Demonstration circuit DC2276A is a 1.5A low dropout
negative linear regulator featuring the
LT
®
3091.
This device
is designed for applications requiring negative output volt-
age, high current without a heat sink, output adjustability
to zero and low dropout voltage.
The LT3091 features fast transient response, high PSRR
and output noise as low as 18µV
RMS
. The LT3091 generates
a wide output voltage range (0V to –32V) while maintain-
ing unity gain operation. This yields virtually constant
bandwidth, load regulation, PSRR and noise, independent
of the programmed output voltage.
The LT3091 supplies 1.5A at a typical dropout voltage of
300mV. Operating quiescent current is nominally 1.2mA
and drops to <<1µA in shutdown. A single resistor adjusts
the LT3091’s precision programmable current limit. The
LT3091’s positive or negative current monitor either
sources a current (0.25mA/A) or sinks a current (0.5mA/A)
proportional to output current.
Built-in protection includes reverse output protection,
internal current limit with foldback and thermal shutdown
with hysteresis.
DC2276A uses the LT3091EDE which is a 14-lead (4mm
x 3mm) plastic DFN package with an exposed pad on the
bottom side of the IC for better thermal performance.
These features make DC2276A an ideal circuit for surface-
mount power supplies, rugged industrial power supplies,
low output voltage supplies and as a post regulator for
switching supplies.
Design files for this circuit board are available at
http://www.linear.com/demo/DC2276A
L,
LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear
Technology Corporation. All other trademarks are the property of their respective owners.
BoarD photo
dc2276af
1
DEMO MANUAL DC2276A
performance summary
CONDITIONS
Shunt at –1.25V for JP1
Shunt at –2.5V for JP1
Shunt at –5.0V for JP1
Shunt at –12V for JP1
Shunt at –15V for JP1
Specifications are at T
A
= 25°C
V
IN
(V)
I
OUT
–10mA
–1.5A
–10mA
–1.5A
–10mA
–1.5A
–10mA
–1.5A
–10mA
–1.5A
MIN*
–36.0
–2.75
–36.0
–4.0
–36.0
–6.50
–36.0
–13.5
–36.0
–16.5
TYP
MAX
–1.55
–1.55
–2.80
–2.80
–5.30
–5.30
–12.3
–12.3
–15.3
–15.3
MIN
–1.30
–1.30
–2.6
–2.6
–5.2
–5.2
–12.5
–12.5
–15.6
–15.6
V
OUT
(V)
TYP
–1.25
–1.25
–2.5
–2.5
–5.0
–5.0
–12.0
–12.0
–15.0
–15.0
MAX
–1.20
–1.20
–2.4
–2.4
–4.8
–4.8
–11.5
–11.5
–14.4
–14.4
* The minimum input voltage for –1.5A load current is set by the 50°C temperature rise of LT3091 on the demo circuit without forced-air cooling. Lower
input voltage can be reached if larger copper area or forced-air cooling is applied. The output current is also limited by the differential voltage of input
and output voltage, please refer the data sheet for details.
2
dc2276af
DEMO MANUAL DC2276A
Quick start proceDure
DC2276A is easy to set up to evaluate the performance
of the LT3091. Refer to Figure 1 for proper measurement
equipment setup and follow the procedure below:
NOTE: When measuring the input or output voltage ripple,
care must be taken to avoid a long ground lead on the
oscilloscope probe. Measure the input or output voltage
ripple by touching the probe tip directly across the terminals
of the input or output capacitors. See Figure 2 for proper
scope probe technique.
1. Use JP2 to set the desired output voltage.
2. Put JP4 on ON position.
3. With power off, connect the input power supply to V
IN
and GND.
+
+
–
–
–
4. Turn on the power at the V
IN
.
NOTE: Make sure that the V
IN
voltage does not exceed
–36V.
5. Check for the proper output voltages:
NOTE: If there is no output, temporarily dis-connect
the load to make sure that the load is not set too high
or is shorted.
6. Once the proper output voltages are established, adjust
the loads within the operating range and observe the out-
put voltage regulation, efficiency and other parameters.
NOTE: Make sure that the power dissipation is limited
below the thermal limit.
POWER SUPPLY
Figure 1. Test Procedure Setup for DC2276A
V
IN
OR V
OUT
GND
Figure 2. Measuring Input or Output Ripple
dc2276af
–
+
–
+
+
+
LOAD
–
3
DEMO MANUAL DC2276A
positive or negative current monitor anD caBle Drop
compensation configuration
There are both positive and negative current monitors on
the LT3091. The configurations for these two monitors are
different. Please check following table for proper configuration.
When the positive current monitor is in use, JP1 needs
to be configured on ON position and JP3 needs to be
configured on +3V position. Meantime, a +3V external
power supply needs to be connected to +3V INPUT turret.
When the negative current monitor is in use, JP1 needs
to be configured on OFF position and JP3 needs to be
configured on IMONN ON position. There is no external
power supply needed.
JP1 (IMONP)
Positive Current Monitor
Negative Current Monitor
Cable Drop Compensation
ON
OFF
OFF
Demo Circuit 2276A also can be configured to evaluate the
cable drop compensation on the LT3091. Note that cable
drop compensation is only using negative current monitor
configuration. JP1, JP3 and R11 shall be configured. JP1
needs to be configured on OFF position. JP3 needs to be
configured on CABLE DROP COMP position. R11 needs
be replaced by a calculated value which has the following
relationship with the total output cable impedance (RCBL)
below. R11 and RCBL are both in Ω: R11 = RCBL
•
2 k.
JP3 (IMONN)
+3V
IMONN ON
CABLE DROP COMP
R11(Ω)
0
0
RCBL(Ω)
•
2 k
4
dc2276af
DEMO MANUAL DC2276A
thermal image
An example thermal image shows the temperature
distribution on the PC board. The test is done in still air
at room temperature with 2.25W power dissipation in
the LT3091 IC. This gives an IC case-to-ambient thermal
resistance of
θ
CA
= 22°C/W on the demo circuit. The IC,
at its highest point, reaches 75°C, at –2.75V
IN
, –1.25V
OUT
and 1.5A load current.
Figure 3. Temperature Rise at 2.25W Dissipation (V
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