LTC1439 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC1439
Linear Technology
'LTC1439' PDF : 32 Pages View PDF
LTC1438/LTC1439
APPLICATIONS INFORMATION
switch resistance is low and it is driven quickly. The only
solution is to limit the rise time of the switch drive so that
the load rise time is limited to approximately (25)(CLOAD).
Thus a 10µF capacitor would require a 250µs rise time,
limiting the charging current to about 200mA.
Automotive Considerations: Plugging into the
Cigarette Lighter
As battery-powered devices go mobile, there is a natural
interest in plugging into the cigarette lighter in order to
conserve or even recharge battery packs during operation.
But before you connect, be advised: you are plugging into
the supply from hell. The main battery line in an automo-
bile is the source of a number of nasty potential transients,
including load dump, reverse battery 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 12 is the most straightfor-
ward 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 LT1438/LT1439 has a maximum
input voltage of 36V, most applications will be limited to
30V by the MOSFET BVDSS.
12V
50A IPK RATING
TRANSIENT VOLTAGE
SUPPRESSOR
GENERAL INSTRUMENT
1.5KA24A
VIN
LTC1438
LTC1439
1438 F12
Figure 12. Automotive Application Protection
22
Design Example
As a design example, assume VIN = 12V(nominal), VIN =
22V(max), VOUT = 3.3V, IMAX = 3A and f = 250kHz, RSENSE
and COSC can immediately be calculated:
RSENSE = 100mV/3A = 0.033Ω
COSC = [1.37(104)/250] – 11 ≈ 43pF
Refering to Figure 3, a 10µH inductor falls within the
recommended range. To check the actual value of the
ripple current the following equation is used :
∆IL
=
VOUT
(f)(L)
⎛⎝⎜1–
VOUT
VIN
⎞
⎠⎟
The highest value of the ripple current occurs at the
maximum input voltage:
∆IL
=
3.3V
250kHz(10µH)
⎛
⎝⎜1–
3.3V ⎞
22V ⎠⎟
=
1.12A
The power dissipation on the topside MOSFET can be
easily estimated. Using a Siliconix Si4412DY for example;
RDS(ON) = 0.042Ω, CRSS = 100pF. At maximum input
voltage with T(estimated) = 50°C:
[ ] PMAIN
=
3.3V
22V
(3)2
1+
(0.005)(50°C
−
25°C )
(0.042Ω )
+ 2.5(22V)1.85(3A)(100pF)(250kHz) = 122mW
The most stringent requirement for the synchronous
N-channel MOSFET is with VOUT = 0V (i.e. short circuit).
During a continuous short circuit, the worst-case dissipa-
tion rises to:
PSYNC = [ISC(AVG)]2(1 + δ)RDS(ON)
With the 0.033Ω sense resistor ISC(AVG) = 4A will result,
increasing the Si4412DY dissipation to 950mW at a die
temperature of 105°C.
CIN will require an RMS current rating of at least 1.5A at
temperature and COUT will require an ESR of 0.03Ω for low
output ripple. The output ripple in continuous mode will be
highest at the maximum input voltage. The output voltage
ripple due to ESR is approximately:
VORIPPLE = RESR(∆IL) = 0.03Ω(1.12A) = 34mVP-P
14389fb
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