LT1932
Constant-Current DC/DC
LED Driver in ThinSOT
FEATURES
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DESCRIPTIO
Up to 80% Efficiency
Inherently Matched LED Current
Adjustable Control of LED Current
Drives Five White LEDs from 2V
Drives Six White LEDs from 2.7V
Drives Eight White LEDs from 3V
Disconnects LEDs In Shutdown
1.2MHz Fixed Frequency Switching
Uses Tiny Ceramic Capacitors
Uses Tiny 1mm-Tall Inductors
Regulates Current Even When V
IN
> V
OUT
Operates with V
IN
as Low as 1V
Low Profile (1mm) ThinSOT
TM
Package
The LT
®
1932 is a fixed frequency step-up DC/DC converter
designed to operate as a constant-current source. Be-
cause it directly regulates output current, the LT1932 is
ideal for driving light emitting diodes (LEDs) whose light
intensity is proportional to the current passing through
them, not the voltage across their terminals.
With an input voltage range of 1V to 10V, the device works
from a variety of input sources. The LT1932 accurately
regulates LED current even when the input voltage is
higher than the LED voltage, greatly simplifying battery-
powered designs. A single external resistor sets LED
current between 5mA and 40mA, which can then be easily
adjusted using either a DC voltage or a pulse width
modulated (PWM) signal. When the LT1932 is placed in
shutdown, the LEDs are disconnected from the output,
ensuring a quiescent current of under 1µA for the entire
circuit. The device’s 1.2MHz switching frequency permits
the use of tiny, low profile chip inductors and capacitors to
minimize footprint and cost in space-conscious portable
applications.
, LTC and LT are registered trademarks of Linear Technology Corporation.
ThinSOT is a trademark of Linear Technology Corporation.
APPLICATIO S
s
s
s
s
s
Cellular Telephones
Handheld Computers
Digital Cameras
Portable MP3 Players
Pagers
TYPICAL APPLICATIO
L1
6.8µH
Li-Ion Driver for Four White LEDs
D1
85
80
75
70
65
60
55
0
1932 TA01
V
IN
2.7V TO 4.2V
C1
4.7µF
6
V
IN
LT1932
5
1
SW
3
15mA
PWM
DIMMING
CONTROL
SHDN
R
SET
4
R
SET
1.50k
LED
GND
2
C2
1µF
EFFICIENCY (%)
C1: TAIYO YUDEN JMK212BJ475
C2: TAIYO YUDEN EMK212BJ105
D1:ZETEX ZHCS400
L1: SUMIDA CLQ4D106R8 OR PANASONIC ELJEA6R8
U
Efficiency
V
IN
= 4.2V
V
IN
= 2.7V
5
10
15
LED CURRENT (mA)
20
1932 TA02
U
U
1932f
1
LT1932
ABSOLUTE
(Note 1)
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
SW 1
GND 2
LED 3
6 V
IN
5 SHDN
4 R
SET
V
IN
Voltage ............................................................. 10V
SHDN Voltage ......................................................... 10V
SW Voltage ............................................................. 36V
LED Voltage ............................................................. 36V
R
SET
Voltage ............................................................. 1V
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
ORDER PART
NUMBER
LT1932ES6
S6 PART MARKING
LTST
S6 PACKAGE
6-LEAD PLASTIC SOT-23
T
JMAX
= 125°C,
θ
JA
= 250°C/ W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
The
q
denotes specifications that apply over the full operating temperature
range, otherwise specifications are at T
A
= 25°C. V
IN
= 1.2V, V
SHDN
= 1.2V, unless otherwise noted.
PARAMETER
Minimum Input Voltage
Quiescent Current
R
SET
Pin Voltage
LED Pin Voltage
LED Pin Current
V
RSET
= 0.2V
V
SHDN
= 0V
R
SET
= 1.50k
R
SET
= 1.50k, V
IN
< V
OUT
(Figure 1)
R
SET
= 562Ω, V
IN
= 1.5V
R
SET
= 750Ω, V
IN
= 1.2V
R
SET
= 1.50k, V
IN
= 1.2V
R
SET
= 4.53k, V
IN
= 1.2V
I
LED
= 15mA
V
IN
= 1V
q
ELECTRICAL CHARACTERISTICS
CONDITIONS
MIN
TYP
1.2
0.1
100
120
MAX
1
1.6
1.0
180
45
36
17.5
UNITS
V
mA
µA
mV
mV
mA
mA
mA
mA
mA/°C
MHz
%
mA
mV
µA
µA
V
V
µA
33
25
12.5
38
30
15
5
– 0.02
1.2
95
550
150
0
15
LED Pin Current Temperature Coefficient
Switching Frequency
Maximum Switch Duty Cycle
Switch Current Limit
Switch V
CESAT
SHDN Pin Current
Start-Up Threshold (SHDN Pin)
Shutdown Threshold (SHDN Pin)
Switch Leakage Current
0.8
90
400
1.6
780
200
0.1
30
0.25
I
SW
= 300mA
V
SHDN
= 0V
V
SHDN
= 2V
0.85
Switch Off, V
SW
= 5V
0.01
5
Note 1:
Absolute Maximum Ratings are those values beyond which the life of
a device may be impaired.
