DEMO MANUAL DC1897B
LTC3605A
20V, 5A Monolithic
Synchronous Step-Down
Regulator
DC1897B can also track another voltage with the LTC3605A
track function. Because of the high switching frequency of
the LTC3605A, which is programmable up to 4MHz, the
DC1897B uses low profile surface mount components. All
these features make the DC1897B an ideal circuit for use
in industrial applications and distributed power systems
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.
DESCRIPTION
Demonstration circuit 1897 is a step-down converter, using
the
LTC
®
3605A
monolithic synchronous buck regulator.
The DC1897B has a maximum input voltage of 20V, and
is capable of delivering up to 5A of output current at a
minimum input voltage of 4V. The output voltage of the
DC1897B can be set as low as 0.6V, the reference volt-
age of the LTC3605A. At low load currents, the DC1897B
operates in discontinuous mode, and during shutdown,
it consumes 11µA of quiescent current typically. The
DC1897B can achieve efficiency over 90%. The LTC3605A
has phase-lock-loop circuits, allowing high current mul-
tiphase operation of several DC1897Bs in parallel. The
PERFORMANCE SUMMARY
Table 1. Performance Summary
PARAMETER
Input Voltage Range
Output Voltage Range
Run/Shutdown
Output Voltage Regulation
V
IN
= 4V to 20V, I
OUT
= 0A to 5A
V
IN
= 4.7V to 20V, I
OUT
= 0A to 5A
V
IN
= 6.4V to 20V, I
OUT
= 0A to 5A
V
IN
= 12V, V
OUT
= 2.5V
I
OUT
= 5A (20MHz BW)
V
IN
= 12V, V
OUT
= 2.5V
V
IN
= 12V, V
OUT
= 3.3V
V
IN
= 12V, V
OUT
= 5V
Phase = INTV
CC
Phase = GND
Phase = Floating
R
T
= 162k
CONDITIONS
VALUE
4V to 20V
0.6V to 5V
GND = Shutdown
V
IN
= Run
2.5V ±2% Typical (2.45V to 2.55V)
3.3V ±2% Typical (3.234V to 3.366V)
5V ±2% Typical (4.9V to 5.1V)
<20mV
P-P
I
OUT
< 1.25A
I
OUT
< 1.45A
I
OUT
< 1.65A
180° Out-of-Phase: 2 Phase
120° Out-of-Phase: 3 Phase
90° Out-of-Phase: 4 Phase
1MHz ±20%
Typical Output Ripple Voltage
Discontinuous Mode
Phase
Nominal Switching Frequency
Table 2. Jumper Description
JUMPER
JP1
JP5
JP6
JP7
FUNCTION
Output Voltage Setting
Phase Mode (PHMODE): 180 Degrees Out-of-Phase (DOP) – 2 Phase, 120 DOP – 3 Phase,
or 90 DOP – 4 Phase
Mode: Forced Continuous Mode (FCM) or Discontinuous Mode (DCM)
Run
RANGE/SETTING (DEFAULT)
2.5V
(2 PHASE) – 3 PHASE – 4 PHASE
(FCM) – DCM
(ON) – OFF
dc1897bf
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DEMO MANUAL DC1897B
QUICK START PROCEDURE
Demonstration Circuit 1897 is easy to set up to evaluate the
performance of the LTC3605A. For proper measurement
equipment configuration, set up the circuit according to
the diagram in Figure 1. Before proceeding to test, check
that the shunts are inserted into these positions: the
2.5V output voltage header JP1, the 180° out-of-phase
(2-PHASE) position of the phase mode (PHMODE) header
JP5, the forced continuous mode (FCM) position of mode
header JP6, and the on position of run header JP7.
When measuring the input or output voltage ripple, care
must be taken to avoid a long ground lead on the oscil-
loscope probe. Measure the input or output voltage ripple
by touching the probe tip directly across the VIN or VOUT
and GND terminals. See Figure 2 for proper scope probe
measurement technique.
