參數(shù)資料
型號(hào)： LLTC1142HV
廠商： Linear Technology Corporation
元件分類： 基準(zhǔn)電壓源/電流源
英文描述： RADIATION HARDENED HIGH EFFICIENCY, 5 AMP SWITCHING REGULATORS
中文描述： 抗輻射高效，5安培開關(guān)穩(wěn)壓器
文件頁(yè)數(shù)： 10/20頁(yè)
文件大?。?/td> 261K
代理商： LLTC1142HV
10
LTC1142/LTC1142L/LTC1142HV
APPLICATIU
both track I
MAX
. Once R
SENSE
has been chosen, I
BURST
and
I
SC(PK)
can be predicted from the following:
W
U
U
A graph for selecting C
T
versus frequency including the
effects of input voltage is given in Figure 3.
As the operating frequency is increased the gate charge
losses will be higher, reducing efficiency (see Efficiency
Considerations
section). The complete expression for
operating frequency of the circuit in Figure 1 is given by:
f
t
V
V
OFF
OUT
IN
=
1
1
where:
t
C
V
V
OFF
T
REG
OUT
=
1 3 10
4
.
V
REG
is the desired output voltage (i.e., 5V, 3.3V). V
OUT
is the measured output voltage. Thus V
REG
/V
OUT
= 1 in
regulation.
Note that as V
IN
decreases, the frequency decreases.
When the input-to-output voltage differential drops below
1.5V for a particular section, the LTC1142 reduces t
OFF
in
that section by increasing the discharge current in C
T
. This
prevents audible operation prior to dropout.
Figure 2. Selecting R
SENSE
L and C
T
Selection for Operating Frequency
Each regulator section of the LTC1142 uses a constant off-
time architecture with t
OFF
determined by an external
timing capacitor C
T
. Each time the P-channel MOSFET
switch turns on, the voltage on C
T
is reset to approximately
3.3V. During the off-time, C
T
is discharged by a current
which is proportional to V
OUT
. The voltage on C
T
is
analogous to the current in inductor L, which likewise
decays at a rate proportional to V
OUT
. Thus the inductor
value must track the timing capacitor value.
The value of C
T
is calculated from the desired continuous
mode operating frequency:
C
f
T
=
1
2 6 10
4
.
Assumes V
IN
= 2V
OUT
, Figure 1 circuit.
Figure 3. Timing Capacitor Value
Once the frequency has been set by C
T
, the inductor L must
be chosen to provide no more than 25mV/R
SENSE
of peak-
to-peak inductor ripple current. This results in a minimum
required inductor value of:
L
MIN
= 5.1 10
5
R
SENSE
C
T
V
REG
As the inductor value is increased from the minimum
value, the ESR requirements for the output capacitor are
I
BURST
SENSE
R
=150mV
R
SC(PK)
SENSE
15mV
I
≈
The LTC1142 automatically extends t
OFF
during a short
circuit to allow sufficient time for the inductor current to
decay between switch cycles. The resulting ripple current
causes the average short-circuit current I
SC(AVG)
to be
reduced to approximately I
MAX
.
0.20
MAXIMUM OUTPUT CURRENT (A)
0
R
S
)
0.15
0.10
0.05
0
4
1142 F02
1
2
3
5
FREQUENCY (kHz)
0
C
50
100
150
200
1142 F03
250
1000
800
600
400
200
0
300
V
IN
= 12V
V
IN
= 10V
V
IN
= 7V
V
SENSE
= V
OUT
= 5V
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