LT3483
Inverting Micropower
DC/DC Converter with
Schottky in ThinSOT Package
FEATURES
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DESCRIPTIO
■
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Internal 40V Schottky Diode
One Resistor Feedback (Other Resistor Inside)
Internal 40V, 200mA Power Switch
Generates Regulated Negative Outputs to –38V
Low Quiescent Current:
40µA in Active Mode
<1µA in Shutdown Mode
Low V
CESAT
Switch: 200mV at 150mA
Wide Input Range: 2.5V to 16V
Uses Small Surface Mount Components
Output Short-Circuit Protected
Available in a 6-Lead SOT-23 Package
APPLICATIO S
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LCD Bias
Handheld Computers
Battery Backup
Digital Cameras
OLED Bias
The LT
®
3483 is a micropower inverting DC/DC converter
with integrated Schottky and one resistor feedback. The
small package size, high level of integration and use of tiny
surface mount components yield a solution size as small
as 40mm
2
. The device features a quiescent current of only
40µA at no load, which further reduces to 0.1µA in
shutdown. A current limited, fixed off-time control scheme
conserves operating current, resulting in high efficiency
over a broad range of load current. A precisely trimmed
10µA feedback current enables one resistor feedback and
virtually eliminates feedback loading of the output. The
40V switch enables voltage outputs up to –38V to be
generated without the use of costly transformers. The
LT3483’s low 300ns off-time permits the use of tiny low
profile inductors and capacitors to minimize footprint and
cost in space-conscious portable applications.
The LT3483 is available in the low profile (1mm) SOT-23
(ThinSOT
TM
) package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
ThinSOT is a trademark of Linear Technology Corporation.
Patent pending.
TYPICAL APPLICATIO
0.22µF
3.6V to –8V DC/DC Converter
75
V
IN
3.6V
10µH
Efficiency and Power Loss
V
IN
= 3.6V
EFFICIENCY
1000
10Ω
V
IN
4.7µF
LT3483
SHDN
FB
GND
D
5pF
806k
EFFICIENCY (%)
70
V
OUT
–8V
25mA
2.2µF
SW
65
POWER
LOSS
60
3483 TA01a
55
0.1
1
10
LOAD CURRENT (mA)
U
100
POWER LOSS (mW)
U
U
10
1
0.1
100
3483 TA01b
3483f
1
LT3483
ABSOLUTE
(Note 1)
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
ORDER PART
NUMBER
TOP VIEW
SW 1
GND 2
FB 3
6 V
IN
5D
4 SHDN
V
IN
Voltage ............................................................. 16V
SW Voltage ............................................................. 40V
D Voltage .............................................................. –40V
FB Voltage ............................................................. 2.5V
SHDN Voltage ......................................................... 16V
Operating Ambient Temperature Range
(Note 2) .................................................. – 40°C to 85°C
Junction Temperature .......................................... 125°C
Storage Temperature Range ................ – 65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
LT3483ES6
S6 PACKAGE
6-LEAD PLASTIC TSOT-23
T
JMAX
= 125°C,
θ
JA
= 256°C/W IN FREE AIR
θ
JA
= 120°C/W ON BOARD OVER GROUND PLANE
S6 PART MARKING
LTBKX
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The
●
denotes specifications which apply over the full operating temperature range, otherwise specifications are T
A
= 25°C.
V
IN
= 3.6V, V
SHDN
= 3.6V unless otherwise specified.
PARAMETER
V
IN
Operating Range
V
IN
Undervoltage Lockout
FB Comparator Trip Voltage to GND (V
FB
)
FB Output Current (Note 3)
FB Comparator Hysteresis
Quiescent Current in Shutdown
Quiescent Current (Not Switching)
I
FB
Line Regulation
Switch Off-Time
Switch Current Limit
Switch V
CESAT
Switch Leakage Current
Rectifier Leakage Current
Rectifier Forward Drop
SHDN Input Low Voltage
SHDN Input High Voltage
SHDN Pin Current
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
The LTC3483E is guaranteed to meet 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 pin.
