Linear Technology DC1345 User manual

DEMO CIRCUIT 1345
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1
WIDE INPUT VOLTAGE RANGE
NO RSENSE STEP-DOWN DC/DC CONVERTER
LTC3879EMSE
DESCRIPTION
Demonstration circuit 1345 is a wide input voltage
range, no Rsense step-down dc/dc converter featuring
the LTC3879EMSE. Its output voltage is 1.2V with a
load rating of 12A and the input voltage range is 4.5V
to 34V.
The fixed on-time valley current mode architecture of
the LTC3879EMSE provides a very fast load step re-
sponse. Other features of the board include a RUN pin,
a TRACK pin, a PGOOD pin and a MODE jumper to
select either CCM or DCM operation at light load.
Design files for this circuit board are available. Call
the LTC factory.
Table 1. Performance Summary (T
A
= 25°C)
PARAMETER CONDITION VALUE
Minimum Input Voltage 4.5V
Maximum Input Voltage 34V
Output Voltage V
OUT
V
IN
= 4.5V to 34V, I
OUT
= 0A to 12A 1.2V ±2%
Maximum Output Current 12A
Typical Output Voltage Ripple V
IN
= 12V, I
OUT
= 12A (20MHz BW) 20mV
P–P
Nominal Switching Frequency V
IN
= 12V, I
OUT
= 12A 300kHz
Efficiency (see Figure 3 for efficiency curves) V
IN
= 12V, I
OUT
= 12A 90% Typical
QUICK START PROCEDURE
Demonstration circuit 1345 is easy to set up to evalu-
ate the performance of the LTC3879EMSE. Refer to
Figure 1 for proper measurement equipment setup and
follow the procedure below:
NOTE: When measuring the output voltage ripple, care
must be taken to avoid a long ground lead on the oscil-
loscope probe. See Figure 2 for proper scope probe
technique. Short, stiff leads should be soldered to the
(+) and (-) terminals of an output capacitor. The
probe’s ground ring needs to touch the (-) lead and the
probe tip needs to touch the (+) lead.
1. Place jumpers in the following positions:
RUN ON
MODE CCM
2. With power off, connect the input power supply to
Vin and GND.
3. Turn on the power at the input.
4. Check for the proper output voltages.
Vout = 1.176V to 1.224V
5. Once the proper output voltages are established,
adjust the loads within the operating range and ob-
serve the output voltage regulation, ripple voltage,
efficiency and other parameters.

DEMO CIRCUIT 1345
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Vin Load
Iout
Vout
Vin
supply
Iin
A
A
V
V
Figure 1. Proper Measurement Equipment Setup
VOUT GND
COUT
+
-
Figure 2. Measuring Output Voltage Ripple

DEMO CIRCUIT 1345
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Fsw = 300kHz, CCM
60
65
70
75
80
85
90
95
0 2 4 6 8 10 12 14
Load current (Amps)
Efficiency
Vin=5V
Vin=12V
Vin=24V
Vin=34V
Parameters:
QT = Infineon BSC093N04LSG
QB = Infineon BSC035N04LSG
L = Wurth 744325072
0.72uH, DCR=1.3mΩ+/- 10%
Ron = 576kΩ
Figure 3. Typical Efficiency Curves

DEMO CIRCUIT 1345
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