Linear DC768 User manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 768
15A POLYPHASE MONOLITHIC SYNCHRONOUS BUCK REGULATOR
1
LTC3415
DESCRIPTION
D emonstration Circuit 768 is a 15A high efficiency,
phase lockable constant frequency buck converter, in-
corporating two LTC3415 polyphase monolithic syn-
chronous regulators. The DC768 has an input voltage
range of 2.5V to 5.5V and an output voltage range from
0.6V to 5V. The reference voltage of 0.6V is +/-1% accu-
rate, producing a more precise output voltage. The op-
erating frequency range of the DC768 is either set by the
LTC3415 internal oscillator to 1.5 MHz, set to 1 MHz or 2
MHz by forcing the PLLPF pin to ground or Vin, respec-
tively, or synchronized to an external clock, with a range
between 750 kHz and 2.25 MHz. The DC768 can deliver
high power for monolithic parts – up to 15A of output
current – due to the high current power switches (28
m Ω of on-state resistance) of the LTC3415. The
LTC3415 also incorporates OPTI-LOOP compensation,
so that the DC768 can be optimized with external com-
ponents to provide fast transient response over a wide
range of line and load conditions. However, the
LTC3415 has internal compensation for those requiring a
simple high power supply only. The DC768 has two
LTC3415s operating in polyphase operation. Due to the
LTC3415 polyphase capability, multiple DC768s can be
paralleled for higher current power supplies. Since the
parallel LTC3415s operate out-of-phase, the amount of
input and output capacitance required will be minimal.
The DC768 offers the three standard modes of opera-
tion: force continuous, pulse-skipping, and Burst-
Mode
TM
, for those circuits that operate during intervals of
low output power. Extra features include tracking, for
easy power supply sequencing, output margining, power
good, and spread spectrum operation, which signifi-
cantly reduces the peak switching noise emitted from
the DC768. All these features make the DC768 perfectly
suited for high current, high power applications, such as
notebook or desktop computers.
Design files for this circuit board are available. Call
the LTC factory.
TM - Burst Mode is a trademark of Linear Technology Corporation
Table 1.
Performance Summary (TA= 25°C)
PARAMETER CONDITIONS VALUE
Minimum Input Voltage 2.5V
Maximum Input Voltage 5.5V
Output Voltage VOUT Regulation VIN = 2.5V to 5.5V, IOUT = 0A to 15A
1.2V ±3% (1.164V to 1.236V)
1.5V ±3% (1.455V to 1.545V)
1.8V ±3% (1.746V to 1.854V)
Typical Output Ripple VOUT VIN = 5V, IOUT = 15A (20 MHz BW) <20mVP–P
Burst Mode - VIN = 3.3V, VOUT = 1.8V <1.5A ±0.1A
Pulse-Skipping - VIN = 3.3V, VOUT = 1.8V < 5A ±0.1A
Operation Modes
Forced Continuous Any Output Current
Nominal Switching Frequency 1.5 MHz

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 768
15A POLYPHASE MONOLITHIC SYNCHRONOUS BUCK REGULATOR
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QUICK START PROCEDURE
D emonstration Circuit 768 is easy to set up to evalu-
ate the performance of the LTC3415. For proper
measurement equipment configuration, set up the
circuit according to the diagram in Figure 1. Before
proceeding to test, make sure the jumper shunts are
in the appropriate header according to the legend in
Figure 1, except for one exception: insert jumper JP5
shunt into the off position, which connects the RUN
pin to ground (GND), and thus, shuts down the cir-
cuit.
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 out-
put voltage ripple by touching the probe tip directly
across the Vin or Vout and GND terminals. See Fig-
ure 2 for proper scope probe technique.
1. With the DC768 set up according to the proper
measurement and equipment in Figure 1, apply
3.3V at Vin (Do not hot-plug Vin or increase Vin
over the rated maximum supply voltage of 5.5V, or
the part may be damaged.). Measure Vout; it
should read 0V. If desired, one can measure the
shutdown supply current at this point. The supply
current will be approximately 400 uA, or less, in
shutdown.
2. Turn on the circuit by inserting the shunt in header
JP5 into the ON position. The output voltage
should be regulating. Measure Vout - it should
measure 1.2V +/- 1.5% (1.182V to 1.218V).
3. Vary the input voltage from 2.5V to 5.5V and adjust
the load current from 0 to 15A. Vout should read
between 1.2V +/- 3% (1.164V to 1.236V).
4. Measure the output ripple voltage at any output
current level; it usually will measure less than 20
mVAC.
5. Observe the voltage waveform at the switch node.
Verify the switching frequency is between 1.3 MHz
and 1.7 MHz (T = 0.769 us and 0.588 us), and that
the switch node waveform is rectangular in shape.
Insert jumper JP5 shunt into the OFF position and move
the 1.2V Vout shunt into any of the remaining output
voltage options: 1.5V or 1.8V. Just as in the 1.2Vout
test, the output voltage should read Vout +/- 1.5% toler-
ance under static line and load conditions, and another
+/- 1.5% tolerance under dynamic line and load condi-
tions (+/- 3% total).
The Burst-Mode
TM
or pulse-skipping operation modes
of the LTC3415 can also be observed now by chang-
ing the position of the shunt in header JP7. 1 or 2
MHz operating frequencies can be activated by re-
positioning the JP6 shunt. The tables below show
the extra features, such as margining and spread
spectrum.
When finished, turn off the circuit (connecting the
RUN pin to ground) by inserting the shunt in header
JP5 into the OFF position.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 768
15A POLYPHASE MONOLITHIC SYNCHRONOUS BUCK REGULATOR
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PARAMETER CONDITIONS VALUE
CLOCKOUT PHMODE Pin Pulled to VIN - VIN = 3.3V, VOUT = 1.8V 1.5 MHz
CLOCKIN Pin Connected to VIN - VIN = 3.3V, VOUT = 1.8V Spread Spectrum (See
Photo)
CLOCKIN
Synchronized to 2Vpk-to-pk, 2 MHZ signal injected into the PLLPF
pin - VIN = 3.3V, VOUT = 1.8V
2 MHZ (Measured at SW
& CLOCKOUT Pins)
PARAMETER CONDITIONS POSITION MGN POSITION BSEL VALUE
5% 1.89V
±4% (1.814V to
1.966V)
10% 1.98V
±4% (1.900V to
2.060V)
MGN+
15% 2.07V
±4% (1.987V to
2.153V)
5% 1.71V
±4% (1.641V to
1.779V)
10% 1.62V
±4% (1.555V to
1.685V)
Margining
V
IN
= 3.3V , V
OUT
= 1.8V, I
OUT
= 5A
MGN-
15% 1.53V
±4% (1.468V to
1.592V)

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 768
15A POLYPHASE MONOLITHIC SYNCHRONOUS BUCK REGULATOR
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Figure 1. Proper Measurement Equipment Setup
Figure 2. Measuring Input or Output Ripple

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 768
15A POLYPHASE MONOLITHIC SYNCHRONOUS BUCK REGULATOR
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Figure 3. Load Step Response
V
IN
= 3.3V, V
OUT
= 1.8V & 10A Load Step
Trace 1: Output Voltage (100mV/div AC)
Trace 2: Output Current (5A/div)

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 768
15A POLYPHASE MONOLITHIC SYNCHRONOUS BUCK REGULATOR
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Figure 4. 180 °
°°
° Out-of-Phase Switching Operation
V
IN
– 3.3V, V
OUT
– 1.2V, I
OUT
– 5A

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 768
15A POLYPHASE MONOLITHIC SYNCHRONOUS BUCK REGULATOR
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