LT1977EFE Linear Technology, LT1977EFE Datasheet - Page 14

• Manufacturer Part Number: LT1977EFE
• Description: IC REG SW 1.5A HV STPDWN 16TSSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LT1977EFE

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 1.2 ~ 54 V
• Current - Output: 1.5A
• Frequency - Switching: 500kHz
• Voltage - Input: 3.3 ~ 60 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-TSSOP Exposed Pad, 16-eTSSOP, 16-HTSSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

LT1977
APPLICATIO S I FOR ATIO
Table 4. Inductor Selection Criteria
VENDOR/
PART NO.
Coiltronics
UP1B-100
UP1B-220
UP2B-220
UP2B-330
UP1B-150
Coilcraft
D01813P-153HC
D01813P-103HC
D53316P-223
D53316P-333
LP025060B-682
Sumida
CDRH4D28-4R7
CDRH5D28-100
CDRH6D28-150
CDRH6D28-180
CDRH6D28-220
CDRH6D38-220
3. Decide if the design can tolerate an “open” core geom-
4. After making an initial choice, consider the secondary
14
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.
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.
V
f = switching frequency, 500kHz
I
PEAK
IN
= maximum input voltage
=
I
OUT
VALUE
+
U
(
6.8
4.7
µ
10
22
22
33
15
15
10
22
33
10
15
18
22
22
H)
V
OUT
2
( )( )( )
U
(
f L V
V
IN
I
(Amps)
DC(MAX)
1.32
1.30
1.40
1.32
1.20
1.30
1.9
1.2
2.0
1.7
1.5
1.5
1.9
1.6
1.4
1.3
–
V
IN
OUT
W
)
(Ohms)
0.111
0.254
0.062
0.092
0.175
0.170
0.111
0.207
0.334
0.165
0.072
0.065
0.084
0.095
0.128
0.096
DCR
U
HEIGHT
(mm)
1.65
5.0
5.0
6.0
6.0
5.0
5.0
5.0
5.1
5.1
3.0
3.0
3.0
3.0
3.0
4.0
Short-Circuit Considerations
The LT1977 is a current mode controller. It uses the V
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
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
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
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
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:
where:
If this condition is not observed, the current will not be
limited at I
higher value. Using the nominal LT1977 clock frequency
of 500kHz, a V
maximum t
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
f = switching frequency
t
V
V
I • R = inductor I • R voltage drop
f t
ON
F
IN
•
= diode forward voltage
= switch on time
= Input voltage
ON
≤
PK
ON
V
F
but will cycle-by-cycle ratchet up to some
to maintain control would be approximately
IN
V
+
IN
of 12V and a (V
I R
•
C
. However, there is finite response
F
+ I • R) of say 0.7V, the
IN
to (V
F
C
+ I • R),
, to its
1977fa
C
C
.