LT3472
Boost and Inverting
DC/DC Converter
for CCD Bias
DESCRIPTIO
The LT
®
3472 dual channel switching regulator generates
positive and negative outputs for biasing CCD imagers.
The device delivers up to –8V at 50mA and 15V at 20mA
from a lithium-ion cell, providing bias for many popular
CCD imagers. Switching at 1.1MHz, the LT3472 uses tiny,
low profile capacitors and inductors and generates low
noise outputs that are easy to filter. Schottky diodes are
internal and the output voltages are set with one resistor
per channel, reducing external component count. The
entire solution is less than 1mm profile and occupies just
50mm
2
.
Internal sequencing circuitry disables the negative chan-
nel until the positive channel has reached 88% of its final
value, ensuring that the sum of the two outputs is always
positive. Separate soft-start capacitors for each output
allow the ramp of each output to be independently
controlled.
The LT3472 is available in a low profile (0.75mm) 10-pin
3mm
×
3mm DFN package.
FEATURES
■
■
■
■
■
■
■
■
■
Generates 15V at 20mA, –8V at 50mA
from a Li-Ion Cell
Internal Schottky Diodes
V
IN
Range: 2.2V to 16V
Output Voltages Up to
±34V
Capacitor-Programmable Soft-Start
Sequencing: Positive Output Reaches 88% of Final
Value Before Negative Output Begins
Requires Only One Resistor to Set Output Voltage
Constant Switching Frequency Ensures Low
Noise Outputs
Available in a 10-Lead (3mm
×
3mm) DFN Package
APPLICATIO S
■
■
■
■
CCD Bias
TFT LCD Bias
OLED Bias
±
Rail Generation for Op Amps
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATIO
V
IN
3V TO 4.2V
22µH
47µH
Li-Ion CCD Bias Supply
Conversion Efficiency
2.2µF
1µF
80
75
POS CHANNEL
85
EFFICIENCY (%)
V
POS
15V
20mA
SWP
V
POS
550k
FBP
4.7pF
V
IN
SWN
DN
70
65
60
55
50
LT3472
320k
FBN
10pF
47µH
V
NEG
–8V
50mA
SHDN
SHDN
SSP
GND
100nF
SSN
100nF
2.2µF
45
40
0
10
30
20
LOAD CURRENT (mA)
40
50
2.2µF
3472 TA01a
U
U
U
NEG CHANNEL
3472 TA01b
3472f
1
LT3472
ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
SWP
V
IN
SHDN
SWN
DN
1
2
3
4
5
11
10 V
POS
9 SSP
8 FBP
7 SSN
6 FBN
V
IN
, SHDN Voltage ................................................... 16V
SWP, SWN, V
POS
Voltage ....................................... 36V
DN Voltage ............................................................ –36V
FBP, FBN, SSP, SSN Voltage ................................... 10V
Maximum Junction Temperature .......................... 125°C
Operating Temperature Range
Extended Commercial ......................... –40°C to 85°C
Storage Temperature Range ................. – 65°C to 125°C
ORDER PART
NUMBER
LT3472EDD
DFN PART
MARKING
LBGC
DD PACKAGE
10-LEAD (3mm
×
3mm) PLASTIC DFN
T
JMAX
= 125°C,
θ
JA
= 43°C/W,
θ
JC
= 3°C/W
EXPOSED PAD IS GND (PIN 11)
MUST BE SOLDERED TO PCB
Consult LTC Marketing for parts specified with wider operating temperature ranges.
The
●
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 3V, SHDN = 3V unless otherwise noted.
PARAMETER
Minimum Operation Voltage
Maximum Operation Voltage
Supply Current
SHDN Voltage High
SHDN Voltage Low
SHDN Pin Bias Current
Positive Feedback Voltage
Negative Feedback Voltage
Positive Feedback Voltage Line Regulation
Negative Feedback Voltage Line Regulation
FBP Current
FBN Current
FBP to Start Negative Channel
Switching Frequency
Maximum Duty Cycle (Both Channels)
Positive Channel Switch Current Limit
Negative Channel Switch Current Limit
Positive Channel Switch V
CESAT
Negative Channel Switch V
CESAT
Switch Leakage Current (Both Channels)
Schottky DP Forward Drop
Schottky DN Forward Drop
Schottky Leakage Current (Both Channels)
I
SWP
= 200mA
I
SWN
= 200mA
V
SW
= 5V
I
DP
= 150mA
I
DN
= 150mA
V
R
= 36V
●
●
●
ELECTRICAL CHARACTERISTICS
CONDITIONS
MIN
2.2
TYP
MAX
16
UNITS
V
V
mA
µA
V
V
µA
V
mV
%/V
mV/V
SHDN = 3V, Not Switching
SHDN = 0V
●
●
2.8
0.1
0.8
1
0.3
SHDN = 3V
●
●
35
1.2
–5
1.25
0
0.01
0.008
1.3
5
FBP = V
FBP
FBN = V
FBN
●
●
24.5
24.5
1.02
0.9
88
250
300
25
25
1.1
1.1
92
350
400
245
400
0.01
700
750
25.3
25.3
1.18
1.4
5
950
1000
4
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
The LT3472E is guaranteed to meet specified performance from
0°C to 70°C. Specifications over the –40°C to 85°C operating range are
assured by design, characterization and correlation with statistical process
controls.
