LTC1174
LTC1174-3.3/LTC1174-5
OPERATIO
The LTC1174 uses a constant off-time architecture to
switch its internal P-channel power MOSFET. The off-time
is set by an internal timing capacitor and the operating
frequency is a function of V
IN
.
The output voltage is set by an internal resistive divider
(LTC1174-3.3 and LTC1174-5) or an external divider re-
turned to V
FB
Pin 1 (LTC1174). A voltage comparator A1
compares the divided output voltage to a reference voltage
of 1.25V.
To optimize efficiency, the LTC1174 automatically switches
between continuous and Burst Mode
®
operation. The volt-
age comparator is the primary control element when the
device is in Burst Mode operation, while the current com-
parator controls the output voltage in continuous 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
T
, begins to discharge 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 automatically
goes into Burst Mode operation. The current mode loop is
interrupted when the output voltage reaches the desired
regulated value. The hysteretic voltage comparator 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.
U
(Refer to Functional Diagram)
In sleep mode, the LTC1174 is in standby 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.
Operating Frequency and Inductor
Since the LTC1174 utilizes a constant off-time architecture,
its operating frequency is dependent on the value of V
IN
. The
frequency of operation can be expressed as:
f
=
1
⎛
V
IN
−
V
OUT
⎞
⎜
⎟
t
OFF
⎝
V
IN
+
V
D
⎠
(
Hz
)
where t
OFF
= 4µs and V
D
is the voltage drop across the diode.
Note that the operating frequency is a function of the input
and ouput voltage.
Although the size of the inductor does not affect the fre-
quency, it does affect the ripple current. The peak-to-peak
ripple current is given by:
+
V
D
⎞
⎛
V
I
RIPPLE
=
4 • 10
−
6
⎜
OUT
⎟
⎝
⎠
L
(
A
P
−
P
)
By choosing a smaller inductor, a low ESR output filter
capacitor has to be used (see C
IN
and C
OUT
). Moreover, core
loss will also increase (see Inductor Core Selection section)
due to higher ripple current.
Burst Mode is a registered trademark of Linear Technology Corporation.
1174fe
7
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