With the DC1897B set up according to the proper measure-
ment configuration and equipment in Figure 1, apply 6.3V
at VIN (do not increase V
IN
over the rated maximum supply
voltage of 20V, or the part may be damaged). Measure
V
OUT
; it should read 2.5V (If desired, the quiescent current
of the circuit can be monitored now by swapping the shunt
in header JP7 into the OFF position). The output voltage
should be regulating. Measure V
OUT
—it should measure
2.5V ±2% (2.45V to 2.55V).
Vary the input voltage from 4V to 20V and adjust the load
current from 0 to 5A. V
OUT
should regulate around 2.5V
±3% (2.425V to 2.575V). Measure the output ripple volt-
age—it should measure less than 30mV AC.
Observe the voltage waveform at the switch pins (the
other side of the inductor from the output). Verify the
switching frequency is between 800kHz and 1.2MHz
(t = 1.25ns and 833ns), and that the switch node waveform
is rectangular in shape.
Change the shunt position on the MODE header from FCM
to DCM (discontinuous mode). Set the input voltage to
12V and the output current to any current less than 1A.
Observe the discontinuous mode of operation at the switch
node, and measure the output ripple voltage. It should
measure less than 100mV AC.
Insert the JP7 shunt into the OFF position and move the
shunt in the 2.5V output JP1 header into any of the two
remaining output voltage option headers: 3.3V (JP2) or
5V (JP3). Just as in the 2.5V VOUT test, the output voltage
should read VOUT ±1% tolerance under static line and
load conditions and ±1% tolerance under dynamic line
and load conditions (±2% total). Also, the circuit operation
in discontinuous mode will be the same. When finished,
turn off the circuit by inserting the shunt in header JP7
into the OFF position.
dc1897bf
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DEMO MANUAL DC1897B
QUICK START PROCEDURE
Figure 1. Proper Equipment Measurement Setup
Figure 2. Measuring Input or Output Ripple
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DEMO MANUAL DC1897B
QUICK START PROCEDURE
Normal Switching Frequency and Output Ripple Voltage
Waveforms
Load Step Response Waveforms
V
IN
= 12V, V
OUT
= 5V, I
OUT
= 5A, f
SW
= 1MHz
Trace 1: Switch Voltage (5V/Div)
Trace 2: Output Ripple Voltage (20mV/Div AC)
Figure 3. Switch Node and Output Ripple Voltage Waveforms
V
IN
= 12V, V
OUT
= 5V, 5A Load Step (0A to 5A)
Forced Continuous Mode, f
SW
= 1MHz
Trace 2: Output Voltage (200mV/Div AC)
Trace 4: Output Current (2A/Div)
Figure 4. Load Step Response
Load Step Response Waveforms
Load Step Response Waveforms
V
IN
= 12V, V
OUT
= 2.5V, 5A Load Step (0A to 5A)
Forced Continuous Mode, f
SW
= 1MHz
Trace 2: Output Voltage (100mV/Div AC)
Trace 4: Output Current (2A/Div)
Figure 5. Load Step Response
V
IN
= 12V, V
OUT
= 3.3V, 5A Load Step (0A to 5A)
Forced Continuous Mode, f
SW
= 1MHz
Trace 2: Output Voltage (100mV/Div AC)
Trace 4: Output Current (2A/Div)
Figure 6. Load Step Response
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DEMO MANUAL DC1897B
QUICK START PROCEDURE
2-Phase Dual Output Waveforms
V
IN
= 12V, V
OUT1
= 2.5V, I
OUT1
= 5A, V
OUT2
= 3.3V, I
OUT2
= 5A,
f
SW
= 1MHz
Trace 1: V
OUT1
Switch Voltage (10V/Div)
Trace 4: L1 Ripple Current (5A/Div)
Trace 3: V
OUT2
Switch Voltage (10V/Div)
Trace 2: L2 Ripple Current (5A/Div)
Figure 7. Switch Node Voltage and Inductor Ripple Current
Waveforms of Two Circuits Operating 180° Out-of-Phase
Circuit Efficiency
V
IN
= 12V
Discontinuous Mode, f
SW
= 1MHz
Figure 8. Efficiency Graph
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