4
DEMO MANUAL DC143
QUICK START GUIDE
Demonstration Board DC143 is easy to set up for
evaluation of the LTC1474/LTC1475. Please follow the
procedure below for proper operation:
●
Connect the input power supply to the V
IN
and GND
terminals.
●
The LBO pin is a current sinking pin. When the LBI pin
goes below 1.23V, the LBO pin will sink 0.7mA of
current.
●
The LBI pin is the low-battery detector input pin.
Normally, its input comes from the input voltage
through a resistor network (the resistor divider is
present only on boards D, E and F).
●
Connect the load between the V
OUT
and GND terminals.
Refer to Figure 6 for proper measurement equipment
setup.
●
For board A, B or C, open jumper JP1 to turn on or short
the jumper to shut down. For Board C, D or E,
momentarily depress the RUN push-button to turn on
or momentarily depress the SHDN push-button to shut
down.
OPERATIO
U
The circuits shown in Figures 1 and 2 operate from input
voltages of 3.3V to 18V. The six different versions of the
demo board provide 3.3V or 5V, as specified in Table 1. For
output voltages other than 3.3V or 5V, use Board A or D
and change the resistive divider R4/R5 to the appropriate
ratio. For output voltages lower than 3V, input voltages as
low as 3V can be used. The demo boards provide two
on/off options—by opening or shorting jumper JP1
(Boards A, B and C) or push-button (Boards D, E and F).
Operation
The LTC1474/LTC1475 use a current mode, constant off-
time architecture shown in Figure 3. Current mode opera-
tion provides the well known advantages of clean start-up
and excellent line and load regulation. Constant off-time
adds to this list simplicity (neither an oscillator nor ramp
compensation is required) and inherent 100% duty cycle
in dropout.
The LTC1474/LTC1475 use Burst Mode operation to keep
the output capacitor charged to the proper output voltage
while minimizing quiescent current. Burst Mode operation
works by using short burst cycles to keep the output
capacitor charged, followed by a “sleep” mode where the
load current is supplied by the output capacitor and the
LTC1474/LTC1475 draw only 9µA of supply current.
Because of Burst Mode operation and the constant
off-time, the frequency changes with input voltage and
load. During sleep mode, the low quiescent current is
achieved by turning on only the voltage comparator and
voltage reference, which are needed to monitor the output
voltage, and the low-battery comparator. The low quies-
cent current and variable frequency minimize losses that
would normally dominate at light loads (DC supply current
losses and switching losses due to the MOSFET switch
gate charge). This results in the high efficiencies down to
extremely light loads and the ultralow supply current
required to maintain the output voltage at no load.
The LTC1474/LTC1475 also provide user-programmable
peak inductor current: the user can set the peak current to
any value between 10mA and 400mA with the appropriate
sense resistor. At the beginning of the burst cycle, the
internal P-channel MOSFET switch is turned on, causing
the inductor current to begin to increase. This current
flows through both an internal and an optional external
sense resistor. The internal current comparator monitors
the voltage drop across the sense resistors and, when the
voltage reaches 100mV, the current comparator trips and
turns the switch off, causing the inductor current to
decrease. At the end of the 4.75µs off-time, the switch
either turns back on or stays off (sleep mode), depending
on the status of the voltage comparator. Without an
external sense resistor (Pins 6 and 7 shorted), the peak
current defaults to the 400mA max due to the internal
sense resistor.
