LT1931/LT1931A
1.2MHz/2.2MHz Inverting
DC/DC Converters in ThinSOT
FEATURES
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DESCRIPTIO
Fixed Frequency 1.2MHz/2.2MHz Operation
Very Low Noise: 1mV
P-P
Output Ripple
– 5V at 350mA from 5V Input
–12V at 150mA from 5V Input
Uses Small Surface Mount Components
Wide Input Range: 2.6V to 16V
Low Shutdown Current: <1µA
Low V
CESAT
Switch: 400mV at 1A
Pin-for-Pin Compatible with the LT1611
Low Profile (1mm) ThinSOT
TM
Package
APPLICATIO S
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The LT
®
1931/LT1931A are the industry’s highest power
inverting SOT-23 current mode DC/DC converters. Both
parts include a 1A integrated switch allowing high current
outputs to be generated in a small footprint. The LT1931
switches at 1.2MHz while the LT1931A switches at 2.2MHz.
These high speeds enable the use of tiny, low cost
capacitors and inductors 2mm or less in height. The
LT1931 is capable of generating – 5V at 350mA or –12V
at 150mA from a 5V supply, while the LT1931A can
generate –5V at 300mA using significantly smaller induc-
tors. Both parts are easy pin-for-pin upgrades for higher
power LT1611 applications.
The LT1931/LT1931A operate in a dual inductor inverting
topology that filters both the input side and output side
current. Very low output voltage ripple approaching 1mV
P-P
can be achieved when ceramic output capacitors are used.
Fixed frequency switching ensures a clean output free
from low frequency noise typically present with charge
pump solutions. The low impedance output remains within
1% of nominal during large load steps. The 36V switch
allows V
IN
to V
OUT
differential of up to 34V.
The LT1931/LT1931A are available in the 5-lead ThinSOT
package.
, LTC and LT are registered trademarks of Linear Technology Corporation. ThinSOT is a
trademark of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
Disk Drive MR Head Bias
Digital Camera CCD Bias
LCD Bias
GaAs FET Bias
Local Low Noise/Low Impedance Negative Supply
TYPICAL APPLICATIO
V
IN
5V
V
IN
SHDN
C1
4.7µF
LT1931
NFB
GND
L1A
10µH
C2
1µF
L1B
10µH
100
95
D1
SW
R1
29.4k
R2
10k
C4
220pF
EFFICIENCY (%)
V
OUT
–5V
350mA
C3
22µF
90
85
80
75
70
65
60
C1: TAIYO YUDEN X5R JMK212BJ475MG
C2: TAIYO YUDEN X5R LMK212BJ105MG
C3: TAIYO YUDEN X5R JMK325BJ226MM
D1: ON SEMICONDUCTOR MBR0520
L1: SUMIDA CLS62-100
1931 F01
55
50
0
50
100 150 200 250
LOAD CURRENT (mA)
300
350
Figure 1. 5V to –5V, 350mA Inverting DC/DC Converter
U
Efficiency
1931 TA01
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LT1931/LT1931A
ABSOLUTE
(Note 1)
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
SW 1
GND 2
NFB 3
4 SHDN
5 V
IN
V
IN
Voltage .............................................................. 16V
SW Voltage ................................................– 0.4V to 36V
NFB Voltage ............................................................. – 2V
Current Into NFB 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
S5 PACKAGE
5-LEAD PLASTIC TSOT-23
T
JMAX
= 125°C,
θ
JA
= 150°C/ W
ORDER PART NUMBER
LT1931ES5
LT1931AES5
LT1931IS5
LT1931AIS5
S5 PART MARKING
LTRA
LTSP
LTBZF
LTBZG
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
The
●
denotes specifications which apply over the full operating temperature range, otherwise specifications are 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
●
CONDITIONS
MIN
LT1931
TYP
2.45
MAX
2.6
16
LT1931A
MIN
TYP
MAX
2.45
2.6
16
–1.275 –1.255 –1.235
–1.280
–1.230
8
5.8
0.01
0.01
1.8
1.6
75
1
2.2
82
1.2
400
0.01
2.4
2.5
600
1
0.5
35
0
70
0.1
16
8
1
0.05
2.6
2.9
UNITS
V
V
V
V
µA
mA
µA
%/V
MHz
MHz
%
A
mV
µA
V
V
µA
µA
– 1.275 – 1.255 – 1.235
– 1.280
– 1.230
4
4.2
0.01
0.01
1
0.85
84
1
1.2
90
1.2
400
0.01
2.4
0.5
2
600
1
8
6
1
0.05
1.4
1.6
NFB Pin Bias Current
Quiescent Current
Quiescent Current in Shutdown
Reference Line Regulation
Switching Frequency
V
NFB
= –1.255V
V
SHDN
= 2.4V, Not Switching
V
SHDN
= 0V, V
IN
= 3V
2.6V
≤
V
IN
≤
16V
●
●
Maximum Duty Cycle
Switch Current Limit
Switch V
CESAT
Switch Leakage Current
SHDN Input Voltage, High
SHDN Input Voltage, Low
SHDN Pin Bias Current
V
SHDN
= 3V
V
SHDN
= 0V
(Note 3)
I
SW
= 1A
V
SW
= 5V
●
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 LT1931E/LT1931AE are 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. LT1931I/LT1931AI are
guaranteed over the –40°C to 85°C temperature range.