Note 2:
The LT1932E 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.
2
U
1932f
W
U
U
W W
W
LT1932
TYPICAL PERFOR A CE CHARACTERISTICS
Switch Saturation Voltage (V
CESAT
)
400
700
SWITCHING FREQUENCY (MHz)
SWITCH SATURATION VOLTAGE (mV)
350
PEAK CURRENT (mA)
300
250
200
150
100
50
0
0
100
T
J
= 125°C
T
J
= 25°C
500
400
300
200
100
T
J
= –50°C
400
500
300
SWITCH CURRENT (mA)
200
LED Pin Voltage
400
350
LED PIN VOLTAGE (mV)
300
LED CURRENT (mA)
250
T
J
= 125°C
200
150
T
J
= –50°C
100
50
0
0
5
10
15 20 25 30
LED CURRENT (mA)
35
40
T
J
= 25°C
30
25
20
15
10
5
0
– 50 – 25
0
R
SET
= 750Ω
LED CURRENT (mA)
Quiescent Current
2.00
1.75
QUIESCENT CURRENT (mA)
1.50
1.25
1.00
0.75
0.50
0.25
0
– 50 – 25
0
SHDN PIN CURRENT
V
IN
= 10V
V
IN
= 1.2V
75
50
25
TEMPERATURE (°C)
U W
1932 G01
Switch Current Limit
2.0
V
IN
= 1.2V
V
IN
= 10V
Switching Frequency
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
V
IN
= 10V
V
IN
= 1.2V
600
600
0
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
0
–50 –25
50
25
0
75
TEMPERATURE (°C)
100
125
1932 G02
1932 G03
LED Current
50
45
40
35
R
SET
= 562Ω
50
45
40
35
30
25
20
15
10
R
SET
= 4.53k
5
0
75
50
25
TEMPERATURE (°C)
100
125
LED Current
R
SET
= 562Ω
R
SET
= 750Ω
R
SET
= 1.50k
R
SET
= 1.50k
R
SET
= 4.53k
0
2
6
INPUT VOLTAGE (V)
4
8
10
1932 G06
1932 G04
1932 G05
SHDN Pin Current
50
45
40
35
30
25
20
15
10
5
100
125
0
0
2
6
8
4
SHDN PIN VOLTAGE (V)
10
1932 G08
Switching Waveforms
T
J
= –50°C
V
SW
10V/DIV
I
L1
200mA/DIV
V
OUT
20mV/DIV
AC COUPLED
I
LED
10mA/DIV
V
IN
= 3V
0.5µs/DIV
4 WHITE LEDs
I
LED
= 15mA
CIRCUIT ON FIRST PAGE
OF THIS DATA SHEET
1093 G09
T
J
= 25°C
T
J
= 125°C
1932 G07
1932f
3
LT1932
PI FU CTIO S
SW (Pin 1):
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.
GND (Pin 2):
Ground Pin. Tie this pin directly to local
ground plane.
LED (Pin 3):
LED Pin. This is the collector of the internal
NPN LED switch. Connect the cathode of the bottom LED
to this pin.
R
SET
(Pin 4):
A resistor between this pin and ground
programs the LED current (that flows into the LED pin).
This pin is also used to provide LED dimming.
SHDN (Pin 5):
Shutdown Pin. Tie this pin higher than
0.85V to turn on the LT1932; tie below 0.25V to turn it off.
V
IN
(Pin 6):
Input Supply Pin. Bypass this pin with a
capacitor to ground as close to the device as possible.
BLOCK DIAGRA
V
IN
C1
OPERATIO
The LT1932 uses a constant frequency, current mode
control scheme to regulate the output current, I
LED
.
