6
LTC1574
LTC1574-3.3/LTC1574-5
For C
OUT
, the RMS current rating should be at least:
I
I
A
RMS
PEAK
RMS
≈
()
=
2
300mA
Absolute Maximum Ratings and Latchup Prevention
The absolute maximum ratings specify that SW
(Pins 3, 14) can never exceed V
IN
(Pin 5) by more than
0.3V. Normally this situation should never occur. It could,
however, if the output is held up while the supply is pulled
down. A condition where this could potentially occur is
when a battery is supplying power to an LTC1574 regula-
tor and also to one or more loads in parallel with the the
regulator’s V
IN
. If the battery is disconnected while the
LTC1574 regulator is supplying a light load and one of the
parallel circuits is a heavy load, the input capacitor of the
LTC1574 regulator could be pulled down faster than the
output capacitor, causing the absolute maximum ratings
to be exceeded. The result is often a latchup which can be
destructive if V
IN
is reapplied. Battery disconnect is pos-
sible as a result of mechanical stress, bad battery contacts
or use of a lithium-ion battery with a built-in internal
disconnect. The user needs to assess his/her application
to determine whether this situation could occur. If so,
additional protection is necessary.
Prevention against latchup can be accomplished by
simply connecting a Schottky diode across the SW and
V
IN
pins as shown in Figure 7. The diode will normally be
reverse biased unless V
IN
is pulled below V
OUT
at which
time the diode will clamp the (V
OUT
– V
IN
) potential to less
than the 0.6V required for latchup. Note that a low
leakage Schottky should be used to minimize the effect
inductor L1 is chosen for its excellent self-shielding prop-
erties. 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.
Design Example
As a design example, assume V
IN
= 9V (nominal),
V
OUT
= 5V and I
OUT
= 350mA maximum. The LTC1574-5
is used for this application with I
PGM
(Pin 6) connected to
V
IN
. The minimum value of L is determined by assuming
the LTC1574-5 is operating in continuous mode.
Figure 6. Continuous Inductor Current
With I
OUT
= 350mA and I
PEAK
= 0.6A (I
PGM
= V
IN
), I
V
= 0.1A.
The peak-to-peak ripple inductor current, I
RIPPLE
, is 0.5A
and is also equal to:
I
VV
L
RIPPLE
OUT D
=
+
()
−
410
6
• A
P-P
Solving for L in the above equation and with V
D
= 0.5V,
L = 44µH. The next higher standard value of L is 50µH
(example: Coiltronics CTX50-4). The operating frequency,
ignoring voltage across diode V
D
is:
f
V
V
kHz
OUT
IN
≈−
=
2 5 10 1
111
5
.•
With the value of L determined, the requirements for C
IN
and C
OUT
are calculated. For C
IN
, its RMS current rating
should be at least:
I
IVVV
V
A
mA
RMS
OUT OUT IN OUT
IN
RMS
=
−
()
[]
()
=
12
174
/
INDUCTOR CURRENT
TIME
I
PEAK
I
V
AVG CURRENT
= I
OUT
=
= 350mA
I
PEAK
+ I
V
2
1574 • F06
APPLICATIO S I FOR ATIO
WUUU
1574 F07
V
IN
V
OUT
LATCHUP
PROTECTION
SCHOTTKY
SW
LTC1574
+
Figure 7. Preventing Absolute Maximum
Ratings from Being Exceeded
