LTC1709 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC1709
Linear Technology
'LTC1709' PDF : 28 Pages View PDF
LTC1709
APPLICATIO S I FOR ATIO
the bandwidth of the feedback loop, so this signal cannot
be used to determine phase margin. This is why it is better
to look at the Ith pin signal which is in the feedback loop
and is the filtered and compensated control loop re-
sponse. The gain of the loop will be increased by increas-
ing RC and the bandwidth of the loop will be increased by
decreasing CC. If RC is increased by the same factor that
CC is decreased, the zero frequency will be kept the same,
thereby keeping the phase the same in the most critical
frequency range of the feedback loop. The output voltage
settling behavior is related to the stability of the closed-
loop system and will demonstrate the actual overall supply
performance.
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-bat-
tery.
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 9 is the most straightforward
approach to protect a DC/DC converter from the ravages
of an automotive power 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 LT1709 has a maximum input voltage of 36V,
most applications will be limited to 30V by the MOSFET
BVDSS.
50A IPK RATING
VIN
12V
TRANSIENT VOLTAGE
SUPPRESSOR
GENERAL INSTRUMENT
1.5KA24A
LTC1709
1709 F09
Figure 9. Automotive Application Protection
Design Example
Asadesignexample,assumeVIN =5V(nominal),VIN␣ =␣ 5.5V
(max), VOUT = 1.8V, IMAX = 20A, TA = 70°C and f␣ =␣ 300kHz.
The inductance value is chosen first based on a 30% ripple
current assumption. The highest value of ripple current
occurs at the maximum input voltage. Tie the PLLFLTR pin
to the INTVCC pin for 300kHz operation. The minimum
inductance for 30% ripple current is:
L
≥
VOUT
f(∆I)
1−
VOUT
VIN
≥
1.8V
(300kHz)(30%)(10A)
1−
15..85VV
≥ 1.35µH
A 1.5µH inductor will produce 27% ripple current. The
peak inductor current will be the maximum DC value plus
one half the ripple current, or 11.4A. The minimum on-
time occurs at maximum VIN:
tON(MIN)
=
VOUT
VINf
=
1.8V
(5.5V)(300kHz)
=
1.1µs
The RSENSE resistors value can be calculated by using the
maximum current sense voltage specification with some
accomodation for tolerances:
RSENSE
=
50mV
11.4A
≈
0.004Ω
22
Share Link: 