3472f
2
U
µA
µA
V
MHz
%
mA
mA
mV
mV
µA
mV
mV
µA
W
U
U
W W
W
LT3472
TYPICAL PERFOR A CE CHARACTERISTICS
Quiescent Current
3.1
3.0
1.13
1.12
1.11
2.9
1.10
1.28
QUIESCENT CURRENT (mA)
FBP VOLTAGE (V)
50
0
TEMPERATURE (°C)
VFB1 (V)
2.8
2.7
2.6
2.5
–50
50
0
TEMPERATURE (°C)
FBN Voltage
10
26.0
25.5
FBN BIAS CURRENT (µA)
FBP BIAS CURRENT (µA)
5
FBN VOLTAGE (mV)
0
–5
–10
–50
50
0
TEMPERATURE (°C)
SHDN Pin Bias Current
300
250
POSITIVE SWITCH SATURATION VOLTAGE (mV)
SHDN CURRENT (µA)
200
150
100
50
0
0
U W
3472 G01
3472 G04
Minimum FBP Voltage to Enable
Inverter
1.30
FBP Voltage
1.26
1.09
1.08
1.07
1.24
1.22
1.06
1.05
–50
1.20
50
100
100
3472 G02
–50
0
100
3472 G03
TEMPERATURE (°C)
FBP Bias Current
26.0
FBN Bias Current
25.5
25.0
25.0
24.5
24.5
100
24.0
–50
50
0
TEMPERATURE (°C)
100
3472 G05
24.0
–50
0
50
100
3472 G06
TEMPERATURE (°C)
Positive Channel Switch V
CESAT
350
90°C
300
250
200
150
100
50
0
25°C
–45°C
5
10
15
SHDN VOLTAGE (V)
20
3472 G07
0
50
100
150
200
250
SWITCH CURRENT (mA)
300
3472 G08
3472f
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LT3472
TYPICAL PERFOR A CE CHARACTERISTICS
Positive Channel Schottky
I-V Characteristic
NEGATIVE SWITCH SATURATION VOLTAGE (mV)
POSITIVE SCHOTTKY FORWARD CURRENT (mA)
400
350
90°C
300
250
200
150
100
50
0
0
0.8
0.6
SCHOTTKY FORWARD DROP (V)
0.2
0.4
1.0
3472 G09
600
500
25°C
400
–45°C
300
200
100
0
90°C
NEGATIVE SCHOTTKY FORWARD CURRENT (mA)
4
U W
25°C
–45°C
Negative Channel Switch V
CESAT
350
300
250
200
150
100
50
0
Negative Channel Schottky
I-V Characteristic
25°C
90°C
–45°C
0
50
100
150
200
SWITCH CURRENT (mA)
250
3472 G10
0
0.8
0.6
SCHOTTKY FORWARD DROP (V)
0.2
0.4
1.0
3472
G11
3472f
LT3472
PI FU CTIO S
SWP (Pin 1):
Switch Pin for Positive (Boost) Channel.
Connect boost inductor here.
V
IN
(Pin 2):
Input Supply Pin. Must be locally bypassed
with a X5R or X7R type ceramic capacitor.
SHDN (Pin 3):
Shutdown Pin. Connect to 0.8V or higher to
enable device, 0.3V or less to disable device.
SWN (Pin 4):
Switch Pin for Negative (Inverter) Channel.
Connect inverter input inductor and flying capacitor here.
DN (Pin 5):
Anode of Internal Schottky for Inverter.
Connect inverter output inductor and flying capacitor
here.
FBN (Pin 6):
Feedback Pin for Inverter. Connect feedback
resistor R2 from this pin to V
O2
. Choose R2 according to
V
O2
= 1.25 • R2/50k. Pin voltage = 0V when regulated.
SSN (Pin 7):
Soft Start-Up Pin for Inverter. Connect a cap
here for soft start-up. Leave open for quick start-up. This
pin is connected to 1.25V with a 50k resistor internally.
FBP (Pin 8):
Feedback Pin for Boost. Connect boost
feedback resistor R1 from this Pin to V
O1
. Choose R1
according to V
O1
= 1.25 • (1 + R1/50k). Pin voltage = 1.25V
when regulated.
SSP (Pin 9):
Soft Start-Up Pin for Boost. Connect a cap
here for soft start-up. Leave open for quick start-up. This
pin is connected to 1.25V with a 50k resistor internally.
V
POS
(Pin 10):
Output Pin for Boost. Connect boost output
capacitor here.
GND (Exposed Pad) (Pin 11):
GND Pin. Tie directly to
ground plane through multiple vias under the package for
optimum thermal performance.
BLOCK DIAGRA
FBP 8
50k
V
IN
2
V
REF
1.25V
FBN 6
SSN
7
50k
1.25V
50k
SSP
9
W
U
U
U
SWP
1
–
A1
COMPARATOR
DP
X1
DRIVER 1
Q
S
Q1
10 V
POS
–
A2
+
+
∑
R
–
+
RAMP
GENERATOR
11 GND
50k
1.2MHz
OSCILLATOR
COMPARATOR
A3
3
SHDN
+
–
–
A4
X2
R
S
Q
DRIVER 2
Q2
4
SWN
+
–
∑
+
RAMP
GENERATOR
DN
5
DN
3472 BD
Figure 1. LT3472 Block Diagram
3472f
5
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