DEMO MANUAL DC2215A
LTC3126UFD
42V, 2.5A Synchronous Step-Down
Regulator with Lossless PowerPath
Description
Demonstration Circuit 2215A, features the
LTC
®
3126,
a
synchronous buck converter with an integrated lossless
input PowerPath™. Pin selectable (JP2) ideal diode-OR
and priority input modes, with programmable undervolt-
age lockout thresholds provide control over the transi-
tion between input sources. The unique all N-channel
architecture provides efficient operation from either of
two input sources to a programmable output voltage.
In this application the valid input voltage range is set
to be from 4.3V to 42V on V
IN1
and from 6V to 8.4V on
V
IN2
providing flexibility and voltage margin for a variety
of applications and power sources. V
IN2
can operate to
42V. Other input voltage ranges are possible by adjusting
V
SET1/2
accordingly.
The LTC3126 uses a low noise, current mode architec-
ture and a resistor programmable (R13) PWM oscillator
frequency which is also synchronizable from 200kHz to
2.2MHz. This minimizes the solution footprint while ensur-
ing high efficiency. For higher efficiency at light loads, the
user can select Burst Mode
®
operation (JP1).
Typical efficiencies for both PWM mode (JP1 = PWM)
and Burst Mode operation (JP1 = BURST) are shown
in Figure 1. Burst Mode operation allows the automatic
transition to PWM operation if the load current increases.
This is shown in Figure 5. More information on Burst Mode
operation and PWM mode can be found in the data sheet.
Input Voltage Range V
IN1
Input Voltage Range V
IN2
V
OUT
I
OUT
In some applications the efficiency can be improved by
setting the EXTV
CC
jumper (JP3) to V
OUT
. This allows the
converter to operate from the fixed V
OUT
voltage. Typical
performance in response to a transient load is shown in
Figure 4.
The system can be monitored through
VALID1, VALID2,
PRIORITY
and PGOOD indicators. There are accurate
V
SET1
and V
SET2
comparators to program independent
UVLO thresholds. Consult the data sheet for information
on start-up and shutdown thresholds. Other features
include a 1μA shut-down current (ENA pin JP4 = OFF),
short-circuit protection, soft-start, inductor current limit
and thermal overload protection.
The LTC3126 data sheet has detailed information about
the operation, specifications, and applications of the part.
The data sheet should be read in conjunction with this
Quick Start Guide.
Design files for this circuit board are available at
http://www.linear.com/demo/DC2215A
L,
LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks
and PowerPath is a trademark of Linear Technology Corporation. All other trademarks are the
property of their respective owners.
4.3V to 42.0V
6.0V to 8.4V (Operational to 42V)
3.3V
2.5A
dc2215af
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DEMO MANUAL DC2215A
100
90
80
70
PERCENTAGE (%)
60
50
40
30
20
10
Burst Mode OPERATION
PWM MODE
0
0.0001
V
IN
= 4.5V
V
IN
= 12V
V
IN
= 25V
0.001
0.01
0.1
LOAD CURRENT(A)
1
10
DC2215A F01
Figure 1. Typical Efficiency as a Function
of Input Voltage and Load Current
PRIORITY MODE OPERATION
V
IN1
= 2V/DIV
V
IN2
= 2V/DIV
V
OUT
= 1V/DIV
V
IN1
TRANSITION
V
IN2
TRANSITION
LOAD = 5
2ms/DIV
DC2215A F02
Figure 2. Demonstration of Priority Operation Where the
Source of the Converter’s Power Is Switched to V
IN1
When
V
IN1
Exceeds the Preset Threshold
2
dc2215af
DEMO MANUAL DC2215A
IDEAL DIODE MODE OPERATION
TRANSITION
TO V
IN2
V
IN2
= 2V/DIV
V
OUT
= 1V/DIV
TRANSITION
TO V
IN1
V
IN1
= 2V/DIV
LOAD = 5
2ms/DIV
DC2215A F03
Figure 3. The LTC3126 Monitors the Voltage on V
IN1
and V
IN2
and Selects the Higher of These Voltages in Ideal Diode
3.3V
200mV/DIV
OUTPUT
VOLTAGE
1A/DIV
OUTPUT
CURRENT
45mA
2.2A
2ms/DIV
DC2215A F04
Figure 4. Typical 2.2A to 45mA to 2.2A Load Transient on
a 3.3V Output with V
IN
= 12V
dc2215af
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DEMO MANUAL DC2215A
AUTOMATIC
TRANSITION FROM
Burst Mode TO PWM
Burst Mode
OPERATION
AUTOMATIC
TRANSITION FROM
PWM TO Burst Mode
OUTPUT VOLTAGE
PWM OPERATION
100mV/DIV
Burst Mode
RIPPLE
600mA
OUTPUT CURRENT
500mA/DIV
100mA
2ms/DIV
DC2215A F05
Figure 5. Output Ripple and Transients in Burst Mode Operation
and Transitioning from and into Burst Mode Operation
4
dc2215af
DEMO MANUAL DC2215A
Quick start proceDure
Using short twisted pair leads for any power connections
and with all loads and power supplies off, refer to Figure 6
for the proper measurement and equipment setup. The
power supplies should not be connected to the circuit until
told to do so in the procedure below.
