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參數(shù)資料
| 型號(hào)： | LTC1266-3.3 |
| 廠商： | Linear Technology Corporation |
| 英文描述： | Synchronous Switching Regulator Controller for N- or P-Channel MOSFETs(3.3V,同步穩(wěn)壓控制器(用于N或P溝道MOS場(chǎng)效應(yīng)管)) |
| 中文描述： | 同步開關(guān)穩(wěn)壓控制器或P溝道MOSFET（3.3伏，同步穩(wěn)壓控制器（用于?馬鞍山或P溝道場(chǎng)效應(yīng)管）的N） |
| 文件頁數(shù)： | 12/20頁 |
| 文件大?。?/td> | 245K |
| 代理商： | LTC1266-3.3 |
12
LTC1266
LTC1266-3.3/LTC1266-5
Driving N-Channel Topside MOSFETs
Driving an N-channel topside MOSFET (PINV, Pin 3, tied to
PWR V
IN
) is a little trickier than driving a P-channel since
the gate voltage must be positive with respect to the
source to turn it on, which means that the gate voltage
must be higher than V
IN
. This requires either a second
supply at least V
GS(ON)
above V
IN
or a bootstrapping circuit
to boost the V
IN
to the proper level. The easiest method is
using a higher supply (see Figure 14) but if one is not
available, the bootstrap method can be used at the ex-
pense of an additional diode (see Figure 1). The bootstrap
works by charging the bootstrap capacitor to V
IN
during
the off-time. During the on-time, the bottom plate of the
capacitor is pulled up to V
IN
so that the voltage at Pin 2 is
now twice V
IN
(plus any ringing on the switch node).
Since the maximum allowable voltage at Pin 2 is 20V, the
Figure 1 bootstrap circuit limits V
IN
to less than 10V. A
higher V
IN
can be achieved if the bootstrap capacitor is
charged to a voltage less than V
IN
, in which case
V
IN(MAX)
= 20 – V
CAP
.
N-channel mode, internal circuitry limits the maximum
on-time to 60
μ
s to guarantee start-up of the bootstrap
circuit. This maximum on-time reduces the maximum
duty cycle to:
Max Duty Cycle =
60
μ
s
60
μ
s + t
OFF
which slightly increases the minimum input voltage at
which dropout occurs. However, because of the superior
on-conductance of the N-channel, the dropout perfor-
mance of an all N-channel regulator is still better (see
Figure 5) even with the duty cycle limitation, except at light
loads.
Low-Battery Comparator
The LTC1266 has an on-chip low-battery comparator
which can be used to sense a low-battery condition when
implemented as shown in Figure 6. The resistor divider
R1, R2 sets the comparator trip point as follows:
)
R1
V
TRIP
= 1.25
)
1 +R2
(V
IN
– V
OUT
) VOLTAGE (V)
0
C
O
μ
F600
800
1000
4
1266 F04
400
200
0
1
2
3
5
L = 50
μ
H
R
SENSE
= 0.02
L = 25
μ
H
R
SENSE
= 0.02
L = 50
μ
H
R
SENSE
= 0.05
Figure 4. Minimum Value of C
OUT
series would normally be in continuous operation. The
output remains in regulation at all times. This minimum
capacitance requirement may be relaxed if Burst Mode
operation is disabled.
N-Channel vs P-Channel MOSFETs
The LTC1266 has the capability to drive either an
N-channel or a P-channel topside switch to give the user
more flexibility. N-channel MOSFETs are superior in per-
formance to P-channel due to their lower R
DS(ON)
and
lower gate capacitance and are typically less expensive;
however, they do have a slightly more complicated gate
drive requirement and a more limited input voltage range
(see following sections).
Driving P-Channel Topside MOSFETs
The P-channel topside switch circuit configuration is the
most straightforward due to the requirement of only one
supply voltage level. This is due to the negative gate
threshold of the P-channel MOSFET which allows the
MOSFET to be switched on and off by swinging the gate
between V
IN
and ground. The phase invert (Pin 3) is tied
to ground to choose this operating mode. Normally, the
converter input (V
IN
) is connected to the LTC1266 supply
Pins 2 and 5 and can go as high as 20V. Pin 2 supplies the
high frequency current pulses to switch the MOSFETs and
should be decoupled with a 0.1
μ
F to 1
μ
F ceramic capaci-
tor. Pin 5 supplies most of the quiescent power to the rest
of the chip.
APPLICATIOU
W
U
U
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