4
DEMO MANUAL DC059
U
To optimize efficiency, the LTC1174 automatically switches
between two modes of operation, burst and continuous.
The voltage comparator is the primary control element
when the device is in Burst Mode
operation, while the
current comparator controls the output voltage in continu-
ous mode.
During the switch ON time, switch current flows through
the 0.1Ω sense resistor. When this current reaches the
threshold of the current comparator A2, its output signal
will change state, setting the flip-flop and turning the
switch off. The timing capacitor, C
TIME
, begins to dis-
charge until its voltage goes below V
TH1
. Comparator A4
will then trip, which resets the flip-flop and causes the
switch to turn on again. Also, the timing capacitor is
recharged. The inductor current will again ramp up until
the current comparator A2 trips. The cycle then repeats.
When the load is relatively light, the LTC1174 automati-
cally goes into Burst Mode
operation. The current mode
loop is interrupted when the output voltage reaches
the desired regulated value. The hysteretic voltage com-
parator A1 trips when V
OUT
is above the desired output
voltage, shutting off the switch and causing the timing
capacitor to discharge. This capacitor discharges past
V
TH1
until its voltage drops below V
TH2
. Comparator A5
then trips and a sleep signal is generated.
In sleep mode, the LTC1174 is inactive and the load
current is supplied by the output capacitor. All unused
circuitry is shut off, reducing quiescent current from
0.45mA to 0.13mA. When the output capacitor discharges
by the amount of the hysteresis of the comparator A1,
the P-channel switch turns on again and the process
repeats itself.
LOW NOISE REGULATOR
In some applications, it is important not to introduce any
switching noise within the audio frequency range.
Due to the Burst Mode operation nature of the LTC1174,
there is a possibility that the regulator will introduce audio
noise at some load currents. To circumvent this problem,
a feed-forward capacitor can be used to shift the noise
spectrum up and out of the audio band. Board C has been
configured for this application. The peak-to-peak output
ripple is reduced to 40mV over the entire load range. A
toroidal surface mount inductor is chosen for its excellent
self-shielding properties. Open magnetic structures such
as drum and rod cores are to be avoided since they inject
high flux levels into their surroundings. This can become
a major source of noise in any converter circuit. Figure 3
shows the efficiency curve of Board C.
V
IN
= 9V
V
OUT
= 3.3V
LOAD CURRENT (mA)
60
70
80
90
85
75
65
EFFICIENCY (%)
500
DC059 • F03
1 10 100
I
PGM
= V
IN
Figure 3. Board C Efficiency
LOW BATTERY DETECTOR
The low battery indicator senses the input voltage through
an external resistive divider. This divided voltage connects
to the (–) input of a voltage comparator (Pin 3) which is
compared with a 1.25V reference voltage. With the current
going into Pin 3 being negligible, the following expression
is used for setting the trip limit:
V
R
R
LB TRIP_
.=+
125 1
4
3
–
+
R4
3
1.25V REFERENCE
DC059 • F04
LTC1174
R3
V
IN
Figure 4. Low Battery Comparator
OPERATIO
