LT3469
Piezo Microactuator Driver
with Boost Regulator
FEATURES
Amplifier
s
Current Limit:
±40mA
Typical
s
Input Common Mode Range: 0V to 10V
s
Output Voltage Range: 1V to (V
CC
– 1V)
s
Differential Gain Stage with High Impedance Output
(g
m
Stage)
s
Quiescent Current (from V ): 2mA
CC
s
Unloaded Gain: 30,000 Typical
Switching Regulator
s
Generates V
CC
Up to 35V
s
Wide Operating Supply Range: 2.5V to 16V
s
High Switching Frequency: 1.3MHz
s
Internal Schottky Diode
s
Tiny External Components
s
Current Mode Switcher with Internal Compensation
s
Low Profile (1mm) SOT-23 Package
DESCRIPTIO
The LT
®
3469 is a transconductance (g
m
) amplifier that can
drive outputs up to 33V from a 5V or 12V supply. An
internal switching regulator generates a boosted supply
voltage for the g
m
amplifier. The amplifier can drive
capacitive loads in the range of 5nF to 300nF. Slew rate is
limited only by the maximum output current. The 35V
output voltage capability of the switching regulator, along
with the high supply voltage of the amplifier, combine to
allow the wide output voltage range needed to drive a
piezoceramic microactuator.
The LT3469 switching regulator switches at 1.3MHz,
allowing the use of tiny external components. The output
capacitor can be as small as 0.22µF, saving space and cost
versus alternative solutions.
The LT3469 is available in a low profile ThinSOT
TM
package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
ThinSOT is a trademark of Linear Technology Corporation.
APPLICATIO S
s
s
s
Piezo Speakers
Piezo Microactuators
Varactor Bias
TYPICAL APPLICATIO
47µH
5V OR 12V
1µF
16V
3
V
IN
Piezo Microactuator Driver
Response Driving a 33nF Load
V
CC
FB
LT3469
GND
9.09k
7
8
10k
100k
4
1
V
OUT
1V TO 33V
PIEZO
ACTUATOR
5nF < C < 300nF
3469 TA03
5
SW
6
453k
2
16.5k
0.47µF
50V
V
OUT
10V/DIV
I
OUT
100mA/DIV
+
INPUT
0V TO 3V
+IN
–IN
+
–
OUT
INPUT
5V/DIV
50µs/DIV
3469 TA04
–
U
3469f
U
U
1
LT3469
ABSOLUTE
(Note 1)
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
OUT 1
FB 2
V
IN
3
GND 4
8 –IN
7 +IN
6 V
CC
5 SW
V
IN
Voltage ............................................................. 16V
SW Voltage ............................................................. 40V
V
CC
Voltage............................................................. 38V
+IN, –IN Voltage ..................................................... 10V
FB Voltage ................................................................ 3V
Current Into SW Pin ................................................. 1A
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
LT3469ETS8
TS8 PART MARKING
LTACA
TS8 PACKAGE
8-LEAD PLASTIC TSOT-23
T
JMAX
= 125°C,
θ
JA
= 250°C/W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. (Note 2) V
IN
= 5V, V
CC
= 35V, unless otherwise noted.
