Datasheet

13
LT3431
sn3431 3431fs
APPLICATIO S I FOR ATIO
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time involved in both the current comparator and turnoff
of the output switch. These result in a minimum on time
t
ON(MIN)
. When combined with the large ratio of V
IN
to
(V
F
+ I • R), the diode forward voltage plus inductor I • R
voltage drop, the potential exists for a loss of control.
Expressed mathematically the requirement to maintain
control is:
ft
VIR
V
ON
F
IN
•
•
≤
+
where:
f = switching frequency
t
ON
= switch minimum on time
V
F
= diode forward voltage
V
IN
= Input voltage
I • R = inductor I • R voltage drop
If this condition is not observed, the current will not be
limited at I
PK
, but will cycle-by-cycle ratchet up to some
higher value. Using the nominal LT3431 clock frequency
of 500KHz, a V
IN
of 12V and a (V
F
+ I • R) of say 0.6V, the
maximum t
ON
to maintain control would be approximately
100ns, an unacceptably short time.
The solution to this dilemma is to slow down the oscilla-
tor when the FB pin voltage is abnormally low thereby
indicating some sort of short-circuit condition. Oscillator
fre quency is unaffected until FB voltage drops to about 2/3
of its normal value. Below this point the oscillator fre-
quency decreases roughly linearly down to a limit of about
100kHz. This lower oscillator frequency during short-
circuit conditions can then maintain control with the
effective minimum on time. Even with frequency foldback,
however, the LT3431 will not survive a permanent output
short at the absolute maximum voltage rating of V
IN
= 60V;
this is defined solely by internal semiconductor junction
breakdown effects.
For the maximum input voltage allowed during an output
short to ground, the previous equation defining minimum
on-time can be used. Assuming V
F
(D1 catch diode) =
0.52V at 2.5A (short-circuit current is folded back to
typical switch current limit • 0.5), I (inductor) • DCR = 2.5A
• 0.027 = 0.068V (L = UP2B-100), typical f = 100kHz
(folded back) and typical minimum on-time = 275ns, the
maximum allowable input voltage during an output short
to ground is typically:
V
IN
= (0.52V + 0.068V)/(100kHz • 275ns)
V
IN(MAX)
= 21V
Increasing the DCR of the inductor will increase the
maximum V
IN
allowed during an output short to ground
but will also drop overall efficiency during normal opera-
tion.
It is recommended that for [V
IN
/(V
OUT
+ V
F
)] ratios > 4, a
soft-start circuit should be used to control the output
capacitor charge rate during start-up or during recovery
from an output short circuit, thereby adding additional
control over peak inductor current. See Buck Converter
with Adjustable Soft-Start later in this data sheet.
OUTPUT CAPACITOR
The LT3431 will operate with either ceramic or tantalum
output capacitors. The output capacitor is normally cho-
sen by its effective series resistance (ESR), because this
is what determines output ripple voltage. The ESR range
for typical LT3431 applications using a tantalum output
capacitor is 0.05Ω to 0.2Ω. A typical output capacitor is an
AVX type TPS, 100µF at 10V, with a guaranteed ESR less
than 0.1Ω. This is a “D” size surface mount solid tantalum
capacitor. TPS capacitors are specially constructed and
tested for low ESR, so they give the lowest ESR for a given
volume. The value in microfarads is not particularly criti-
cal, and values from 22µF to greater than 500µF work well,
but you cannot cheat mother nature on ESR. If you find a
tiny 22µF solid tantalum capacitor, it will have high ESR,
and output ripple voltage will be terrible. Table 3 shows
some typical solid tantalum surface mount capacitors.
Table 3. Surface Mount Solid Tantalum Capacitor ESR
and Ripple Current
ESR (Max., Ω ) Ripple Current (A)
E Case Size
AVX TPS, Sprague 593D 0.1 to 0.3 0.7 to 1.1
D Case Size
AVX TPS, Sprague 593D 0.1 to 0.3 0.7 to 1.1
C Case Size
AVX TPS 0.2 (typ) 0.5 (typ)