LTC1174
LTC1174-3.3/LTC1174-5
O U
W
U
PPLICATI
A
S I FOR ATIO
the maximum RMS current must be used. The CIN RMS
current is given by:
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 IPEAK 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 Schot-
tky diode.
1/2
IOUT VOUT V − V
(
)
IN
OUT
[
]
IRMS
≈
A
RMS
(
)
V
IN
This formula has a maximum at VIN = 2VOUT, where IRMS
=
IOUT/2.Thissimpleworstcaseiscommonlyusedfordesign
becauseevensignificantdeviationsdonotoffermuchrelief.
Note that ripple current directly affects capacitor’s lifetime.
DONOTUNDERSPECIFYTHISCOMPONENT.Anadditional
0.1µF ceramic capacitor is also required on VIN for high
frequency decoupling.
Short-Circuit Protection
The LTC1174 is protected from output short by its internal
current limit. Depending on the condition of IPGM 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).
COUT
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:
IPGM = VIN
I
PEAK
2
I
≈
A
RMS
(
)
RMS
= 170mA or 300mA
IPGM = 0
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
ahigherrippleoutputvoltageandatthesametimecausethe
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.
GND
1174 F02
L = 100µH
IN = 13.5V
20µs/DIV
V
Figure 2. Inductor's Current with Output Shorted
Low-Battery Detector
Thelow-batteryindicatorsensestheinputvoltagethrough
anexternalresistivedivider. Thisdividedvoltageconnects
to the “–” input of a voltage comparator (pin 3) which is
comparedwitha1.25Vreferencevoltage. Withthecurrent
going into pin 3 being negligible, the following expression
is used for setting the trip limit:
Catch Diode Selection
Thecatchdiodecarriesloadcurrentduringtheoff-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 VIN approaches VOUT
R4
R3
V
= 1.25 1+
LBTRIP
8
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