2-PHASE DUAL SYNCHRONOUS STEP-DOWN CONVERTER WITH SPREAD SPECTRUM
AND OUTPUT TRACKING
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 803
LTC3736EUF-1
DESCRIPTION
Demonstration circuit 803 is a high efficiency 2-phase
dual synchronous step-down DC/DC converter with
2.75V to 8V input range. It has two outputs: 2.5V (5A
maximum at 5V input) and 1.8V (5A maximum at 5V
input). The demo circuit features the LTC®3736EUF-1
controller. The constant frequency current mode ar-
chitecture with MOSFET V
DS
sensing eliminates the
need for sense resistors and improves efficiency. Out
of phase operation significantly reduces input ripple
current as well as the input capacitor size.
Switching frequency is internally set at 550kHz. The
frequency can also be adjusted using FREQ pin (with
spread spectrum operation disabled). Tying FREQ pin
Table 1. Performance Summary (T
A
= 25°C)
PARAMETER
Input Voltage Range
V
OUT1
V
IN
= 2.75-8V, I
OUT1
= 0A to 5A,
I
OUT2
= 0A to 5A
V
OUT2
V
IN
= 2.75-8V, I
OUT1
= 0A to 5A,
I
OUT2
= 0A to 5A
Typical Output Ripple V
OUT1
Typical Output Ripple V
OUT2
Typical Switching Frequency
Typical Spread Spectrum Frequency Range
V
IN
= 3.3V, I
OUT1
= 3A (20MHz BW)
V
IN
= 3.3V, I
OUT2
= 3A (20MHz BW)
FREQ Pin Floating; Spread Spectrum: Disable
Spread Spectrum: Enable
20mV
P–P
14mV
P–P
550kHz
450kHz- 580kHz
1.8V ±3%
CONDITION
VALUE
2.75V to 8V
2.5V ±3%
to GND selects 300kHz operation; tying FREQ pin to
VIN selects 750kHz operation.
The demo board can be selected to operate in spread
spectrum mode (JP1: Enable) with significantly re-
duced peak switching noise. The board has tracking
function too, allowing V
OUT2
to track V
OUT1
during start-
up.
Design files for this circuit board are available. Call
the LTC factory.
LTC is a trademark of Linear Technology Corporation
QUICK START PROCEDURE
Demonstration circuit 803 is easy to set up to evalu-
ate the performance of LTC3736-1. Refer to Figure 1.
for proper measurement equipment setup and follow
the procedure below:
measuring the input or output voltage
ripple, care must be taken to avoid a long ground lead
on the oscilloscope probe. (it’s recommended to
NOTE:
When
measure the output ripple directly at the main output
capacitor).
1.
2.
With power off, connect the input power supply to
+Vin (2.75V-8V) and GND (input return).
Connect the 2.5V load (Load 1 in Figure 1) between
Vout1 and GND; connect the 1.8V load (Load 2 in
1
2-PHASE DUAL SYNCHRONOUS STEP-DOWN CONVERTER WITH SPREAD SPECTRUM
AND OUTPUT TRACKING
Figure 1) between Vout2 and GND. (Initial loads:
0A)
3.
4.
5.
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 803
Once the proper output voltages are established,
adjust the loads within the operating range and ob-
serve the output voltage regulation, ripple voltage
and other parameters.
Connect the DVMs to the input and outputs.
Turn on the input power supply and check for the
proper output voltages. Vout1 should be 2.5V+/-
3%. Vout2 should be 1.8V+/-3%.
Figure 1. Proper Measurement Equipment Setup
2
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