5
DEMO MANUAL DC194
MICROPOWER BOOST REGULATOR
V
OUT
AC COUPLED
100mV/DIV
I
L1
200mA/DIV
V
SW
5V/DIV
5
µ
s/DIV
DC194 F03a
1
µ
s/DIV
DC194 F03b
V
OUT
AC COUPLED
100mV/DIV
I
L1
200mA/DIV
V
SW
5V/DIV
Figure 3. DC194’s Operating Waveforms. In the Upper Photo,
the LT1317 Is in Burst Mode, Delivering 50mA to the Load. In
the Lower Photo, the Load Curent Is 160mA and the LT1317 Is
Switching at 600kHz. V
IN
= 2.5V, V
OUT
= 3.3V
OPERATIO
U
graph of maximum load appears in the Typical Perfor-
mance section of this manual. The lower curve shows the
guaranteed load capability based on the minimum current
limit specification in the LT1317 data sheet. The upper
curve shows the load capability of a typical DC194. As load
current is increased beyond this level, the output voltage
will sag as the LT1317 reaches its current limit. Again, be
aware that L1 and D1 provide a direct path between the
input and output and that this circuit does not limit the
output current. As an increasing load drags the output
voltage below the input, a larger current will flow, limited
only by the impedance of the power source, inductor L1
and diode D1.
Shutdown Mode
The SHDN pin of the LT1317 is tied directly to the SHDN
terminal of the DC194 and has been left floating. An
internal current source will pull up on this pin and the
LT1317 will run in this condition. The LT1317 will also
operate if this pin is pulled above 1.4V by an external
signal. The SHDN pin can be pulled as high as V
IN
+ 0.3V.
The LT1317 is placed in shutdown mode by pulling this pin
below 0.4V; you can do this by tying the SHDN terminal of
the DC194 to the GND terminal. The current consumption
of the LT1317 in shutdown mode is typically 25
µ
A. How-
ever, the load can draw additional current through the
inductor and catch diode, raising the power consumption
in shutdown. The LT1317’s low-battery detector remains
active in shutdown. Applications of the low-battery detec-
tor are discussed below.
Efficiency
The efficiency of the DC194 is plotted in the Typical
Performance section of this manual. Efficiency measure-
ments should be made with care, as there is plenty of
opportunity for errors to creep in.
The efficiency is defined as the power delivered to the load
divided by the power drawn from the input supply. Nor-
mally, the average input voltage, input current, output
voltage and output current are measured under steady-
state conditions and the efficiency is calculated from these
values. Each should be measured with the highest accu-
racy and precision possible.
Figure 2 shows connections for the proper measurement
of efficiency and output regulation. The input and output
voltages are measured at the DC194 in order to avoid
including voltage drops across ammeters and terminal
connections. It is best to take all of these measurements
at one time. Be aware that most digital multimeters drop
significant voltage when they are used as ammeters, so
you must measure the input voltage while the ammeter is
in the circuit—the input voltage will be lower than the
voltage at the output of your bench-top supply. Another
difficulty occurs at low power when the LT1317 is in Burst
Mode operation. Here, the part will be drawing a few
hundred milliamperes while switching, but only a few
hundred microamperes average. An ammeter set to a
sensitive scale will have too much resistance to allow
these pulses of current to pass without large voltage
drops. The result is that the power delivered to the LT1317
is not equal to the average current times the average
