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DEMO MANUAL DC309
NO-DESIGN SWITCHER
dc309fa
LTC1773 Low Input Voltage
Synchronous Current Mode
Step-Down DC/DC Converter
, LTC and LT are registered trademarks of Linear Technology Corporation.
SYMBOL PARAMETER CONDITIONS VALUE
V
IN
Input Supply Voltage Range V
OUT
= 1.8V 2.8V to 8.5V
V
OUT
Output Voltage I
OUT
= 1A 3.3V ±0.066V
I
OUT
= 1A 2.5V ±0.050V
I
OUT
= 1A 1.8V ±0.036V
I
OUT
= 1A 1.5V ±0.030V
Demo board DC309 is a step-down (buck) regulator using
the LTC
®
1773. The exclusive use of surface mount com-
ponents results in a highly efficient application in a very
small board space. It is ideal for cell phones and other
portable electronics operating from one or two Li-Ion cells
or three to six NiCd cells. DC309 is capable of providing
2.5A at an output voltage of 1.8V with an input supply of
3.3V. This demo board highlights the capabilities of the
LTC1773, which uses a current mode PWM architecture to
synchronously drive an external pair of P- and N-channel
power MOSFETs. The result is a high performance power
supply that has low output voltage ripple. A constant
operating frequency of 550kHz makes the LTC1773 attrac-
tive for noise-sensitive applications while allowing for
smaller external components such as the inductor and the
output capacitor. In addition, its high efficiency over a
wide load current range and its low quiescent current
make the LTC1773 ideal for battery-powered applications.
In dropout, the external P-channel MOSFET is turned on
continuously (100% duty cycle), providing low dropout
operation when V
IN
approaches V
OUT
. Under light load
conditions, efficiency can be improved by activating Burst
Mode
TM
operation, which is done by connecting the SYNC/
FCB pin to V
IN
or leaving it floating. Tying this pin to ground
will force continuous operation, regardless of the load.
The SYNC/FCB pin can also be used to synchronize the
LTC1773 to frequencies of up to 750kHz. At input voltages
below 2.5V, the LTC1773’s undervoltage lockout feature is
activated, shutting down the output. Soft-start is provided
by connecting an external capacitor to the RUN/SS pin.
Gerber files for this circuit board are available. Call the
LTC factory.
Burst Mode is a trademark of Linear Technology Corporation.
DESCRIPTIO
U
PERFOR A CE SU ARY
UWWW
TYPICAL PERFOR A CE CHARACTERISTICS A D BOARD PHOTO
U
U
W
Efficiency
OUTPUT CURRENT (mA)
1
EFFICIENCY (%)
100
90
80
70
60
50
DC309 TA01
10
100 1000
V
OUT
= 2.5V
SEE FIGURE 1
V
IN
= 5V
V
IN
= 3.3V
V
IN
= 8V
10,000
