LT1618
Constant-Current/
Constant-Voltage 1.4MHz
Step-Up DC/DC Converter
FEATURES
■
■
■
■
■
■
■
DESCRIPTIO
Accurate Input/Output Current Control:
±5%
Over
Temperature
Accurate Output Voltage Control:
±1%
Wide V
IN
Range: 1.6V to 18V
1.4MHz Switching Frequency
High Output Voltage: Up to 35V
Low V
CESAT
Switch: 200mV at 1A
Available in (3mm
×
3mm
×
0.8mm) 10-Pin DFN and
10-Pin MSOP Packages
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 (3mm
×
3mm) Exposed Pad DFN
and 10-pin MSOP packages, the LT1618 provides a com-
plete solution for constant-current applications.
, LTC and LT are registered trademarks of Linear Technology Corporation.
APPLICATIO S
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■
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LED Backlight Drivers
USB Powered Boost/SEPIC Converters
Input Current Limited Boost/SEPIC Converters
Battery Chargers
TYPICAL APPLICATIO
USB to 12V Boost Converter
(with Selectable 100mA/500mA Input Current Limit)
V
IN
5V
0.1Ω
L1
10µH
D1
V
OUT
12V
7
SW
LT1618
8
3.3V
OFF ON
0V
20k
9
V
IN
SHDN
I
ADJ
GND
4
3.3V
100mA 500mA
0V
5
V
C
10
2k
13k
10nF
FB
90
85
2
C1
4.7µF
3
ISN
ISP
1
R2
107k
C2
4.7µF
EFFICIENCY (%)
R1
909k
80
75
70
65
60
0
20
C1: TAIYO YUDEN JMK212BJ475
C2: TAIYO YUDEN EMK316BJ475
D1: ON SEMICONDUCTOR MBR0520
L1: SUMIDA CR43-100
1618 TA01a
U
Efficiency Curve
60 80 100 120 140 160
40
LOAD CURRENT (mA)
1618 TA01b
U
U
sn1618 1618fas
1
LT1618
ABSOLUTE
AXI U RATI GS
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
PACKAGE/ORDER I FOR ATIO
TOP VIEW
FB
ISN
ISP
I
ADJ
GND
1
2
3
4
5
11
10 V
C
9 SHDN
8 V
IN
7 SW
6 SW
ORDER PART
NUMBER
LT1618EDD
DD PART
MARKING
LAFQ
FB
ISN
ISP
I
ADJ
GND
1
2
3
4
5
DD PACKAGE
10-LEAD (3mm
×
3mm) PLASTIC DFN
T
JMAX
= 125°C,
θ
JA
= 43°C/W,
θ
JC
= 3°C/W
EXPOSED PAD (PIN 11) IS GND AND
MUST BE SOLDERED TO PCB
Consult LTC marketing for parts specified with wider operating temperature ranges.
The
●
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
Input Voltage
Quiescent Current
Reference Voltage
Reference Voltage Line Regulation
FB Pin Bias Current
Error Amplifier Voltage Gain
Error Amplifier Transconductance
Error Amplifier Sink Current
Error Amplifier Source Current
Current Sense Voltage (ISP, ISN)
ISP, ISN Pin Bias Currents (Note 3)
(ISP, ISN) Common Mode Minimum Voltage
Switching Frequency
Maximum Switch Duty Cycle
Switch Current Limit
(Note 4)
V
FB
= 1V
V
FB
= 0V
●
ELECTRICAL CHARACTERISTICS
CONDITIONS
V
SHDN
= 1.6V, Not Switching
V
SHDN
= 0V
Measured at FB Pin
●
1.6V < V
IN
< 18V
V
FB
= 1.263V, V
IN
= 1.8V
∆I
C
=
±
5µA
V
FB
= 1.35V, V
C
= 1V
V
FB
= 1.10V, V
C
= 1V
V
FB
= 0V, V
IADJ
= 0V
V
ISP
= 1.85V, V
ISN
= 1.80V, V
IADJ
= 0V
1.25
88
1.5
●
●
2
U
U
W
W W
U
W
(Note 1)
Junction Temperature ........................................... 125°C
Operating Temperature Range (Note 2) .. – 40°C to 85°C
Storage Temperature Range
MSOP ............................................... – 65°C to 150°C
DFN ................................................. – 65°C to 125°C
Lead Temperature (Soldering, 10 sec) (MSOP) .... 300°C
TOP VIEW
10
9
8
7
6
V
C
SHDN
V
IN
SW
NC
ORDER PART
NUMBER
LT1618EMS
MS PART
MARKING
LTNH
MS PACKAGE
10-LEAD PLASTIC MSOP
T
JMAX
= 125°C,
θ
JA
= 160°C/W
MIN
1.6
TYP
1.8
0.1
MAX
18
2.7
1
1.276
1.283
0.03
±12
UNITS
V
mA
µA
V
V
%/V
nA
V/V
µmho
µA
µA
1.250
1.243
1.263
1.263
0.01
±2
180
160
15
30
47.5
50
50
1.4
550
92
2.1
52.5
80
1.8
1.6
mV
µA
V
MHz
kHz
%
A
sn1618 1618fas
2.8
LT1618
The
●
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
Switch V
CESAT
Switch Leakage Current
SHDN Pin Current
Shutdown Threshold (SHDN Pin)
Start-Up Threshold (SHDN Pin)
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
1
CONDITIONS
I
SW
= 1A (Note 4)
Switch Off, V
SW
= 5V
V
SHDN
= 1.6V
MIN
TYP
200
0.01
5
MAX
260
5
20
0.3
UNITS
mV
µA
µA
V
V
ELECTRICAL CHARACTERISTICS
temperature range are assured by design, characterization, and correlation
with statistical process controls.
