Linear Technology 453' LT1970 User manual

LT1970
1
DESCRIPTION
Demonstration circuit 453 is useful for evaluating the
LT1970, a pow er amplifier w ith ad ustable current limit-
ing. The demo circuit is available in tw o versions;
DC453A-A w ith up to 500mA of output current and
DC453A-B for applications w ith up to 5 Amps of output
current. For the 5 Amp version a Class B M OSFET based
current boost stage has been added to the output section
w ith current limit control still provided by the LT1970.
The maximum output current of the board is set by tw o
on-board potentiometers to permit independent control
of the sinking and sourcing current limit. The current
limit ad ustment can also be provided by external voltage
sources. Symmetrical sourcing and sinking current limit
w ith a single control is also possible.
LEDs provide an indication of faults detected by the
LT1970. Separate indicators illuminate w hen the amplifier
enters sourcing or sinking current limit and if the LT1970
overheats and enters thermal shutdow n protection.
Several umpers enable flexible amplifier operation. Con-
figurations include:
■
Single or dual pow er supplies
■
Separate or common input stage and pow er output
stage supplies
■
AC or DC coupled inputs
■
Non-Inverting gains of 1 or 2, Inverting gain of –1
■
Separate or common current limit control
Design files for this circuit board are available. Call
the LTC factory.
, LTC and LT are registered trademarks of Linear Technology Corporation. ThinSOT and
Pow erPath are trademarks of Linear Technology Corporation.
TYPICAL PERFORMANCE SUMMARY
T
A
= 25°C, Com m on ±12V Supplies
SYM BOL PARAM ETER CON ITIONS M IN TYP M AX UNITS
V
SUPPLY
Pow er Supply Range Single Supply
Dual Supply
8
±8
36
±18
V
V
I
OUT
Ad ustable Output Current Limit Range DC453A-A (LT1970 basic amplifier), Vcc ≥ 8V
DC453A-B (Boosted)
±4
±0.04
±500
±5
mA
A
BW
S
Small Signal –3dB Bandw idth
DC453A-A (LT1970 basic amplifier)
A
V
= +1, Vout=200mV
P-P
, R
LOAD
=100 Ω ,
DC453A-B (Boosted)
A
V
= +1, Vout=200mV
P-P
, R
LOAD
=10 Ω
1.6
7.2
M Hz
KHz
BW
L
Large Signal –3dB Bandw idth
DC453A-A (LT1970 basic amplifier)
A
V
= +1, Vout=20V
P-P
, R
LOAD
=100 Ω
DC453A-B (Boosted)
A
V
= +1, Vout=10V
P-P
, R
LOAD
=10 Ω ,
58
7
KHz
KHz
SR Slew Rate 1.6 V/µS
V
SATH
Output Sat Voltage High DC453A-A (LT1970 basic amplifier), V
SAT
=V+-V
OUT
Common ±12V Supplies, R
LOAD
=250 Ω
Common ±12V Supplies, R
LOAD
=25 Ω
Vcc/Vee=±12V and V+/V-=±5V, R
LOAD
=100 Ω
Vcc/Vee=±12V and V+/V-=±5V, R
LOAD
=10 Ω
DC453A-B (Boosted), See Operational Notes
1.4
2.1
0.1
0.8
V
V
V
V
DEMO CIRCUIT
453
QUICK START GUIDE
LT1970
Power Amplifier with
Adjustable Current Limiting

