.2
×
1.8
1946fb
5
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