LTC1174-3.3 Linear Technology, LTC1174-3.3 Datasheet - Page 8

• Manufacturer Part Number: LTC1174-3.3
• Description: High Efficiency Step-Down and Inverting DC/DC Converter
• Manufacturer: Linear Technology

LTC1174
LTC1174-3.3/LTC1174-5
the maximum RMS current must be used. The C
current is given by:
This formula has a maximum at V
I
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
0.1 F ceramic capacitor is also required on V
frequency decoupling.
C
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:
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.
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
8
A
OUT
OUT
PPLICATI
I
I
RMS
RMS
/2. This simple worst case is commonly used for design
170
I
OUT
I
PEAK
2
mA
V
OUT
O
or 300mA
U
A
V
V
IN
RMS
S
IN
I FOR ATIO
V
U
OUT
IN
1 2
= 2V
W
IN
OUT
approaches V
A
RMS
, where I
IN
U
for high
IN
RMS
RMS
OUT
=
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:
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
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.
Short-Circuit Protection
The LTC1174 is protected from output short by its internal
current limit. Depending on the condition of I
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).
V
LBTRIP
I
PGM
I
PGM
Figure 2. Inductor's Current with Output Shorted
PEAK
= V
GND
= 0
IN
at close to 100% duty cycle. A fast switching
L = 100 H
V
IN
1 25 1
= 13.5V
.
R
R
3
4
20 s/DIV
PGM
pin, the
1174 F02