DEMO MANUAL DC2041A
LTC3255EMSE
Wide V
IN
Range Fault Protected
50mA Step-Down Charge Pump
Description
Demonstration circuit 2041A is a switched-capacitor
step-down power supply featuring the
LTC
®
3255.
The
LTC3255 has the ability to step down voltage 2:1 to ef-
ficiently regulate output voltages less than half the V
IN
voltage reducing the input current needed. As V
IN
reduces
below twice V
OUT
, the LTC3255 will automatically switch
to 1:1 mode.
The LTC3255 also has an integrated shunt regulator al-
lowing it to provide a regulated output from a 4mA to
20mA current loop.
The LTC3255 is a robust device that can survive reverse-
polarity input supplies with current limit and overtem-
perature protection.
Refer to the LTC3255 data sheet for more details on the
electrical and timing specifications.
Design files for this circuit board are available at
http://www.linear.com/demo
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.
performance summary
PARAMETER
Input Supply Range
V
OUT
I
OUT
Specifications are at T
A
= 25°C
MIN
4
2.4
TYP
MAX
48
12.5
7.8
UNITS
V
V
mA
mA
CONDITIONS
Shunt Jumper = OFF (SHUNT = GND)
Shunt Jumper = ON (SHUNT = BIAS) I
IN
= 4mA
50
7.4
7.4mA DC Supply from 4mA to 20mA Current Loop
1µF
AVAILABLE OUTPUT CURRENT (mA)
40
37
34
31
28
25
22
19
16
13
10
7
4
4
6
C
–
OUTPUT
3.3V
7.4mA
Available Output Current
vs Input Current
+
4mA TO 20mA INPUT
>10V COMPLIANCE
V
IN
EN
1µF
C
+
V
OUT
220k
PGOOD
2.15M
LTC3255
PGOOD
SHUNT
BIAS
GND
FB
0.1µF
10µF
AVAILABLE
I
OUT
INCREASED
I
OUT
CAPABILITY
1.21M
DC2041A F01
–
INPUT
CURRENT
10 12 14 16
8
INPUT CURRENT (mA)
18
20
DC2041A F02
dc2041af
1
DEMO MANUAL DC2041A
Quick start proceDure
The DC2041 is easy to set up to evaluate the performance
of the LTC3255 with the shunt disabled. Refer to Figure 1
for proper measurement equipment setup with the shunt
disabled and follow the evaluation procedure below. To
evaluate the LTC3255 with the shunt enabled, go to the
SHUNT ENABLED section.
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 V
IN
or
V
OUT
and GND terminals. See Figure 2 for proper scope
probe technique.
1. Set the EN jumper, JP1, on the DC2041 board to the
ON position.
2. Set the SHUNT jumper, JP3, to the OFF position.
3. Set the V
OUT
jumper, JP2, to the desired output voltage.
NOTE. Do not set the V
OUT
jumper to the OPT setting un-
less the appropriate resistor is installed in R7. The V
OUT
jumper OPT position is to allow an alternate V
OUT
setting.
This can be accomplished by adding a resistor to R7 to
set the output to a desired voltage. Refer to the LTC3255
data sheet to calculate the appropriate value for R7.
4. With power off, connect a 0V to 48V, 100mA power
supply to V
IN
and GND with a voltmeter as shown in
Figure 1.
5. Turn on the V
IN
input power supply and increase the
supply until the V
OUT
voltage is in regulation and the
PGOOD voltage is pulled to V
OUT
.
NOTE. Make sure that the input voltages do not exceed 60V.
6. With V
IN
at least 1V above V
OUT
, increase the load on
V
OUT
from 0mA to 50mA and observe the input current.
The output ripple may also be observed using an oscil-
loscope with the probe connected as shown in Figure 2.
7. With a 50mA load on V
OUT
, increase V
IN
until the cur-
rent on V
IN
drops to approximately ½ the V
OUT
current.
8. Refer to the LTC3255 data sheet for more details on
how the LTC3255 operates.
9. When done, turn off all loads and power supplies.
2
dc2041af
DEMO MANUAL DC2041A
Quick start proceDure
+ +
+
–
–
–
PS1
0V TO 48V
100mA Supply
+
–
+
+ –
L1
0mA TO
100mA Load
DC2041 F01
–
Figure 1. Proper Measurement Equipment Setup with Shunt Disabled
Figure 2. Measuring Input or Output Ripple
VIN
GND
dc2041af
3
DEMO MANUAL DC2041A
sHunt enaBLeD
The DC2041 is easy to set up to evaluate the performance
of the LTC3255 with the shunt enabled. Refer to Figure 3
for proper measurement equipment setup of the LTC3255
with the shunt enabled and follow the evaluation procedure.
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 V
IN
or
V
OUT
and GND terminals. See Figure 2× for proper scope
probe technique.
1. Set the EN jumper, JP1, on the DC2041 board to the
ON position.
2. Set the SHUNT jumper, JP3, to the ON position.
3. Set the V
OUT
jumper, JP2, to the desired output voltage.
NOTE. Do not set the V
OUT
jumper to the OPT setting un-
less the appropriate resistor is installed in R7. The V
OUT
jumper OPT position is to allow an alternate V
OUT
setting.
This can be accomplished by adding a resistor to R7 to
set the output to a desired voltage. Refer to the LTC3255
data sheet to calculate the appropriate value for R7.
4. With power off, connect a 0V to 48V, 100mA power
supply to a controlled current source, a 250Ω ¼ Watt
resistor, V
IN
and GND with a voltmeter and ammeter
as shown in Figure 3.
5. Turn on the V
IN
input power supply, PS1, and increase
the supply above twice the V
OUT
voltage setting plus
9V. (VPS1 > 2V
OUT
+ 2V + VCS1 + VRS1 + Vammeter).
For example, if V
OUT
is set to 3.3V, set PS1 above
3.3V
• 2 + 9V
or 15.6V.
NOTE. Make sure that the input voltages do not exceed 60V.
6. Turn on and set the input current source to 4mA. Ob-
serve the voltage across the V
IN
and GND terminals
and across the V
OUT
and GND terminals. The output
ripple may also be observed using an oscilloscope
with the probe connected as shown in Figure 2.
7. Increase the load on V
OUT
to about 7.4mA and observe
the input current, input voltage and output voltage.
8. Increase the input current source from 4mA to 20mA.
Observe the voltage across the V
IN
and GND terminals
and across the V
OUT
and GND terminals. Adjust PS1
as per equation described in step 5 if needed.
9. Increase the load on V
OUT
to about 38mA and observe
the output voltage. Observe the input current and input
voltage as the load on V
OUT
is changed.
10. Refer to the LTC3255 data sheet for more details on
how the LTC3255 operates.
11. When done, turn off all loads and power supplies.
4
dc2041af
DEMO MANUAL DC2041A
sHunt enaBLeD
+ –
+
–
+
+
PS1 SUPPLY
V > (2V
OUT
+24V)
+ VCS1 + VRS1
VMAX = 60V
CS1
4mA TO 20mA
CURRENT SOURCE
–
+
–
–
+
–
+
+ –
L1
0mA TO
40mA Load
DC2041 F03
–
Figure 3. Proper Measurement Equipment Setup with Shunt Enabled
dc2041af
5
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