LT1970
2
TYPICAL PERFORMANCE SUMMARY
T
A
= 25°C, Com m on ±12V Supplies
SYM BOL PARAM ETER CON ITIONS M IN TYP M AX UNITS
V
SATL
Output Sat Voltage Low DC453A-A (LT1970 basic amplifier), V
SAT
= V
OUT-
V-
Common ±12V Supplies, R
LOAD
=250 Ω
Common ±12V Supplies, R
LOAD
=25 Ω
Vcc/Vee=±12V and V+/V-=±5V, R
LOAD
=100 Ω
Vcc/Vee=±12V and V+/V-=±5V, R
LOAD
=10 Ω
DC453A-B (Boosted), See Operational Notes
2.1
2.8
0.3
1.3
V
V
V
V
I
S
Total Q uiescent Supply Current No Load
DC453A-A (LT1970 basic amplifier)
DC453A-B (Boosted)
9
12
mA
mA
QUICK START PROCEDURE
Demonstration circuit 453 is easy to set up to evaluate
the performance of the LT1970. Before applying pow er to
the amplifier, configure all on-board umpers for the de-
sired operation. Figure 1 provides an overview of how the
placement of each of the umpers ad usts the configura-
tion. Position each umper for the follow ing results:
DC coupling, non-inverting, applies the input signal di-
rectly to the amplifier + input w ith a 5K Ω input imped-
ance, inverting, directly connects to a 10K Ω resistor to
the amplifier – input.
AC coupling connects the input signal through a series
10µF capacitor. Depending on the amplifier topology the
low er –3dB corner frequency is in the range of 1.6Hz to
6.2Hz.
Directs the input signal for an Inverting or Non-Inverting
amplifier.
DUAL sets the DC bias of the inputs and output to
ground w hen symmetrical dual + and - pow er supplies
are used.
SINGLE sets the DC bias of the inputs and output to ½
Vcc w hen a single pow er supply is used. This setting also
biases the inputs and output to the midpoint of asymmet-
rical dual supplies.
Removing the shunt from this umper can allow the
pow er amplifier to bias at the same dc potential as the
input signal for DC coupled single supply uses.
The four positions of this umper configure the amplifier
to one of three gain values. The inset diagram on the
schematic, Figure 4, show s how the amplifier feedback is
arranged for each umper position. A small table is also
included on the printed circuit board to serve as a ready
reference for the umper settings.
The umper position shorting pins 2 and 3 provides the
same configuration as shorting pins 3 and 4.
Separate (S) position allow s for the independent setting
of the amplifier sourcing current limit and sinking current
limit through tw o 10K potentiometers and an on-board
5V regulator. Any voltage betw een 0V and 5V applied to
the LT1970 pins VCSRC and VCSNK sets the output cur-
rent limit value.
JP1: Input Signal Coupling, AC or C
JP2: Inverting (INV) or Non
-
Inverting (NI)
JP3: Input biasing for SINGLE or UAL Pow er Su
p
plies
JP4: Gain Configuration Setting
JP5: Separate (S) or Com m on (C) Current Lim it Co
n
t
rol

LT1970
3
Common (C) position connects the VCSRC and VCSN K
input pins together to force the sourcing and sinking cur-
rent limit values to be the same. W ith this setting, only
the potentiometer labeled VCSRC ad usts the current
limit control voltage.
SINGLE setting connects the Vee supply of the LT1970
directly to ground.
DUAL setting connects the Vee supply to the VEE input
ack.
W ith the Common (COM ) selection ust one pow er
source, a single positive supply, or one pair of plus
and minus supplies, is required. This supply pow ers both
the input stage supplies of the LT1970, Vcc and Vee, and
the output stage supplies, V+ and V-.
W ith the Separate (SEP) selection the output stage sup-
plies must be provided separately to the V+ and Vinput
acks. This feature reduces pow er dissipation in the out-
put stage by running the supplies at a low er value than
the main amplifier input stage.
.
Figure 1. C453A Jum per Configuration
JP7: Com m on (COM ) or Separate (SEP) Pow er Su
p
plies
JP6: Vee biasing for SINGLE or UAL Pow er Su
p
plies
DC AC
COM SEP
53 2 1
S C
IN V NO N SIN G LE DU AL
SIN G LE DU AL
Inverting or Non-
Inverting
Amplifier
Single Supply (DC Vout=Vcc/2) or
Dual Supply (DC Vout=0V)
DC or AC
Coupled Input
JP1
JP2 JP3
JP
JP6
JP7
JP5
Amplifier Gain
Configuration
G ain Setting Table
Single Supply (LT1970
Vee=0V) or Dual Supply
(LT1970 Vee=Vee
Voltage)
Common or Separate Input
Stage and O utput Stage
Supplies
Separate or Common Source
and Sink Current Limit Control
Note proper shunt
orientation.