Note 3:
Current limit guaranteed by design and/or correlation to static test.
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W
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W W
W
LT1931/LT1931A
TYPICAL PERFOR A CE CHARACTERISTICS
Quiescent Current
7.0
6.5
QUIESCENT CURRENT (mA)
NOT SWITCHING
–1.27
6.0
5.5
5.0
4.5
4.0
3.5
3.0
–50
–25
LT1931A
SHDN PIN CURRENT (µA)
FEEDBACK VOLTAGE (V)
LT1931
50
25
TEMPERATURE (°C)
0
Current Limit
1.6
1.4
CURRENT LIMIT (A)
1.2
V
CESAT
(V)
0.30
1.0
0.8
0.6
0.4
0.2
0
10
20
30
40 50 60 70
DUTY CYCLE (%)
80
90
FREQUENCY (MHz)
PI FU CTIO S
SW (Pin 1):
Switch Pin. Connect inductor/diode here.
Minimize trace area at this pin to keep EMI down.
GND (Pin 2):
Ground. Tie directly to local ground plane.
NFB (Pin 3):
Feedback Pin. Reference voltage is –1.255V.
Connect resistive divider tap here. Minimize trace area.
The NFB bias current flows out of the pin. Set R1 and R2
according to:
For LT1931: R1
=
| V
OUT
| – 1.255
1.255
+
4 • 10
– 6
R2
For LT1931A: R1
=
| V
OUT
| – 1.255
1.255
+
8 • 10
– 6
R2
U W
75
1931 G01
Feedback Pin Voltage
–1.28
90
80
70
60
50
40
30
20
10
0
Shutdown Pin Current
T
A
= 25°C
LT1931A
–1.26
–1.25
–1.24
–1.23
–1.22
–50
LT1931
100
–25
0
25
50
TEMPERATURE (°C)
75
100
1931 G02
–10
0
1
3
4
2
SHDN PIN VOLTAGE (V)
5
6
1931 G03
Switch Saturation Voltage
0.45
Oscillator Frequency
2.5
2.3
2.1
1.9
1.7
1.5
1.3
1.1
0.9
0.7
LT1931
LT1931A
T
A
= 25°C
T
A
= 25°C
0.40
0.35
0.25
0.20
0.15
0.10
0.05
0
0
0.2
0.4
0.6
0.8
SWITCH CURRENT (A)
1.0
1.2
0.5
–50
–25
25
50
0
TEMPERATURE (°C)
75
100
1931 G06
1931 G04
1931 G05
U
U
U
(
)
SHDN (Pin 4):
Shutdown Pin. Tie to 2.4V or more to enable
device. Ground to shut down.
V
IN
(Pin 5):
Input Supply Pin. Must be locally bypassed.
(
)
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LT1931/LT1931A
BLOCK DIAGRA
V
IN
5
–
Q1
V
OUT
R1
(EXTERNAL)
NFB
R2
(EXTERNAL)
Q2
x10
R3
30k
R4
150k
3 NFB
C
C
C
PL
(OPTIONAL)
1.2MHz
OSCILLATOR
SHDN
4
SHUTDOWN
Figure 2
OPERATIO
The LT1931 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 2. At the start of each oscillator
cycle, the SR latch is set, turning on the power switch Q3.
A voltage proportional to the switch current 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 is set by the error amplifier (g
m
) and
is simply an amplified version of the difference between
the feedback voltage and the reference voltage of –1.255V.
In this manner, the error amplifier sets the correct peak
current level to keep the output in regulation. If the error
amplifier’s output increases, more current is taken from
the output; if it decreases, less current is taken. One
function not shown in Figure 2 is the current limit. The
switch current is constantly monitored and not allowed to
exceed the nominal value of 1.2A. If the switch current
reaches 1.2A, the SR latch is reset regardless of the state
of comparator A2. This current limit protects the power
switch as well as various external components connected
to the LT1931.