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 the PWM comparator
A2 is proportional to the switch current, summed to-
gether with a portion of the oscillator ramp. When this
signal reaches the level set by the output of error amplifier
A1, comparator A2 resets the latch and turns off the
4
W
U
U
U
U
L1
SHDN
V
IN
SW
D1
V
OUT
1
Q1
C2
5
6
DRIVER
+
0.04Ω
×5
–
1.2MHz
OSCILLATOR
S
Q
R
A2
+
+
Σ
+
DRIVER
Q2
3
LED
I
LED
–
–
A1
+
LED CURRENT
REFERENCE
2
GND
4
I
SET
1932 F01
R
SET
R
SET
Figure 1. LT1932 Block Diagram
power switch. In this manner, A1 sets the correct peak
current level to keep the LED current in regulation. If A1’s
output increases, more current is delivered to the output;
if it decreases, less current is delivered. A1 senses the
LED current in switch Q2 and compares it to the current
reference, which is programmed using resistor R
SET
. The
R
SET
pin is regulated to 100mV and the output current,
I
LED
, is regulated to 225 • I
SET
. Pulling the R
SET
pin higher
than 100mV will pull down the output of A1, turning off
power switch Q1 and LED switch Q2.
1932f
LT1932
APPLICATIO S I FOR ATIO
Inductor Selection
Several inductors that work well with the LT1932 are listed
in Table 1. Many different sizes and shapes are available.
Consult each manufacturer for more detailed information
and for their entire selection of related parts. As core
losses at 1.2MHz are much lower for ferrite cores that for
the cheaper powdered-iron ones, ferrite core inductors
should be used to obtain the best efficiency. Choose an
inductor that can handle at least 0.5A and ensure that the
inductor has a low DCR (copper wire resistance) to mini-
mize I
2
R power losses. A 4.7µH or 6.8µH inductor will be
a good choice for most LT1932 designs.
Table 1. Recommended Inductors
L
(µH)
4.7
6.8
4.7
10
4.7
6.8
4.7
6.8
4.7
6.8
MAX
DCR
(mΩ)
180
250
260
300
250
350
216
296
162
195
MAX
HEIGHT
(mm)
2.2
2.2
2.2
2.2
1.6
1.6
0.8
0.8
1.2
1.2
PART
ELJEA4R7
ELJEA6R8
LQH3C4R7M24
LQH3C100M24
LB2016B4R7
LB2016B100
CMD4D06-4R7
CMD4D06-6R8
CLQ4D10-4R7
CLQ4D10-6R8
VENDOR
Panasonic
(714) 373-7334
www.panasonic.com
Murata
(814) 237-1431
www.murata.com
Taiyo Yuden
(408) 573-4150
www.t-yuden.com
Sumida
(847) 956-0666
www.sumida.com
EFFICIENCY (%)
Inductor Efficiency Considerations
Many applications have thickness requirements that re-
strict component heights to 1mm or 2mm. There are 2mm
tall inductors currently available that provide a low DCR
and low core losses that help provide good overall effi-
ciency. Inductors with a height of 1mm (and less) are
becoming more common, and a few companies have
introduced chip inductors that are not only thin, but have
a very small footprint as well. While these smaller induc-
tors will be a necessity in some designs, their smaller size
gives higher DCR and core losses, resulting in lower
efficiencies. Figure 2 shows efficiency for the Typical
Application circuit on the front page of this data sheet, with
several different inductors. The larger devices improve
EFFICIENCY (%)
U
efficiency by up to 12% over the smaller, thinner ones.
Keep this in mind when choosing an inductor.
The value of inductance also plays an important role in the
overall system efficiency. While a 1µH inductor will have
a lower DCR and a higher current rating than the 6.8µH
version of the same part, lower inductance will result in
higher peak currents in the switch, inductor and diode.
Efficiency will suffer if inductance is too small. Figure 3
shows the efficiency of the Typical Application on the front
page of this data sheet, with several different values of the
same type of inductor (Panasonic ELJEA). The smaller
values give an efficiency 3% to 5% lower than the 6.8µH
value.
85
80
75
70
65 TAIYO YUDEN
LB2016B6R8
60
TAIYO YUDEN
LB2012B6R8
55
0
5
SUMIDA
CLQ4D10-6R8
SUMIDA
CMD4D06-6R8
PANASONIC
ELJEA6R8
V
IN
= 3.6V
4 WHITE LEDs
ALL ARE 10µH
INDUCTORS
20
1932 F02
W
U U
10
15
LED CURRENT (mA)
Figure 2. Efficiency for Several Different Inductor Types
85
80
6.8µH
75
70
65
60
55
0
5
V
IN
= 3.6V
4 WHITE LEDs
PANASONIC ELJEA
INDUCTORS
10
15
LED CURRENT (mA)
20
1932 F03
22µH
4.7µH
2.2µH
Figure 3. Efficiency for Several Different Inductor Values
1932f
5
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