LT1970
4
POWER CONNECTIONS
Figure 2 show s how to properly connect pow er to the
453 demo circuit.
Figure 2. Pow er Supply Connections
OPTIONAL EXTERNAL
CONNECTIONS
Several test point turrets have been added, see Figure
3, to make it easy to use external equipment to control
the operation of the pow er amplifier.
Figure 3. External Control Options
Input Signal
M anual Current
Lim it Adjustm ents
Source
Sink
Load
External Current Lim it
Control
(Use Source Supply
Only for Single
Symmetrical Current
Limit Control)
0V to 5Vdc
0V to 5Vdc
Sink Source
External
Enable(5V)/Disable(0V)
Pulse
Thermal Shutdow n
Sink Current Limit
Source Current Limit
Error Flag M onitoring
TP11
TP3
TP4
(TP14)
(TP15)
(TP16)
TP10
J4 J6
Output Signal
Dual Supplies Separate Dual Supplies Single Supply

LT1970
5
OPERATIONAL NOTES
M inim um Supply Voltage
The LT1970 can operate w ith a total supply voltage of
only 5 Volts. For convenience, a 5V regulator is included
on DC453 to provide the control voltage for the current
limit ad ustments. To keep this regulator properly biased
the minimum positive Vcc supply must be at least 8 Volts
w hen using either a single or dual pow er supply. Low er
supply voltage is possible if external current limit control
voltages are provided.
Current Lim it Control
To ensure proper operation of the LT1970 tw o 100k Ω
resistors to ground are connected to the tw o current limit
control inputs. These resistors prevent open circuit con-
trol inputs w ith umper JP5 removed. The effect of these
resistors slightly attenuates the current limit control volt-
ages provided by the on-board potentiometers. W ith
separate limit control, the maximum voltage is 4.6V.
W hen tied together for common control the maximum
voltage is 4.2V.
The actual current limit of DC453 is actually slightly
greater than the expected nominal value at higher output
current levels (greater than 400mA for the –A version
and 4A for the –B version). The reason for this is the in-
clusion of diode package D5. The back-to-back diodes
across the current sense inputs prevent erratic behavior
in the unlikely event of an abrupt output short circuit
condition. These diodes limit the maximum voltage dif-
ference seen at the sense amplifier inputs. W hen the
voltage across the sense resistor, Rsense, exceeds 0.4
Volts, the diodes begin to conduct current and decrease
the actual voltage difference seen by the sense amplifier.
Boosted Current Version
DC453A-B contains complimentary P and N channel
pow er M OSFETs for output current up to ±5A. The same
easy voltage control of the output current is provided by
the LT1970. The current boost stage is a class B design
intended for DC and low frequency applications.
The crossover distortion of this typical Class B design is
apparent at frequencies greater than 7kHz. Above this
frequency, the total time in crossover becomes 10% or
more of the period of a sine w ave input.
The frequency response of the current boosted amplifier
w ill vary as a function of the load resistance. Resistor R9,
100 Ω , and the load resistor create an attenuation net-
w ork inside the feedback loop of the amplifier. This
causes the LT1970 to run at a higher closed loop gain
than the overall amplifier gain, limiting the observed out-
put closed loop frequency response. The –3dB corner
frequency of the boosted amplifier is:
W here R
F
(10K Ω ) and C
24
(220pf) are feedback compo-
nents already provided on the board. Refer to the Figure
4 schematic. W ith no load, the non-inverting unity gain
bandw idth is 72kH z. Depending on signal levels the us-
able bandw idth may be less due to the 1.6V/µs slew rate.
The large pow er M OSFETs in the output stage can pull
the output voltage very near the supply voltage rails. This
can cause the sense amplifier inputs to exceed their input
common mode voltage range, w hich is 1V aw ay from
either the Vcc or Vee supply rail. This can cause w hat
appears to be a latch-up condition w here the output goes
to one rail or the other and illuminates the current limit
indicators. Reducing the input voltage and cycling the
pow er supplies w ill reset the amplifier back to normal. To
prevent this from occurring, places for resistor divider
netw orks to reduce the voltages seen at the sense ampli-
fier inputs are provided. These are resistors R12 through
R15 on the board. The identical divider netw orks should
ensure that the peak voltages at the sense amplifier in-
puts are never w ithin 1 Volt of the Vcc or Vee supply
rails.







+•π
=
−
load
F
dB
R
R
CR
F
9
24
3
12
1

LT1970
6