The Block Diagram for the LT1931A is identical except that
the oscillator is 2.2MHz and resistors R3 to R6 are one-half
the LT1931 values.
4
+
R
C
RAMP
GENERATOR
Σ
–
W
V
IN
R5
80k
R6
80k
1 SW
COMPARATOR
A1
g
m
LATCH
S
DRIVER
Q
Q3
+
A2
R
+
0.01Ω
–
2 GND
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LT1931/LT1931A
APPLICATIO S I FOR ATIO
LT1931A AND LT1931 DIFFERENCES:
Switching Frequency
The key difference between the LT1931A and LT1931 is
the faster switching frequency of the LT1931A. At 2.2MHz,
the LT1931A switches at nearly twice the rate of the
LT1931. Care must be taken in deciding which part to use.
The high switching frequency of the LT1931A allows
smaller cheaper inductors and capacitors to be used in a
given application, but with a slight decrease in efficiency
and maximum output current when compared to the
LT1931. Generally, if efficiency and maximum output
current are critical, the LT1931 should be used. If applica-
tion size and cost are more important, the LT1931A will be
the better choice. In many applications, tiny inexpensive
chip inductors can be used with the LT1931A, reducing
solution cost.
Duty Cycle
The maximum duty cycle (DC) of the LT1931A is 75%
compared to 84% for the LT1931. The duty cycle for a
given application using the dual inductor inverting topol-
ogy is given by:
DC
=
| V
OUT
|
| V
IN
|
+
| V
OUT
|
For a 5V to –5V application, the DC is 50% indicating that
the LT1931A can be used. A 5V to –16V application has a
DC of 76.2% making the LT1931 the right choice. The
LT1931A can still be used in applications where the DC, as
calculated above, is above 75%. However, the part must
be operated in the discontinuous conduction mode so that
the actual duty cycle is reduced.
INDUCTOR SELECTION
Several inductors that work well with the LT1931 are listed
in Table 1 and those for the LT1931A are listed in Table 2.
Besides these, there are many other inductors that can be
used. Consult each manufacturer for detailed information
and for their entire selection of related parts. Ferrite core
inductors should be used to obtain the best efficiency, as
U
core losses at frequencies above 1MHz are much lower for
ferrite cores than for powdered-iron units. When using
coupled inductors, choose one that can handle at least 1A
of current without saturating, and ensure that the inductor
has a low DCR (copper-wire resistance) to minimize I
2
R
power losses. If using uncoupled inductors, each inductor
need only handle one-half of the total switch current so
that 0.5A per inductor is sufficient. A 4.7µH to 15µH
coupled inductor or a 15µH to 22µH uncoupled inductor
will usually be the best choice for most LT1931 designs.
For the LT1931A, a 2.2µH to 4.7µH coupled inductor or a
3.3µH to 10µH uncoupled inductor will usually suffice. In
certain applications such as the “Charge Pump” inverting
DC/DC converter, only a single inductor is used. In this
case, the inductor must carry the entire 1A switch current.
Table 1. Recommended Inductors—LT1931
PART
CLS62-100
CR43-150
CR43-220
CTX10-1
CTX15-1
LQH3C100K24
LQH4C150K04
L
(µH)
10
15
22
10
15
10
15
Size
(L
×
W
×
H) mm
6.8
×
6.6
×
2.5
4.5
×
4.0
×
3.2
8.9
×
11.4
×
4.2
VENDOR
Sumida
(847) 956-0666
www.sumida.com
Coiltronics
(407) 241-7876
www. coiltronics.com
Murata
(404) 436-1300
www.murata.com
3.2
×
2.5
×
2.0
W
U U
Table 2. Recommended Inductors—LT1931A
PART
ELJPC3R3MF
ELJPC4R7MF
CLQ4D10-4R7
1
CLQ4D10-6R8
2
LB20164R7M
LB20163R3M
LQH3C4R7K24
LQH4C100K24
L
(µH)
3.3
4.7
4.7
6.8
4.7
3.3
4.7
10
Size
(L
×
W
×
H) mm
2.5
×
2.0
×
1.6
VENDOR
Panasonic
(408) 945-5660
www.panasonic.com
Sumida
(847) 956-0666
www.sumida.com
Taiyo Yuden
(408) 573-4150
www.t-yuden.com
Murata
(404) 436-1300
www.murata.com
7.6
×
4.8
×
1.8
2.0
×
1.6
×
1.6
3.2
×
2.5
×
2.0
1
Use drawing #5382-T039
2
Use drawing #5382-T041
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