Note 3:
Bias currents flow into the ISP and ISN pins.
Note 4:
Switch current limit and switch V
CESAT
for the DD package is
guaranteed by design and/or correlation to static test.
TYPICAL PERFOR A CE CHARACTERISTICS
Switch Saturation Voltage
(V
CE, SAT
)
500
1.270
SATURATION VOLTAGE (mV)
400
T
J
= 125°C
300
T
J
= 25°C
200
T
J
= –50°C
FEEDBACK VOLTAGE (V)
VOLTAGE
PEAK CURRENT (A)
100
0
0
1.0
0.5
1.5
SWITCH CURRENT (A)
Current Sense Voltage
(I
ADJ
Pin = 0V)
52
CURRENT SENSE VOLTAGE (mV)
CURRENT SENSE VOLTAGE (mV)
51
QUIESCENT CURRENT (mA)
50
49
48
– 50 – 25
25
50
75
0
TEMPERATURE (°C)
U W
2.0
1618 G01
FB Pin Voltage and Bias Current
4
Switch Current Limit
2.5
1.265
2
2.0
FB PIN BIAS CURRENT (nA)
1.5
1.260
CURRENT
0
1.0
1.255
–2
0.5
1.250
– 50 – 25
75
25
50
0
TEMPERATURE (°C)
100
–4
125
0
– 50 – 25
25
50
75
0
TEMPERATURE (°C)
100
125
1618 G02
1618 G03
Current Sense Voltage
(V
ISP, ISN
)
60
50
40
30
20
10
0
0.0
Quiescent Current
2.5
2.0
V
IN
= 18V
V
IN
= 1.6V
1.5
1.0
0.5
100
125
0.2
0.4 0.6 0.8 1.0 1.2
I
ADJ
PIN VOLTAGE (V)
1.4
1.6
0
– 50 – 25
25
50
75
0
TEMPERATURE (°C)
100
125
1618 G04
1618 G05
1618 G06
sn1618 1618fas
3
LT1618
TYPICAL PERFOR A CE CHARACTERISTICS
Switching Frequency
1.8
1.7
SWITCHING FREQUENCY (MHz)
1.6
T
J
= 25°C
1.4
SWITCHING FREQUENCY (MHz)
SHDN PIN CURRENT (µA)
1.6
1.5
1.4
1.3
1.2
1.1
1.0
– 50 – 25
75
25
50
0
TEMPERATURE (°C)
V
IN
= 18V
V
IN
= 1.6V
PIN FUNCTIONS
(MS/DD)
FB (Pin 1/Pin 1):
Feedback Pin. Set the output voltage by
selecting values for R1 and R2 (see Figure 1):
SW (NA/Pin 6):
Switch Pin for DD Package. Connect this
pin to Pin 7.
SW (Pin 7/Pin 7):
Switch Pin. This is the collector of the
internal NPN power switch. Minimize the metal trace area
connected to this pin to minimize EMI.
V
IN
(Pin 8/Pin 8):
Input Supply Pin. Bypass this pin with
a capacitor to ground as close to the device as possible.
SHDN (Pin 9/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/Pin 10):
Compensation Pin for Error Amplifier.
Connect a series RC from this pin to ground. Typical values
are 2kΩ and 10nF.
Exposed Pad (NA/Pin 11):
The Exposed Pad on the DD
package is GND and must be soldered to the PCB GND for
optimum thermal performance.
⎛
V
⎞
R1
=
R2
⎜
OUT
– 1
⎟
⎝
1.263V
⎠
ISN (Pin 2/Pin 2):
Current Sense (–) Pin. The inverting
input to the current sense amplifier.
ISP (Pin 3/Pin 3):
Current Sense (+) Pin. The noninverting
input to the current sense amplifier.
I
ADJ
(Pin 4/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/Pin 5):
Ground Pin. Tie this pin directly to local
ground plane.
NC (Pin 6/NA):
No Connection for MS Package.
4
U W
100
1618 G07
Frequency Foldback
50
45
40
35
30
25
20
15
10
5
0
SHDN Pin Current
1.2
1.0
0.8
0.6
0.4
0.2
0
0.8
0.4
0.6
1.0
FEEDBACK PIN VOLTAGE (V)
T
J
= – 50°C
T
J
= 25°C
T
J
= 125°C
125
0
0.2
1.2
1618 G08
0
5
15
10
SHUTDOWN PIN VOLTAGE (V)
20
1618 G09
U
U
U
sn1618 1618fas
LT1618
BLOCK DIAGRA
V
IN
C1
9
SHDN
8
V
IN
7
SW
C2
DRIVER
S
Q
R
A3
5
GND
OPERATIO
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
W
L1
D1
R
SENSE
V
OUT
Q1
ISP
3
ISN
2
I
ADJ
+
0.02Ω
×5
A1
×25
+
–
–
1.4MHz
OSCILLATOR
+
+
+
–
A2
4
Σ
–
–
+
R1
1
1.263V
R2
FB
10
V
C
R
C
C
C
Figure 1. LT1618 Block Diagram
U
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).
sn1618 1618fas
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