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DEMO MANUAL DC242
DC/DC CONVERTER
LT1777 High Voltage,
Low Noise Step-Down
DC/DC Converter
Demonstration Circuit DC242 is a high input voltage, low
noise step-down (buck) regulator using the LT
®
1777
switching regulator IC. Typical applications are automo-
tive cellular and GPS receivers, low noise telecom and
industrial-instrument power supplies. This controller in-
cludes an onboard 700mA peak-current switch and is
optimized for use with a high supply voltage. The input
voltage can range from 7.4V to 48V. In order to achieve low
noise, the LT1777 is equipped with dI/dt limiting circuitry,
which is programmed via a small external inductor in the
power path. It also contains internal circuitry to limit the
dV/dt turn-on and turn-off ramp rates. By slowing down
the sharp edges during turn-on and turn-off of the power
switch, conducted and radiated EMI are significantly re-
duced. The output voltage is jumper selectable to either
3.3V or 5V. The LT1777 uses a 100kHz switching fre-
quency and current mode control, which offers excellent
dynamic input supply rejection and short-circuit protec-
tion. Additionally, the supply current can be shut down to
less than 30µA in standby mode. Gerber files for this
circuit are available. Call the LTC factory.
, LTC and LT are registered trademarks of Linear Technology Corporation.
PARAMETER CONDITIONS VALUE
Input Voltage Range I
OUT
= 0mA to 500mA 7.4V to 48V
Output Voltage (Jumper Selectable) 3.3V, 5V
Maximum Output Load Current V
IN
> 12V, V
OUT
= 5V I
O
= 500mA*
Typical Output Ripple Voltage I
OUT
= 500mA, V
IN
= 24V 17mV
P-P
Nominal Operating Frequency V
IN
= 7.4V to 48V 100kHz
I
Q
, Supply Current in Shutdown 30µA
*I
O
= 450mA for V
IN
≤ 12V and V
OUT
= 5V
T
A
= 25°C
V
SW
Switching Waveforms
500ns/DIV
DC242 TA01
V
SW
CURRENT
200mA/DIV
V
SW
VOLTAGE
10V/DIV
Component Side
DESCRIPTIO
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PERFOR A CE SU ARY
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TYPICAL PERFOR A CE CHARACTERISTICS A D BOARD PHOTO
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