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參數(shù)資料
| 型號： | LTC1267CG |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | RADIATION HARDENED HIGH EFFICIENCY, 5 AMP SWITCHING REGULATORS |
| 中文描述： | 0.05 A DUAL SWITCHING CONTROLLER, 400 kHz SWITCHING FREQ-MAX, PDSO28 |
| 封裝： | PLASTIC, SSOP-28 |
| 文件頁數(shù)： | 11/16頁 |
| 文件大小： | 341K |
| 代理商： | LTC1267CG |
11
LTC1267
LTC1267-ADJ/LTC1267-ADJ5
APPLICATIOU
the voltage comparator. This causes Burst Mode
opera-
tion to be activated when the LTC1267 would normally be
in continuous operation. The effect is most pronounced
with low values of R
SENSE
and can be improved by oper-
ating at higher frequencies with lower values of L. The
output remains in regulation at all times.
W
U
U
EXT V
CC
Pin Connection
The LTC1267 contains an internal PNP switch connected
between the EXT V
CC
and V
CC
pins. The switch closes and
supplies the V
CC
power whenever the EXT V
CC
pin is higher
in voltage than the 4.5V internal regulator. This allows the
MOSFET driver and control power to be derived from the
output during normal operation and from the internal
regulator when the output is out of regulation (start-up,
short circuit).
Significant efficiency gain can be realized by powering V
CC
from the output, since the V
IN
current resulting from the
driver and control currents will be scaled by a factor of
Duty Cycle/Efficiency. For LTC1267, LTC1267-ADJ or
LTC1267-ADJ5 this simply means connecting the EXT
V
CC
pin directly to V
OUT
of the 5V regulator.
The following list summarizes the four possible connec-
tions for EXT V
CC
:
1. EXT V
CC
left open. This will cause V
CC
to be powered
only from the internal 4.5V regulator, resulting in re-
duced MOSFET gate drive levels and an efficiency
penalty of up to 10% at high input voltages.
2. EXT V
CC
connected directly to highest V
OUT
of the two
regulators. This is the normal connection for LTC1267/
LTC1267-ADJ/LTC1267-ADJ5 and provides the high-
est efficiency.
3. EXT V
CC
connected to an output-derived boost net-
work. For 3.3V and other low voltage regulators, effi-
ciency gains can still be realized by connecting EXT V
CC
to an output-derived voltage which has been boosted to
greater than 4.5V. This can be done either with the
inductive boost winding shown in Figure 5a or the
capacitive charge pump shown in Figure 5b. The charge
pump has the advantage of simple magnetics and
generally provides the highest efficiency at the expense
of a slightly higher parts count.
+
V
IN
V
IN
C
IN
L
1:1
R
SENSE
1N4148
+
C
OUT
V
3.3V
+
1
μ
F
P-CH
LTC1267 F05A
N-CH
EXT V
CC
PGATE 3
PDRIVE 3
LTC1267
NGATE 3
PGND3
Figure 5a. Inductive Boost Circuit for EXT V
CC
+
V
IN
V
IN
C
IN
R
SENSE
L
+
C
OUT
V
OUT
3.3V
BAT 85
0.22
μ
F
VN2222LL
BAT 85
BAT 85
1
μ
F
P-CH
LTC1267 F05B
N-CH
EXT V
CC
PGATE 3
PDRIVE 3
LTC1267
NGATE 3
PGND3
+
Figure 5b. Capacitive Charge Pump for EXT V
CC
4. EXT V
CC
connected to an external supply. If an external
supply is available in the 5V to 10V range it may be used
to power EXT V
CC
providing it is compatible with the
MOSFET gate drive requirements. When driving stan-
dard threshold MOSFETs, the external supply must
always be present during operation to prevent MOSFET
failure due to insufficient gate drive.
Under the condition that EXT V
CC
is connected to V
OUT1
which is greater than 5.5V, to power down the whole
regulator, both the pins MSHDN and SHDN1 have to be
pulled high. If SHDN1 is left floating or grounded the
EXT V
CC
may self-power from V
OUT1
, preventing com-
plete shutdown.
LTC1267 Adjustable Applications
When an output voltage other than 3.3V or 5V is required,
the LTC1267-ADJ and LTC1267-ADJ5 adjustable ver-
sions are used with an external resistive divider from V
OUT
to the V
FB1, 2
pins. This is shown in Figure 6. The regulated
voltage is determined by:
)
V
OUT
=
1.25V
)
1 +R2
R1
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