LT1618
1
APPLICATIO S
U
FEATURES
DESCRIPTIO
U
TYPICAL APPLICATIO
U
The LT
1618 step-up DC/DC converter combines a tradi-
tional voltage feedback loop and a unique current feedback
loop to operate as a constant-current, constant-voltage
source. This fixed frequency, current mode switcher oper-
ates from a wide input voltage range of 1.6V to 18V, and
the high switching frequency of 1.4MHz permits the use of
tiny, low profile inductors and capacitors. The current
sense voltage is set at 50mV and can be adjusted using the
I
ADJ
pin.
Available in the 10-Pin MSOP package, the LT1618 pro-
vides a complete solution for constant-current applica-
tions.
s
LED Backlight Drivers
s
USB Powered Boost/SEPIC Converters
s
Input Current Limited Boost/SEPIC Converters
s
Battery Chargers
, LTC and LT are registered trademarks of Linear Technology Corporation.
s
Accurate Input/Output Current Control:
5% Over
Temperature
s
Accurate Output Voltage Control:
1%
s
Wide V
IN
Range: 1.6V to 18V
s
1.4MHz Switching Frequency
s
High Output Voltage: Up to 35V
s
Low V
CESAT
Switch: 200mV at 1A
s
Tiny 10-Pin MSOP Package
Constant-Current/
Constant-Voltage 1.4MHz
Step-Up DC/DC Converter
Efficiency Curve
USB to 12V Boost Converter
(with Selectable 100mA/500mA Input Current Limit)
LOAD CURRENT (mA)
EFFICIENCY (%)
1618 TA01b
90
85
80
75
70
65
60
0
40
80
100
20
60
120 140
160
OFF ON
SHDN
I
ADJ
V
C
V
IN
V
OUT
12V
SW
ISN
ISP
FB
LT1618
R1
909k
R2
107k
V
IN
5V
C1
4.7
F
10nF
C2
4.7
F
1618 TA01a
D1
L1
10
H
3
2
1
9
8
7
GND
5
10
4
C1: TAIYO YUDEN JMK212BJ475
C2: TAIYO YUDEN EMK316BJ475
D1: ON SEMICONDUCTOR MBR0520
L1: SUMIDA CR43-100
0.1
2k
13k
20k
0V
3.3V
100mA 500mA
0V
3.3V
LT1618
2
V
IN
, SHDN Voltage ................................................... 18V
SW Voltage .............................................................. 36V
ISP, ISN Voltage ...................................................... 36V
I
ADJ
Voltage ............................................................... 6V
FB Voltage .............................................................. 1.5V
V
C
Voltage .............................................................. 1.5V
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
MS10 PART
MARKING
T
JMAX
= 125
C,
JA
= 160
C/W
Consult LTC marketing for parts specified with wider operating temperature ranges.
LT1618EMS
ABSOLUTE AXI U
RATI GS
W
W
W
U
PACKAGE/ORDER I FOR ATIO
U
U
W
(Note 1)
ELECTRICAL CHARACTERISTICS
The
q
denotes specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25
C. V
IN
= 1.6V, V
SHDN
= 1.6V, unless otherwise noted.
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Input Voltage
1.6
18
V
Quiescent Current
V
SHDN
= 1.6V, Not Switching
1.8
2.7
mA
V
SHDN
= 0V
0.1
1
A
Reference Voltage
Measured at FB Pin
1.250
1.263
1.276
V
q
1.243
1.263
1.283
V
Reference Voltage Line Regulation
1.6V < V
IN
< 18V
0.01
0.03
%/V
FB Pin Bias Current
V
FB
= 1.263V, V
IN
= 1.8V
q
2
12
nA
Error Amplifier Voltage Gain
180
V/V
Error Amplifier Transconductance
I
C
=
5
A
160
mho
Error Amplifier Sink Current
V
FB
= 1.35V, V
C
= 1V
15
A
Error Amplifier Source Current
V
FB
= 1.10V, V
C
= 1V
30
A
Current Sense Voltage (ISP, ISN)
V
FB
= 0V, V
IADJ
= 0V
q
47.5
50
52.5
mV
ISP, ISN Pin Bias Currents (Note 3)
V
ISP
= 1.85V, V
ISN
= 1.80V, V
IADJ
= 0V
50
80
A
(ISP, ISN) Common Mode Minimum Voltage
1.8
V
Switching Frequency
V
FB
= 1V
1.25
1.4
1.6
MHz
V
FB
= 0V
550
kHz
Maximum Switch Duty Cycle
q
88
92
%
Switch Current Limit
1.5
2.1
2.8
A
Switch V
CESAT
I
SW
= 1A
200
260
mV
ORDER PART
NUMBER
LTNH
1
2
3
4
5
FB
ISN
ISP
I
ADJ
GND
10
9
8
7
6
V
C
SHDN
V
IN
SW
N/C
TOP VIEW
MS10 PACKAGE
10-LEAD PLASTIC MSOP
LT1618
3
TYPICAL PERFOR A CE CHARACTERISTICS
U
W
TEMPERATURE (
C)
50
FEEDBACK VOLTAGE (V)
125
1618 G02
0
75
1.270
1.265
1.260
1.255
1.250
4
2
0
2
4
25
25
50
100
VOLTAGE
CURRENT
FB PIN BIAS CURRENT (nA)
TEMPERATURE (
C)
50
PEAK CURRENT (A)
125
1618 G03
0
75
2.5
2.0
1.5
1.0
0.5
0
25
25
50
100
TEMPERATURE (
C)
50
CURRENT SENSE VOLTAGE (mV)
125
1618 G04
0
75
52
51
50
49
48
25
25
50
100
I
ADJ
PIN VOLTAGE (V)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
CURRENT SENSE VOLTAGE (mV)
1618 G05
60
50
40
30
20
10
0
TEMPERATURE (
C)
50
QUIESCENT CURRENT (mA)
125
1618 G06
0
75
2.5
2.0
1.5
1.0
0.5
0
25
25
50
100
V
IN
= 18V
V
IN
= 1.6V
FB Pin Voltage and Bias Current
Switch Current Limit
Current Sense Voltage
(I
ADJ
Pin = 0V)
Current Sense Voltage
(V
ISP, ISN
)
Quiescent Current
Switch Saturation Voltage
(V
CE, SAT
)
SWITCH CURRENT (A)
0
SATURATION VOLTAGE (mV)
2.0
1618 G01
0.5
1.0
1.5
500
400
300
200
100
0
T
J
= 125
C
T
J
= 50
C
T
J
= 25
C
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: The LT1618 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: Bias currents flow into the ISP and ISN pins.