1.5
6
10
I
SW
= 150mA to GND
SW = 40V
D = – 40V
I
D
= 150mA to GND
0.64
0.4
170
FB Falling
FB = V
FB
– 5mV
FB Rising
V
SHDN
= GND
FB = –0.05V
2.5V
≤
V
IN
≤
16V
300
200
200
1
4
230
40
●
●
CONDITIONS
MIN
2.5
TYP
2
MAX
16
2.4
12
–9.75
1
50
0.07
UNITS
V
V
mV
µA
mV
µA
µA
%/V
ns
mA
mV
µA
µA
V
V
V
µA
0
–10.15
5
–10
10
2
U
3483f
W
U
U
W W
W
LT3483
TYPICAL PERFOR A CE CHARACTERISTICS
V
FB
Current
10.2
12
SWITCH OFF TIME (ns)
10.1
V
FB
CURRENT (µA)
V
FB
VOLTAGE (mV)
10.0
9.9
9.8
–50
–20
40
10
TEMPERATURE (°C)
Switch Current Limit
230
220
210
200
190
180
170
–50
QUIESCENT CURRENT (µA)
40
SHDN PIN BIAS CURRENT (µA)
SWITCH CURRENT LIMIT (mA)
–20
10
40
TEMPERATURE (°C)
PI FU CTIO S
SW:
Switch. Connect to external inductor L1 and positive
terminal of transfer capacitor.
GND:
Ground.
FB:
Feedback. Place resistor to negative output here. Set
resistor value R1 = V
OUT
/10µA.
SHDN:
Shutdown. Connect to GND to turn device off.
Connect to supply to turn device on.
D:
Anode Terminal of Integrated Schottky Diode. Con-
nect to negative terminal of transfer capacitor and exter-
nal inductor L2 (flyback configuration) or to cathode of
external Schottky diode (inverting charge pump
configuration).
V
IN
:
Input Supply. Must be locally bypassed with 1µF or
greater.
U W
70
3483 G01
V
FB
Voltage
400
350
9
300
250
200
150
100
50
100
0
–50
–20
40
10
TEMPERATURE (°C)
70
100
3483 G02
Switch Off Time
6
3
0
–50
–20
10
40
TEMPERATURE (°C)
70
100
3483 G03
Quiescent Current
50
10
SHDN Pin Bias Current
T
A
= 25°C
8
30
6
20
4
10
NOT SWITCHING
V
FB
= –0.05V
2
70
100
3483 G04
0
–50
–20
40
10
TEMPERATURE (°C)
70
100
3483 G05
0
0
8
4
12
SHDN PIN VOLTAGE (V)
16
3483 G06
U
U
U
3483f
3
LT3483
BLOCK DIAGRA
W
L1A
V
IN
L1B
•
C
FLY
•
V
OUT
C
OUT
6
V
IN
1.250V
REFERENCE
125k
R1
V
OUT
3
FB
300ns
DELAY
S
R
Q
Q
1
SW
5
D
Q1
D1
25mV
+
A3
+
A2
–
A1
+
0.1Ω
0.1Ω
–
20mV
–
GND
2
OPTIONAL CHARGE PUMP CONFIGURATION.
L1B REPLACED WITH:
D2
D
R2
V
OUT
3483 BD
OPERATIO
The LT3483 uses a constant off-time control scheme to
provide high efficiency over a wide range of output cur-
rents. Operation can be best understood by referring to the
Block Diagram. When the voltage at the FB pin is approxi-
mately 0V, comparator A3 disables most of the internal cir-
cuitry. Output current is then provided by external capacitor
C
OUT
, which slowly discharges until the voltage at the FB
pin goes above the hysteresis point of A3. Typical hyster-
esis at the FB pin is 10mV. A3 then enables the internal
circuitry, turns on power switch Q1, and the currents in
external inductors L1A and L1B begin to ramp up. Once the
4
U
switch current reaches 200mA, comparator A1 resets the
latch, which turns off Q1 after about 80ns. Inductor cur-
rent flows through the internal Schottky D1 to GND, charg-
ing the flying capacitor. Once the 300ns off-time has
elapsed, and internal diode current drops below 250mA
(as detected by comparator A2), Q1 turns on again and
ramps up to 200mA. This switching action continues until
the output capacitor charge is replenished (until the FB pin
decreases to 0V), then A3 turns off the internal circuitry
and the cycle repeats. The inverting charge pump topology
replaces L1B with the series combination D2 and R2.
