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LTC1174CN8#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20
7
LTC1174
LTC1174-3.3/LTC1174-5
1174fe
(Refer to Functional Diagram)
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.
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
t
VV
VV
Hz
OFF
IN OUT
IN D
=
−
+
⎛
⎝
⎜
⎞
⎠
⎟
()
1
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:
I
VV
L
A
RIPPLE
OUT D
PP
=
+
⎛
⎝
⎜
⎞
⎠
⎟
()
−
−
410
6
•
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.
OPERATIO
U
Burst Mode is a registered trademark of Linear Technology Corporation.
8
LTC1174
LTC1174-3.3/LTC1174-5
1174fe
Inductor Core Selection
With the value of L selected, the type of inductor must be
chosen. Basically there are two kinds of losses in an
inductor, core and copper
Core losses are dependent on the peak-to-peak ripple
current and the core material. However it is independent of
the physical size of the core. By increasing the inductance
the inductor’s peak-to-peak ripple current will decrease,
therefore reducing core loss. Utilizing low core loss mate-
rial, such as molypermalloy or Kool Mµ will allow users to
concentrate on reducing copper loss and preventing satu-
ration. Figure 1 shows the effect of different core material on
the efficiency of the LTC1174. The CTX core is Kool Mµ and
the CTXP core is powdered iron (material 52).
Although higher inductance reduces core loss, it increases
copper loss as it requires more windings. When space is not
Figure 1. Efficiency Using Different Types of
Inductor Core Material
APPLICATIO S I FOR ATIO
WUUU
LOAD CURRENT (mA)
1
EFFICIENCY (%)
10 100 500
100
90
80
70
60
50
V
IN
= 5V
V
OUT
= 3.3V
I
PGM
= V
IN
CTX50-4
CTX50-4P
1174 F01
LOAD CURRENT (mA)
1
EFFICIENCY (%)
10 100 500
100
90
80
70
60
50
V
IN
= 5V
V
OUT
= 3.3V
I
PGM
= V
IN
CTX100-4
CTX100-4P
a premium larger gauge wire can be used to reduce the wire
resistance. This also prevents excessive heat dissipation.
C
IN
In continuous mode the source current of the P-channel
MOSFET is a square wave of duty cycle V
OUT
/V
IN
. To prevent
large voltage transients, a low ESR input capacitor sized for
the maximum RMS current must be used. The C
IN
RMS
current is given by:
I
IVVV
V
A
RMS
OUT OUT IN OUT
IN
RMS
≈
−
()
[]
()
12/
This formula has a maximum at V
IN
= 2V
OUT
, where I
RMS
=
I
OUT
/2. This simple worst case is commonly used for design
because even significant deviations do not offer much relief.
Note that ripple current directly affects capacitor’s lifetime.
DO NOT UNDERSPECIFY THIS COMPONENT. An additional
0.1µF ceramic capacitor is also required on V
IN
for high
frequency decoupling.
C
OUT
To avoid overheating, the output capacitor must be sized to
handle the ripple current generated by the inductor. The
worst case RMS ripple current in the output capacitor is
given by:
I
I
A
mA
RMS
PEAK
RMS
≈
()
=
2
170
or 300mA
Although the output voltage ripple is determined by the
hysteresis of the voltage comparator, ESR of the output
capacitor is also a concern. Too high of an ESR will create
a higher ripple output voltage and at the same time cause the
LTC1174 to sleep less often. This will affect the efficiency of
the LTC1174. For a given technology, ESR is a direct
function of the volume of the capacitor. Several small-sized
capacitors can also be paralleled to obtain the same ESR as
one large can. Manufacturers such as Nichicon, Chemicon
and Sprague should be considered for high performance
capacitors. The OS-CON semiconductor dielectric capaci-
tor available from Sanyo has the lowest ESR for its size, at
a higher price.
9
LTC1174
LTC1174-3.3/LTC1174-5
1174fe
Catch Diode Selection
The catch diode carries load current during the off-time. The
average diode current is therefore dependent on the
P-channel switch duty cycle. At high input voltages the
diode conducts most of the time. As V
IN
approaches V
OUT
the diode conducts only a small fraction of the time. The
most stressful condition for the diode is when the output is
short-circuited. Under this condition the diode must safely
handle I
PEAK
at close to 100% duty cycle. A fast switching diode
must also be used to optimize efficiency. Schottky diodes are
a good choice for low forward drop and fast switching times.
Most LTC1174 circuits will be well served by either a 1N5818,
a MBRS140T3 or a MBR0520L Schottky diode.
Short-Circuit Protection
The LTC1174 is protected from output short by its internal
current limit. Depending on the condition of I
PGM
pin, the
limit is either set to 340mA or 600mA. In addition, the off-
time of the switch is increased to allow the inductor’s
current to decay far enough to prevent any current build-up
(see Figure 2).
APPLICATIO S I FOR ATIO
WUUU
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
LBTRIP
=+
⎛
⎝
⎜
⎞
⎠
⎟
125 1
4
3
.
When the LTC1174 is shut down, the low-battery detector
is inactive.
I
PGM
= V
IN
I
PGM
= 0
GND
L = 100µH
V
IN
= 13.5V
20µs/DIV
1174 F02
Figure 2. Inductor's Current with Output Shorted
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
Figure 3. Low-Battery Comparator
LTC1174
–
+
1.25V
REFERENCE
R4
R3
3
V
IN
1174 F03
LTC1174 Adjustable/Low Noise Applications
The LTC1174 develops a 1.25V reference voltage between
the feedback (Pin 1) terminal and ground (see Figure 4). By
selecting resistor R1, a constant current is caused to flow
through R1 and R2 to set the overall output voltage. The
regulated output voltage is determined by:
V
R
R
OUT
=+
⎛
⎝
⎜
⎞
⎠
⎟
125 1
2
1
.
For most applications, a 30k resistor is suggested for R1.
To prevent stray pickup, a 100pF capacitor is suggested
across R1 located close to the LTC1174. Alternatively, a
capacitor across R2 can be used to increase the switching
frequency for low noise operation.
Inverting Applications
The LTC1174 can easily be set up for a negative output
voltage. If –5V is desired, the LTC1174-5 is ideal for this
application as it requires the least components. Figure 5
shows the schematic for this application. Note that the
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LTC1174CN8#PBF

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Description:
Switching Voltage Regulators High Efficiency Step-Down and Inverting DC/DC Converter
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