ELECTRICAL CHARACTERISTICS
The
q
denotes specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25
C. V
IN
= 1.6V, V
SHDN
= 1.6V, unless otherwise noted.
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Switch Leakage Current
Switch Off, V
SW
= 5V
0.01
5
A
SHDN Pin Current
V
SHDN
= 1.6V
5
20
A
Shutdown Threshold (SHDN Pin)
0.3
V
Start-Up Threshold (SHDN Pin)
1
V
LT1618
4
PI
N
FU
N
CTIO
N
S
U
U
U
FB (Pin 1): Feedback Pin. Set the output voltage by
selecting values for R1 and R2 (see Figure 1):
R
R
V
V
OUT
1
2
1 263
1
=
.
ISN (Pin 2): Current Sense () Pin. The inverting input to
the current sense amplifier.
ISP (Pin 3): Current Sense (+) Pin. The noninverting input
to the current sense amplifier.
I
ADJ
(Pin 4): Current Sense Adjust Pin. A DC voltage
applied to this pin will reduce the current sense voltage. If
this adjustment is not needed, tie this pin to ground.
GND (Pin 5): Ground Pin. Tie this pin directly to local
ground plane.
SW (Pin 7 ): 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 8): Input Supply Pin. Bypass this pin with a
capacitor to ground as close to the device as possible.
SHDN (Pin 9): Shutdown Pin. Tie this pin higher than 1V
to turn on the LT1618; tie below 0.3V to turn it off.
V
C
(Pin 10): Compensation Pin for Error Amplifier. Con-
nect a series RC from this pin to ground. Typical values are
2k
and 10nF.
TYPICAL PERFOR A CE CHARACTERISTICS
U
W
TEMPERATURE (
C)
50
SWITCHING FREQUENCY (MHz)
125
1618 G07
0
75
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1.0
25
25
50
100
V
IN
= 18V
V
IN
= 1.6V
FEEDBACK PIN VOLTAGE (V)
SWITCHING FREQUENCY (MHz)
1.2
1618 G08
0
0.2
0.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0.4
0.6
1.0
T
J
= 25
C
SHUTDOWN PIN VOLTAGE (V)
0
SHDN PIN CURRENT (
A)
20
1618 G09
5
10
15
50
45
40
35
30
25
20
15
10
5
0
T
J
= 50
C
T
J
= 25
C
T
J
= 125
C
Switching Frequency
Frequency Foldback
SHDN Pin Current
LT1618
5
1.4MHz
OSCILLATOR
DRIVER
+
+
+
+
+
5
Q
Q1
S
R
+
A1
25
5
10
3
2
8
7
9
1.263V
4
1
SHDN
I
ADJ
GND
V
C
R
C
C
C
V
IN
SW
ISN
ISP
FB
A2
A3
0.02
C1
C2
R
SENSE
V
OUT
V
IN
R1
R2
L1
D1
Figure 1. LT1618 Block Diagram
BLOCK DIAGRA
W
OPERATIO
U
The LT1618 uses a constant frequency, current mode
control scheme to provide excellent line and load regula-
tion. Operation can be best understood by referring to the
Block Diagram in Figure 1. At the start of each oscillator
cycle, the SR latch is set, turning on power switch Q1. The
signal at the noninverting input of PWM comparator A3 is
a scaled-down version of the switch current (summed
together with a portion of the oscillator ramp). When this
signal reaches the level set by the output of error amplifier
A2, comparator A3 resets the latch and turns off the power
switch. In this manner, A2 sets the correct peak current
level to keep the output in regulation. If the error amplifier's
output increases, more current is delivered to the output;
if it decreases, less current is delivered. A2 has two
inverting inputs, one from the voltage feedback loop, and
one from the current feedback loop. Whichever inverting
input is higher takes precedence, forcing the converter
into either a constant-current or a constant-voltage mode.
The LT1618 is designed to transition cleanly between the
two modes of operation. Current sense amplifier A1 senses
the voltage between the ISP and ISN pins and provides a
25
level-shifted version to error amplifier A2. When the
voltage between ISP and ISN reaches 50mV, the output of
A1 provides 1.263V to one of the noninverting inputs of A2
and the converter is in constant-current mode. If the
current sense voltage exceeds 50mV, the output of A1 will
increase causing the output of A2 to decrease, thus
reducing the amount of current delivered to the output. In
this manner the current sense voltage is regulated to
50mV. Similarly, if the FB pin increases above 1.263V, the
output of A2 will decrease to reduce the peak current level
and regulate the output (constant-voltage mode).
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