LTC1439 Linear Technology, LTC1439 Datasheet - Page 22

• Manufacturer Part Number: LTC1439
• Description: Dual High Efficiency/ Low Noise/ Synchronous Step-Down Switching Regulators
• Manufacturer: Linear Technology

APPLICATIONS
LTC1438/LTC1439
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)(C
Thus a 10 F capacitor would require a 250 s 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 BV
22
12V
Figure 12. Automotive Application Protection
GENERAL INSTRUMENT
TRANSIENT VOLTAGE
50A I PK RATING
SUPPRESSOR
1.5KA24A
U
DSS
INFORMATION
U
.
V
IN
W
LTC1438
LTC1439
1438 F12
U
LOAD
).
Design Example
As a design example, assume V
22V(max), V
and C
Refering to Figure 3, a 10 H inductor falls within the
recommended range. To check the actual value of the
ripple current the following equation is used :
The highest value of the ripple current occurs at the
maximum input voltage:
The power dissipation on the topside MOSFET can be
easily estimated. Using a Siliconix Si4412DY for example;
R
voltage with T(estimated) = 50 C:
The most stringent requirement for the synchronous
N-channel MOSFET is with V
During a continuous short circuit, the worst-case dissipa-
tion rises to:
With the 0.033 sense resistor I
increasing the Si4412DY dissipation to 950mW at a die
temperature of 105 C.
C
temperature and C
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:
DS(ON)
IN
R
C
P
V
P
will require an RMS current rating of at least 1.5A at
MAIN
SENSE
OSC
SYNC
ORIPPLE
I
I
L
L
OSC
= [1.37(10
= 0.042 , C
= [I
250
V
can immediately be calculated:
( )( )
= 100mV/3A = 0.033
2 5 22
f L
OUT
3 3
22
.
= R
OUT
.
SC(AVG)
kHz
V
3 3
V
ESR
.
= 3.3V, I
– 1
V
3
(
OUT
V
10
4
1 85
( I
2
)/250] – 11 43pF
]
.
V
2
1
V
OUT
(1 + )R
will require an ESR of 0.03 for low
RSS
L
H
IN
) = 0.03 (1.12A) = 34mV
3
)
MAX
A
0 005 50
= 100pF. At maximum input
1
.
100
–
= 3A and f = 250kHz, R
OUT
3 3
DS(ON)
22
.
pF
IN
= 0V (i.e. short circuit).
SC(AVG)
V
V
= 12V(nominal), V
250
C
1 12
kHz
25
.
= 4A will result,
C
A
122
0 042
.
P-P
mW
SENSE
IN
=