LTC1625IGN Linear Technology, LTC1625IGN Datasheet - Page 17

• Manufacturer Part Number: LTC1625IGN
• Description: IC SW REG STEP-DOWN SYNC 16-SSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1625IGN

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 1.19 ~ 36 V
• Current - Output: 50mA
• Frequency - Switching: 150kHz
• Voltage - Input: 3.7 ~ 36 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

APPLICATIONS
automobile is the source of a number of nasty potential
transients, including load dump, reverse and double
battery.
Load dump is the result of a loose battery cable. When the
cable breaks connection, the field collapse in the alternator
can cause a positive spike as high as 60V which takes
several hundred milliseconds to decay. Reverse battery is
just what it says, while double battery is a consequence of
tow truck operators finding that a 24V jump start cranks
cold engines faster than 12V.
The network shown in Figure 8 is the most straightforward
approach to protect a DC/DC converter from the ravages
of an automotive battery line. The series diode prevents
current from flowing during reverse battery, while the
transient suppressor clamps the input voltage during load
dump. Note that the transient suppressor should not
conduct during double-battery operation, but must still
clamp the input voltage below breakdown of the converter.
Although the LTC1625 has a maximum input voltage of
36V, most applications will be limited to 30V by the
MOSFET V
Design Example
As a design example, take a supply with the following
specifications: V
3.3V, I
can immediately be estimated:
A 0.042 Siliconix Si4412DY MOSFET (
close to this value.
R
DS ON
O(MAX)
(
Figure 8. Automotive Application Protection
)
(BR)DSS
LTC1625
= 2A, and f = 225kHz. The required R
(
PGND
2
120
A
V
IN
IN
)( . )
.
mV
U
= 12V to 22V (15V nominal), V
1 3
INFORMATION
U
0 046
RATING
50A I
TRANSIENT VOLTAGE
SUPPRESSOR
GENERAL INSTRUMENT
1.5KA24A
.
PK
W
JA
= 50 C/W) is
1625 F08
12V
U
DS(ON)
OUT
=
For 40% ripple current at maximum V
should be:
Choosing a standard value of 15 H results in a maximum
ripple current of:
Next, check that the minimum value of the current limit is
acceptable. Assume a junction temperature close to a
70 C ambient with
This is comfortably above I
the assumed T
Since (76 C)
A short circuit to ground will result in a folded back
current of:
with a typical value of R
resulting power dissipated in the bottom MOSFET is:
which is less than under full load conditions.
T
I
I
L
P
P
LIMIT
SC
J
TOP
BOT
I
L MAX
= 70 C + (120mW)(50 C/W) = 76 C
(
(
225
( .
0 03
( . )( ) ( .
43
15
3 3
)
22
1 7 22 2 3
kHz
( .
30
.
0 042
mW
V
3 3
V
V
J
15
(
mV
150
.
)( . )(
:
225
– .
( .
)( . )
2 3
V
0 4 2
V
3 3
(80 C), the solution is self-consistent.
1 1
mV
kHz
2
77
80 C
)( . )
A
3 3
V
.
) ( . )( .
1 3
mW
2
( .
)(
A
1 2
V
1 3 0 042
DS(ON)
= 1.3.
)
15
A
O(MAX)
2
1
)(
–
A
1
180
) ( . )( .
(
–
H
2
120
15
2
1
)
3 3
1 1 0 03
22
and (50 C) = 1.1. The
.
pF
V
= 2A. Now double-check
15
1
0 83
mW
V
)( .
V
.
–
)(
0 5
3 3
225
22
H
)
A
.
V
16
V
LTC1625
IN
s
kHz
)
2 3
)
the inductor
.
H
)
1 2
A
0 83
37
.
.
mW
17
A
A