QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 769
SYNCHRONOUS STEP-DOWN CONVERTER WITH OUTPUT TRACKING
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LTC3808EDE
DESCRIPTION
Demonstration circuit 769A is a high efficiency synchro-
nous step-down DC/DC converter with output tracking
and spread spectrum functions featuring the
LTC3808EDE controller. The demo board is capable of
providing 5A output current at 5V input. Burst
mode/Pulse skipping/Forced continuous operation is
selectable. The constant frequency current mode archi-
tecture with MOSFET V
DS
sensing eliminates the need for
sense resistor and improves efficiency. The maximum
peak current sense threshold can be easily selected with
IPRG pin (See the table on the board).
Switching frequency is internally set at 550KHz. The fre-
quency can be adjusted using PLLLPF pin. Tying PLLLPF
pin to GND selects 300KHz operation; tying PLLLPF pin
to V
IN
selects 750KHz operation. The switching frequency
can also be externally synchronized from 250KHz to
850KHz using SYNC terminal (JP5 selected).
The demo board can be selected to operate in spread
spectrum mode (JP2 selected) with significantly reduced
EMI noise.
The demo board has tracking function, allowing Vout to
track an external voltage signal at TRACK terminal dur-
ing start-up (JP6: Track).
Design files for this circuit board are available. Call
the LTC factory.
Table 1.
Performance Summary (T
A
= 25°C)
PARAMETER CONDITION VALUE
Input Voltage Range 2.7V to 8V
V
OUT
V
IN
= 2.7-8V, I
OUT
= 0A to 5A 1.8V ±2.5%
Maximum load current I
OUT
V
IN
= 5V, V
OUT
= 1.8V 5A
Typical Output Ripple Voltage
V
IN
= 3.3V, I
OUT
= 3A, Fs=550KHz (20MHz BW) 23mV
P–P
Typical Switching Frequency PLLLPF Pin Floating; JP4 selected 550kHz
Typical Spread Spectrum Frequency Range JP2 selected (Spread Spectrum) 460kHz- 635kHz
QUICK START PROCEDURE
Demonstration circuit 769A is easy to set up to evaluate
the performance of LTC3808. Refer to Figure 1 for
proper measurement equipment setup and follow the
procedure below: (Initial jumper positions: JP3 se-
lected; JP6: Soft Start)
NOTE:
When measuring the input or output voltage rip-
ple, care must be taken to avoid a long ground lead on
the oscilloscope 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 technique.
1.
With power off, connect the input power supply to
+Vin (2.7V-8V) and GND (input return).
2.
Connect the 1.8V load between Vout and GND (Initial
load: 0 A).
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 769
SYNCHRONOUS STEP-DOWN CONVERTER WITH OUTPUT TRACKING
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3.
Connect the DVMs to the input and output.
4.
Turn on the input power supply and check for the
proper output voltage. Vout should be 1.8V+/-2.5%.
5.
Once the proper output voltage is established, adjust
the load within the operating range and observe the
output voltage regulation, ripple voltage and other pa-
rameters.
Figure 1.
Proper Measurement Equipment Setup
VIN
GND
Figure 2.
Measuring Input or Output Ripple
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 769
SYNCHRONOUS STEP-DOWN CONVERTER WITH OUTPUT TRACKING
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40
50
60
70
80
90
100
1 10 100 1k 10k
LO A D C URRE NT (mA )
EFFICIENCY (%)
Vin=3.3V
Vo=1.8V
Figure 3.
Efficiency vs load current (Burst mode, 550KHz)
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 769
SYNCHRONOUS STEP-DOWN CONVERTER WITH OUTPUT TRACKING
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