PARAMETER
g
m
Amplifier
Input Offset Voltage
Input Offset Current
Input Bias Current
Input Resistance—Differential Mode
Input Resistance—Common Mode
Common Mode Rejection Ratio
Power Supply Rejection Ratio—V
IN
Power Supply Rejection Ratio—V
CC
Gain
Transconductance
Maximum Output Current
Maximum Output Voltage, Sourcing
Minimum Output Voltage, Sinking
Output Resistance
Supply Current—V
CC
Switching Regulator
Minimum Operating Voltage
Maximum Operating Voltage
Feedback Voltage
FB Pin Bias Current
FB Line Regulation
Supply Current—V
IN
Switching Frequency
Maximum Duty Cycle
Switch Current Limit (Note 3)
Switch V
CESAT
CONDITIONS
V
OUT
= V
CC
/2
q
q
q
ELECTRICAL CHARACTERISTICS
MIN
TYP
3
10
150
1
200
100
120
85
30
20
220
±40
34.5
34.9
200
10
100
2
MAX
10
100
500
UNITS
mV
nA
nA
MΩ
MΩ
dB
dB
dB
V/mV
V/mV
µA/mV
µA/mV
mA
mA
V
V
mV
mV
kΩ
mA
V
V
V
nA
%/V
mA
MHz
%
mA
mV
3469f
V
CM
= 0V to 10V
V
IN
= 2.5V to 16V
V
CC
= 15V to 35V
No Load, V
OUT
= 2V to 33V
R
L
= 200k, V
OUT
= 2V to 33V
I
OUT
=
±100µA
q
V
OUT
= V
CC
/2
q
V
CC
= 35V, I
OUT
= 10mA
V
CC
= 35V, I
OUT
= 0mA
I
OUT
= –10mA
I
OUT
= 0mA
V
CC
= 35V, V
OUT
= 2V to 33V
V
CC
= 35V
70
80
65
15
10
160
140
±30
±23
34.0
34.5
260
300
±55
±58
1000
500
2.5
2.5
1.5
q
q
16
1.19
2.5V < V
IN
< 16V
q
q
q
0.8
88
165
I
SW
= 100mA
1.23
45
0.03
1.9
1.3
91
220
350
1.265
200
2.6
1.7
500
2
U
W
U
U
W W
W
LT3469
ELECTRICAL CHARACTERISTICS
PARAMETER
Switch Leakage Current
Diode V
F
Diode Reverse Leakage Current
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. (Note 2) V
IN
= 5V, V
CC
= 35V, unless otherwise noted.
CONDITIONS
V
SW
= 5V
I
D
= 100mA
V
R
= 5V
MIN
TYP
0.01
740
0.1
MAX
1
1100
1
UNITS
µA
mV
µA
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
The LT3469E is guaranteed to meet performance specifications
from 0°C to 85°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 limit is guaranteed by design and/or correlation to static
test. Slope compensation reduces current limit at higher duty cycles.
TYPICAL PERFOR A CE CHARACTERISTICS
(Switching Regulator)
V
IN
Quiescent Current
2.4
2.2
2.0
1.6
I
Q
(mA)
–50°C
100°C
200
CURRENT LIMIT (mA)
1.8
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0
2
4
6
25°C
SCHOTTKY CURRENT (mA)
8
V
IN
(V)
10
12
Schottky Reverse Leakage
25
V
R
= 5V
SWITCHING FREQUENCY (MHz)
LEAKAGE CURRENT (µA)
20
FB VOLTAGE (V)
15
10
5
0
–50
–25
0
25
50
TEMPERATURE (°C)
U W
14
3469 G05
Current Limit vs Duty Cycle
250
T
A
= 25°C
250
Schottky Forward Voltage
200
150
25°C
100°C
100
–50°C
50
150
100
50
16
0
0
20
60
40
DUTY CYCLE (%)
80
100
3469 G06
0
200 300 400 500 600 700 800 900 1000
FORWARD VOLTAGE (mV)
3469 G07
Switching Frequency
1.4
1.2
FB Pin Voltage and Bias Current
1.275
50
1.255
1.0
0.8
0.6
0.4
40
FB BIAS CURRENT (nA)
CURRENT
1.235
VOLTAGE
1.215
20
30
1.195
0.2
0
–50
10
75
100
3469 G09
–25
50
25
0
TEMPERATURE (°C)
75
100
3469 G10
1.175
–50
–25
0
25
50
TEMPERATURE (°C)
75
0
100
3469 G11
3469f
3
LT3469
TYPICAL PERFOR A CE CHARACTERISTICS
(g
m
Amplifier)
V
CC
Quiescent Current
2.5
100°C
2.0
–50°C
I
Q
(mA)
OUTPUT CURRENT (mA)
25°C
1.5
g
m
(µA/mV)
1.0
0.5
0
15
18
21
27
24
V
CC
(V)
30
33
36
PI FU CTIO S
OUT (Pin 1):
Output of the g
m
Amplifier. There must be at
least 5nF of capacitive load at the output in a gain of 10
configuration. Capacitive loads up to 300nF can be con-
nected to this pin. Piezo actuators below 5nF can be driven
if capacitance is placed in parallel to bring the total
capacitance to 5nF.
FB (Pin 2):
Feedback Pin. Reference voltage is 1.23V.
Connect feedback resistor divider here.
V
IN
(Pin 3):
Input Supply Pin. Must be locally bypassed.