3483f
LT3483
APPLICATIO S I FOR ATIO
CHOOSING A REGULATOR TOPOLOGY
Inverting Charge Pump
The inverting charge pump regulator combines an induc-
tor-based step-up with an inverting charge pump. This
configuration usually provides the best size, efficiency and
output ripple and is applicable where the magnitude of
V
OUT
is greater than V
IN
. Negative outputs to –38V can be
produced with the LT3483 in this configuration. For cases
where the magnitude of V
OUT
is less than or equal to V
IN
,
use a 2-inductor or transformer configuration such as the
inverting flyback.
In the inverting charge pump configuration, a resistor is
added in series with the Schottky diode between the
negative output and the D pin of the LT3483. The purpose
of this resistor is to smooth/reduce the current spike in the
flying capacitor when the switch turns on. A 10Ω resistor
works well for a Li
+
to –8V application, and the impact to
converter efficiency is less than 3%. The resistor values
recommended in the applications circuits also limit the
switch current during a short-circuit condition at the
output.
Inverting Flyback
The inverting flyback regulator, shown in the –5V applica-
tion circuit, uses a coupled inductor and is an excellent
choice where the magnitude of the output is less than or
equal to the supply voltage. The inverting flyback also
performs well in a step-up/invert application, but it occu-
pies more board space compared with the inverting charge
pump. Also, the maximum |V
OUT
| using the flyback is less
than can be obtained with the charge pump—it is reduced
from 38V by the magnitudes of V
IN
and ringing at the
switch node. Under a short-circuit condition at the output,
a proprietary technique limits the switch current and
prevents damage to the LT3483 even with supply voltage
as high as 16V. As an option, a 0.47µF capacitor may be
added between terminals D and SW of LT3483 to suppress
ringing at SW.
U
Inductor Selection
Several recommended inductors that work well with the
LT3483 are listed in Table 1, although there are many other
manufacturers and devices that can be used. Consult each
manufacturer for more detailed information and for their
entire selection of related parts. Many different sizes and
shapes are available. For inverting charge pump regula-
tors with input and output voltages below 7V, a 4.7µH or
6.8µH inductor is usually the best choice. For flyback
regulators or for inverting charge pump regulators where
the input or output voltage is greater than 7V, a 10µH
inductor is usually the best choice. A larger value inductor
can be used to slightly increase the available output
current, but limit it to around twice the value recom-
mended, as too large of an inductance will increase the
output voltage ripple without providing much additional
output current.
Table 1. Recommended Inductors
MAX
L
I
DC
PART
(
µ
H) (mA)
LQH2MCN4R7M02L 4.7 300
LQH2MCN6R8M02L 6.8 255
LQH2MCN100M02L 10 225
SDQ12
10 980
Coupled
15 780
Inductor
744876
10 550
Coupled
Inductor
DCR
(
Ω
)
0.84
1.0
1.2
0.72
1.15
0.46
HEIGHT
(mm) MANUFACTURER
0.95 Murata
www.murata.com
1.2
Cooper Electronics
Tech
www.cooperet.com
Würth Elektronik
www.we-online.com
1.2
W
U U
Capacitor Selection
The small size and low ESR of ceramic capacitors make
them ideal for LT3483 applications. Use of X5R and X7R
types is recommended because they retain their capacitance
over wider voltage and temperature ranges than other di-
electric types. Always verify the proper voltage rating. Table
2 shows a list of several ceramic capacitor manufacturers.
Consult the manufacturers for more detailed information
on their entire selection of ceramic capacitors.
A 4.7µF ceramic bypass capacitor on the V
IN
pin is
recommended where the distance to the power supply or
battery could be more than a couple inches. Otherwise, a
1µF is adequate.
3483f
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