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參數(shù)資料
| 型號： | LTC1435A |
| 廠商： | Linear Technology Corporation |
| 英文描述： | 14-bit, 5V, 200KSPS, 8-Channel Unipolar ADC 24-TSSOP -40 to 85 |
| 中文描述： | 高效率低噪聲同步降壓型開關(guān)穩(wěn)壓器 |
| 文件頁數(shù)： | 14/20頁 |
| 文件大?。?/td> | 427K |
| 代理商： | LTC1435A |
14
LTC1435A
APPLICATIO
S I
N
FOR
ATIO
U
By powering EXTV
CC
from an output-derived source, the
additional V
IN
current resulting from the driver and
control currents will be scaled by a factor of
Duty Cycle/Efficiency. For example, in a 20V to 5V ap-
plication, 10mA of INTV
CC
current results in approxi-
mately 3mA of V
IN
current. This reduces the midcurrent
loss from 10% or more (if the driver was powered di-
rectly from V
IN
) to only a few percent.
3. I
2
R losses are predicted from the DC resistances of the
MOSFET, inductor and current shunt. In continuous
mode the average output current flows through L and
R
SENSE
, but is “chopped” between the topside main
MOSFET and the synchronous MOSFET. If the two
MOSFETs have approximately the same R
DS(ON)
, then
the resistance of one MOSFET can simply be summed
with the resistances of L and R
SENSE
to obtain I
2
R
losses. For example, if each R
DS(ON)
= 0.05
,
R
L
= 0.15
, and R
SENSE
= 0.05
, then the total resis-
tance is 0.25
. This results in losses ranging from 3%
to 10% as the output current increases from 0.5A to
2A. I
2
R losses cause the efficiency to drop at high output
currents.
4. Transition losses apply only to the topside MOSFET(s),
and only when operating at high input voltages (typically
20V or greater). Transition losses can be estimated from:
Transition Loss = 2.5 (V
IN
)
1.85
(I
MAX
)(C
RSS
)(f)
Other losses, including C
IN
and C
OUT
ESR dissipative
losses, Schottky conduction losses during dead-time,
and inductor core losses, generally account for less than
2% total additional loss.
W
U
Checking Transient Response
The regulator loop response can be checked by looking at
the load transient response. Switching regulators take sev-
eral cycles to respond to a step in DC (resistive) load cur-
rent. When a load step occurs, V
OUT
immediately shifts by
an amount equal to (
I
LOAD
)(ESR), where ESR is the ef-
fective series resistance of C
OUT
.
I
LOAD
also begins to
charge or discharge C
OUT
which generates a feedback error
signal. The regulator loop then acts to return V
OUT
to its
steady-state value. During this recovery time V
OUT
can be
monitored for overshoot or ringing, which would indicate
a stability problem. The I
TH
external components shown in
the Figure 1 circuit will provide adequate compensation for
most applications.
A second, more severe transient is caused by switching in
loads with large (>1
μ
F) supply bypass capacitors. The
discharged bypass capacitors are effectively put in parallel
with C
OUT
, causing a rapid drop in V
OUT
. No regulator can
deliver enough current to prevent this problem if the load
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
LOAD
).
Thus a 10
μ
F capacitor would require a 250
μ
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 9 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
Figure 9. Automotive Application Protection
1435A F09
50A IPK RATING
LTC1435A
TRANSIENT VOLTAGE
SUPPRESSOR
GENERAL INSTRUMENT
1.5KA24A
V
IN
12V
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