When measuring the input or output voltage ripple, care
must be taken to avoid a long ground lead on the oscil-
loscope probe.
The indicators for
PRIORITY, VALID1
and
VALID2
are
negative logic while the indicator for PGOOD is positive
logic. PGOOD indicates the output voltage is good if it is
pulled to V
CC
and
VALID1
and
VALID2
indicate that the
input voltage on the respective sources are good if the
pins are pulled to ground. With the unit enabled (JP4,
(ENA = ON) and JP2 (DIODE =
PRIORITY))
the unit is
operating in PRIORITY mode. This means that if there is
enough voltage on V
IN1
then V
IN1
will be supplying power
to the converter independent of the voltage on V
IN2
and
VALID1
will be low.
If JP2 (DIODE) is connected to IDEAL DIODE the unit is
operating in IDEAL DIODE mode and power will be drawn
from whichever source has enough voltage to be enabled
and is the higher voltage. Both
VALID1
and
VALID2
will be
low if V
IN1
and V
IN2
are above their operating thresholds.
Turrets
Left side of board:
V
IN1
:
4.3V to 42V supply input: do not exceed +42V.
GND:
Ground connection for input supply.
V
IN2
:
6V to 8.4V supply input: operational to +42V.
Right side of board:
GND:
Ground connection for LOAD.
V
OUT
:
3.3V/2.5A first of two terminals in parallel.
V
OUT
:
3.3V/2.5A second of two terminals in parallel.
GND:
Ground connection for LOAD.
Jumpers
JP1/PWM/SYNCH:
Selects PMW or Burst Mode opera-
tion. Leaving the jumper open and applying a square
wave signal to the SYNC pin allows the converter to
be synchronized. Default position is PWM which puts
the unit in fixed frequency PWM operation.
JP2/DIODE:
Selects PRIORITY or DIODE-OR mode.
JP3/EXTV
CC
:
connects EXTV
CC
to V
OUT
through a
diode or leaves EXTV
CC
open. If EXTV
CC
is open, the
LTC3126 derives bias from V
IN
. The default position
is V
OUT
.
JP4/ENA:
Selects ENA = ON/OFF The default position
.
is “ON”.
1. JP1 through JP4 settings to start:
JP1
JP2
JP3
JP4
PWM/SYNC
DIODE
EXTVcc
ENA
=
=
=
=
PWM
PRIORITY
VOUT
ON
2. With the power supplies (V
IN1
, V
IN2
) OFF connect them
as shown in Figure 6. If accurate current measurements
are desired (for efficiency calculations for example)
then connect the ammeters in series with the supplies
as shown. The ammeters, however, are not required.
3. Connect the load, set at 5Ω, (660mA) to V
OUT
as shown
in Figure 6. Again, connect an ammeter if accurate
current measurement or monitoring is desired.
4. Turn on the Power Supply, V
IN1
, and increase the volt-
age. The converter will start at approximately 4.3V.
5. Verify V
OUT
is ~3.3V.
6. V
IN1
can now be varied between 4.3V and 42V. V
OUT
will remain in regulation for load currents up to 2.5A.
7. Set V
IN1
to 0V and turn off V
IN1
.
dc2215af
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