GND (Pin 4):
Ground Pin. Connect directly to local ground
plane.
SW (Pin 5):
Switch Pin. Connect inductor here. Minimize
trace area at this pin to reduce EMI.
V
CC
(Pin 6):
Output of Switching Regulator and Supply
Rail for g
m
Amp. There must be 0.22µF or more of
capacitance here.
+IN (Pin 7):
Noninverting Terminal of the g
m
Amplifier.
–IN (Pin 8):
Inverting Terminal of the g
m
Amplifier.
BLOCK DIAGRA
+IN
7
–IN
8
+
g
m
–
4
U W
3469 G01
Output Current
vs Differential Input Voltage
30
25
20
15
10
5
0
–5
–10
–15
–20
–25
–30
–50 –40 –30 –20 –10 0 10 20 30 40 50
DIFFERENTIAL INPUT VOLTAGE (mV)
3469 G02
g
m
vs V
CC
250
–50°C
25°C
200
100°C
150
100°C
–50°C
25°C
100
50
0
15
20
25
V
CC
(V)
30
35
3469 G14
W
U
U
U
V
IN
V
CC
FB
OUT
1
1.23V
2
V
IN
3
SW
5
V
CC
6
–
A1
SWITCH
CONTROLLER
Q1
+
4
GND
3469 F01
Figure 1. LT3469 Block Diagram
3469f
LT3469
OPERATIO
g
m
Amplifier
The LT3469 is a wide output voltage range g
m
amplifier
designed to drive capacitive loads. Input common mode
range extends from 10V to ground. The output current is
proportional to the voltage difference across the input
terminals. When the output voltage has settled, the input
terminals will be at the same voltage; supply current of the
amplifier will be low and power dissipation will be low. If
presented with an input differential, however, the output
current can increase significantly, up to the maximum
output current (typically 40mA). The output voltage slew
rate is determined by the maximum output current and the
output capacitance, and can be quite high. With a 10nF
load, the output slew rate will typically be 4V/µs. The
capacitive load compensates the g
m
amplifier and must be
present for stable operation. The gain capacitance product
of the amplifier must be at least 50nF. For example, if the
amplifier is operated in a gain of 10 configuration, a
minimum capacitance of 5nF is necessary. In a gain of 20
configuration, a minimum of 2.5nF is necessary. Closed
loop –3dB bandwidth is set by the output capacitance.
Typical closed loop bandwidth is approximately:
g
m
2π • A
V
• C
OUT
where g
m
= 200µA/mV
For example, an amplifier in a gain of 10 configuration with
10nF of output capacitance will have a closed loop –3dB
bandwidth of approximately 300kHz. Figure 3 shows typi-
100
SLEW RATE (V/µs)
10
BANDWIDTH (kHz)
1
0.1
2
20
CAPACITANCE (nF)
200
3469 F02
Figure 2. Slew Rate vs Capacitance
U
cal bandwidth of a gain of 10 configuration per output
capacitance.
In applications where negative phase contributions below
crossover frequency must be minimized, a phase boost
capacitor can be added, as shown in Figure 4. Larger val-
ues of C
BOOST
will further reduce the closed-loop negative
phase contribution, however, the amplifier phase margin
will be reduced. For an amplifier phase margin of approxi-
mately 55°, select C
BOOST
as follows:
C
BOOST
=
C
OUT
(
R1 / R2
+
1
)
g
m
(
R1|| R2
)
where g
m
= 200µA/mV.
In a gain of 10 configuration, choosing C
BOOST
as de-
scribed will lead to nearly zero closed-loop negative phase
contribution at 3kHz for values of C
OUT
from 10nF to
200nF. The phase boost capacitor should not be used if
C
OUT
is less than twice the minimum for stable operation.
The gain capacitance product should therefore be higher
than 100nF if a phase boost capacitor is used.
Switching Regulator
The LT3469 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. The switch controller sets the
peak current in Q1 proportional to its input. The input to the
switch controller is set by the error amplifier, A1, and is
1000
WITH PHASE
BOOST CAPACITOR
100
WITHOUT PHASE
BOOST CAPACITOR
10
2
20
CAPACITANCE (nF)
200
3469 F03
Figure 3. Closed Loop –3dB Bandwidth
vs Capacitance in a Gain of 10 Configuration
3469f
5
