Linear Technology LTC3773EG User manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 908A
POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING
1
LTC3773EG
DESCRIPTION
Demonstration circuit 908A is a Polyphase 3-
Output Step-Down Supply with Tracking featuring
the LTC3773EG. The output voltages are 3.3V,
2.5V and 1.8V and the maximum load current for
each is 15A. The input voltage range is 7V to 24V.
Typical applications include s ervers, communica-
tions, computers and industrial power supplies.
The board can be programmed for either continu-
ous-conduction mode (CCM), pulse skip mode
(DCM) or Burst Mode (BM). Other features of the
board include an SDB pin which can be used for
rail sequencing, a phase mode pin, and a dynamic
load circuit for each rail.
An on-board LT1933 (5V / 500mA buck converter)
provides current for the LTC3773EG’s internal
logic and gate drive circuitry. The 5V bias can also
be supplied externally. If an external bias voltage
is applied it needs to be between 4.5V and 7.0V.
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 7V
Maximum Input Voltage 24V
V
OUT1
V
IN
= 7V to 24V, I
OUT
= 0A to 15A 3.3V ±3%
V
OUT2
V
IN
= 7V to 24V, I
OUT
= 0A to 15A 2.5V ±3%
V
OUT3
V
IN
= 7V to 24V, I
OUT
= 0A to 15A 1.8V ±3%
Typical output voltage ripple for VOUT1 VIN = 12V, IOUT1 = 15A, 20MHz BW 25mVp-p TYP
Typical output voltage ripple for VOUT2 VIN = 12V, IOUT2 = 15A, 20MHz BW 25mVp-p TYP
Typical output voltage ripple for VOUT3 VIN = 12V, IOUT3 = 15A, 20MHz BW 25mVp-p TYP
Nominal Switching Frequency 220kHz TYP
Efficiency
(see Figure 4 for typical efficiency curves)
V
IN
= 12V, V
OUT1
= 3.3V, I
OUT1
= 15A
V
IN
= 12V, V
OUT2
= 2.5V, I
OUT2
= 15A
V
IN
= 12V, V
OUT3
= 1.8V, I
OUT3
= 15A
92.2% TYP
90.9% TYP
88.9% TYP

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 908A
POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING
2
QUICK START PROCEDURE
Demonstration circuit 908A is easy to set up to evaluate
the performance of the LTC3773EG. 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. Measure the output voltage ripple by
touching the probe tip directly across the Vout and GND
terminals. See Figure 2 for proper scope probe tech-
nique.
1. Place the jumpers as shown below:
Description JP# Position
INT5V JP5 ON
5V SELECT JP3 INT5V
SDB JP1 ON
SYNC JP2 CCM
PHMD JP6 LOW
2. With power off, connect the input power supply to
Vin and GND.
3. Turn on the input voltage and set it to a voltage be-
tween 7V and 24V. The input voltage source should
be able to supply at least 20A.
NOTE: Make sure the input voltage does not exceed
24V.
4. Check for the proper output voltages:
Vout1 = 3.201V to 3.399V
Vout2 = 2.425V to 2.575V
Vout3 = 1.746V to 1.854V
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.
LOAD STEP RESPONSE
The load step response for each rail can be tested with
an on-board dynamic load circuit and a pulse generator.
Refer to Figure 3 and follow the steps below.
1. Set the output of the pulse generator for a duty cycle
of less than 5% and an amplitude of 1V or below.
2. Connect the output of pulse generator from Pulse
Gen+ to Pulse Gen-.
3. Connect the LOAD STEP BNC connector to an oscil-
loscope with a coaxial cable to monitor the load step
current waveform. 10mV = 1A.
4. Turn-on the 908A and apply the desired amount of
static load to the output.
5. Increase the amplitude of the pulse generator output
to obtain the desired load step height.
6. Monitor the output voltage across the turrets as
shown.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 908A
POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING
3
RAIL TRACKING AND SEQUENCING
The DC908A is setup for coincident rail tracking, where
both Vout2 and Vout3 track Vout1 directly through their
TRACK pins and TRACK dividers. Refer to Figure 5 for
the turn-on waveforms. The ramp-rate for Vout1 is de-
termined by a 10nF capacitor on the TRACK1 pin at C8
on the demo board. If desired, Vout1 can be setup to
track an external signal. To do this, remove C8 and stuff
the appropriate values for the TRACK 1 divider at R40
and R41 and apply the tracking signal to the TRACK1
turret.
If desired, the rails can be configured to be turned on
and off independently. To do this, remove the TRACK2
and TRACK3 dividers at R33, R35, R38 and R39, and
stuff TRACK capacitors at C31 and C32 to set the de-
sired ramp-rate. Rail sequencing can be implemented by
placing a capacitor on the SDB pin. As the voltage on the
SDB pin ramps-up, Vout1 will turn on 1
st
, Vout2 will turn
on 2
nd
and Vout3 will turn-on last. Refer to the data sheet
for more details on rail tracking and sequencing.
V
A
+
-
Vin
Iin
Vin
supply
Vout1
load
A
Iout1
+
-
V
-
Vout1
Vout2
load
A
Iout2
+
V
-
Vout2
Vout3
load
A
Iout3
+
V
-
Vout3
Figure 1. Proper Measurement Equipment Setup

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 908A
POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING
4
VOUT GND
Figure 2. Measuring Output Voltage Ripple
+
-
Oscilloscope
Vout1
load
A
Iout1
Pulse
generator
COAX cable
Figure 3. Proper Measurement Equipment Setup for Load Step Testing. Only the setup for the 3.3V rail is shown.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 908A
POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING
5
Efficiency at Vin = 12V, Fsw = 220kHz
84%
86%
88%
90%
92%
94%
96%
0 3 6 9 12 15 18
Load current (Amps)
Efficiency
3.3V
1.8V
2.5V
Efficiency at Vin = 24V, Fsw = 220kHz
80%
82%
84%
86%
88%
90%
92%
0 3 6 9 12 15 18
Load current (Amps)
Efficiency
3.3V
1.8V
2.5V
Figure 4. Typical efficiency curves

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 908A
POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING
6
3.3V
2.5V
1.8V
SDB
2V / div
2msec / div
Vo1,2,3
1V / div
Figure 5. Turn-on waveforms at Vin = 12V after SDB pin is released. Demo board is setup for coincident rail tracking.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 908A
POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING
7

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 908A
POLYPHASE 3-OUTPUT STEP-DOWN SUPPLY WITH TRACKING
8

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