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參數(shù)資料
| 型號(hào)： | LTC1267-ADJ |
| 廠商： | Linear Technology Corporation |
| 英文描述： | XTAL MTL T/H HC49/US |
| 中文描述： | 雙通道高效率同步降壓型開關(guān)穩(wěn)壓器 |
| 文件頁數(shù)： | 12/16頁 |
| 文件大小： | 341K |
| 代理商： | LTC1267-ADJ |
12
LTC1267
LTC1267-ADJ/LTC1267-ADJ5
APPLICATIU
W
U
U
1. LTC1267 V
IN
current is the DC supply current given in
the electrical characteristics which excludes MOSFET
driver and control currents. V
IN
currents results in a
small (<1%) loss which increases with V
IN
.
2. LTC1267 V
CC
current is the sum of the MOSFET driver
and control circuits currents. The MOSFET driver cur-
rent results from switching the gate capacitance of the
power MOSFETs. Each time a MOSFET gate is switched
from low to high to low again, a packet of charge dQ
moves from V
CC
to ground. The resulting dQ/dt is a
current out of V
CC
which is typically much larger than
the control circuit current. In continuous mode I
GATECHG
≈
f
O
(Q
P
+Q
N
), where Q
P
and Q
N
are the gate charges of
the two MOSFETs.
By powering EXT V
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 application,
10mA of V
CC
current results in approximately 3mA of
V
IN
current. This reduces the mid-current loss from
10% or more (if the driver was powered directly from
V
IN
) to only a few percent.
3. I
2
R losses are easily predicted from the DC resistances
of the MOSFET, inductor, and current shunt. In continu-
ous mode all the output current flows through L and
R
SENSE
, but is “chopped” between the P-channel and N-
channel MOSFETs. If the two MOSFETs have approxi-
mately 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)
= 0.1
, R
L
= 0.15
, and R
SENSE
= 0.05
, then
the total resistance is 0.3
. This results in losses
ranging from 3% to 12% as the output current increases
from 0.5A to 2A. I
2
R losses cause the efficiency to roll
off at high output currents.
4. Transition losses apply only to the P-channel MOSFET
and only when operating at high input voltages (typically
20V or greater). Transition losses can be estimated
from:
Transition Loss
≈
5
×
V
IN2
×
I
MAX
×
C
RSS
×
f
O
Other losses including C
IN
and C
OUT
ESR dissipative
losses, Schottky conduction losses during dead-time,
The V
FB1, 2
pin is extremely sensitive to pickup from the
inductor switching node. Care should be taken to isolate
the feedback network from the inductor and a 100pF
capacitor should be connected between the V
FB1, 2
and
SGND pins next to the package.
The circuit in Figure 6 cannot be used to regulate a V
OUT
which is greater than the maximum voltage allowed on the
LTC1267 EXT V
CC
pin (10V). In applications with
V
OUT
> 10V, R
SENSE
must be moved to the ground side of
the output capacitor and load. This operates the current
sense comparator at 0V common mode, increasing the
off-time approximately 40% and requiring the use of a
smaller timing capacitor C
T
.
Efficiency Considerations
The percent efficiency of a switching regulator is equal to
the output power divided by the input power times 100%.
It is often useful to analyze individual losses to determine
what is limiting the efficiency and which change would
produce the most improvement. Percent efficiency can be
expressed as:
% Efficiency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc., are the individual losses as a percent-
age of input power. (For high efficiency circuits, only small
errors are incurred by expressing losses as a percentage
of output power.)
Although all dissipative elements in the circuit produce
losses, four main sources usually account for most of the
losses in LTC1267 circuits:
1. LTC1267 V
IN
current
2. LTC1267 V
CC
current
3. I
2
R losses
4. P-channel transition losses
Figure 6. LTC1267-ADJ/LTC1267-ADJ5
External Feedback Network
R
SENSE
100pF
R2
R1
+
C
OUT
V
OUT
V
FB1, 2
SGND
LTC1267 F06
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