Datasheet

LT1977
14
1977fa
APPLICATIO S I FOR ATIO
WUUU
II
VVV
fLV
PEAK OUT
OUT IN OUT
IN
=+
()
()( )( )
–
2
V
IN
= maximum input voltage
f = switching frequency, 500kHz
3. Decide if the design can tolerate an “open” core geom-
etry like a rod or barrel, which have high magnetic field
radiation, or whether it needs a closed core like a toroid
to prevent EMI problems. This is a tough decision
because the rods or barrels are temptingly cheap and
small and there are no helpful guidelines to calculate
when the magnetic field radiation will be a problem.
4. After making an initial choice, consider the secondary
things like output voltage ripple, second sourcing, etc.
Use the experts in the Linear Technology’s applications
department if you feel uncertain about the final choice.
They have experience with a wide range of inductor
types and can tell you about the latest developments in
low profile, surface mounting, etc.
Short-Circuit Considerations
The LT1977 is a current mode controller. It uses the V
C
node voltage as an input to a current comparator which
turns off the output switch on a cycle-by-cycle basis as
this peak current is reached. The internal clamp on the V
C
node, nominally 2.2V, then acts as an output switch peak
current limit. This action becomes the switch current limit
specification. The maximum available output power is
then determined by the maximum specified switch current
limit.
A potential control problem could occur under short-
circuit conditions. If the power supply output is short
circuited, the feedback amplifier responds to the low
output voltage by raising the control voltage, V
C
, to its
peak current limit value. Ideally, the output switch would
be turned on, and then turned off as its current exceeded
the value indicated by V
C
. However, there is finite response
time involved in both the current comparator and turn-off
of the output switch. These result in a typical minimum on
time of 300ns (see Typical Performance Characteristics)
.
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 math-
ematically the requirement to maintain control is:
ft
VIR
V
ON
F
IN
•
•
≤
+
where:
f = switching frequency
t
ON
= switch 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 LT1977 clock frequency
of 500kHz, a V
IN
of 12V and a (V
F
+ I • R) of say 0.7V, the
maximum t
ON
to maintain control would be approximately
116ns, an unacceptably short time.
The solution to this dilemma is to slow down the oscillator
to allow the current in the inductor to drop to a sufficiently
Table 4. Inductor Selection Criteria
VENDOR/ VALUE I
DC(MAX)
DCR HEIGHT
PART NO. (
µ H) (Amps) (Ohms) (mm)
Coiltronics
UP1B-100 10 1.9 0.111 5.0
UP1B-220 22 1.2 0.254 5.0
UP2B-220 22 2.0 0.062 6.0
UP2B-330 33 1.7 0.092 6.0
UP1B-150 15 1.5 0.175 5.0
Coilcraft
D01813P-153HC 15 1.5 0.170 5.0
D01813P-103HC 10 1.9 0.111 5.0
D53316P-223 22 1.6 0.207 5.1
D53316P-333 33 1.4 0.334 5.1
LP025060B-682 6.8 1.3 0.165 1.65
Sumida
CDRH4D28-4R7 4.7 1.32 0.072 3.0
CDRH5D28-100 10 1.30 0.065 3.0
CDRH6D28-150 15 1.40 0.084 3.0
CDRH6D28-180 18 1.32 0.095 3.0
CDRH6D28-220 22 1.20 0.128 3.0
CDRH6D38-220 22 1.30 0.096 4.0