LTC1625 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1625' PDF : 24 Pages View PDF
LTC1625
APPLICATIONS INFORMATION
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(BR)DSS.
VIN
LTC1625
PGND
50A IPK
12V
RATING
TRANSIENT VOLTAGE
SUPPRESSOR
GENERAL INSTRUMENT
1.5KA24A
1625 F08
Figure 8. Automotive Application Protection
Design Example
As a design example, take a supply with the following
specifications: VIN = 12V to 22V (15V nominal), VOUT =
3.3V, IO(MAX) = 2A, and f = 225kHz. The required RDS(ON)
can immediately be estimated:
For 40% ripple current at maximum VIN the inductor
should be:
L
≥
3.3V
(225kHz)(0.4)(2A)
1–
3.3V
22V
=
16µH
Choosing a standard value of 15µH results in a maximum
ripple current of:
∆IL(MAX)
=
3.3V
(225kHz)(15µH)
1–
3.3V
22V
=
0.83A
Next, check that the minimum value of the current limit is
acceptable. Assume a junction temperature close to a
70°C ambient with ρ80°C = 1.3.
ILIMIT
≥
150mV
(0.042Ω)(1.3)
–
1
2
0.83A
=
2.3A
This is comfortably above IO(MAX) = 2A. Now double-check
the assumed TJ:
PTOP
=
3.3V (2.3A)2(1.3)(0.042Ω)
22V
+
(1.7)(22)2(2.3A)(180pF)(225kHz)
= 43mW + 77mW = 120mW
TJ = 70°C + (120mW)(50°C/W) = 76°C
Since ρ(76°C) ≅ ρ(80°C), the solution is self-consistent.
A short circuit to ground will result in a folded back
current of:
ISC
=
30mV
(0.03Ω)(1.1)
+
1
2
(15V)(0.5µs)
15µH
=
1.2A
with a typical value of RDS(ON) and ρ(50°C) = 1.1. The
resulting power dissipated in the bottom MOSFET is:
RDS(ON)
=
120mV
(2A)(1.3)
=
0.046Ω
A 0.042Ω Siliconix Si4412DY MOSFET (θJA = 50°C/W) is
close to this value.
PBOT
=
15V – 3.3V
15V
(1.2A)2(1.1)(0.03Ω)
=
37mW
which is less than under full load conditions.
17
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