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參數(shù)資料
| 型號(hào)： | LTC1700 |
| 廠商： | Linear Technology Corporation |
| 英文描述： | Quadruple 2-Input Exclusive-OR Gates 14-SO 0 to 70 |
| 中文描述： | 無檢測(cè)電阻同步升壓型DC / DC控制器 |
| 文件頁數(shù)： | 8/16頁 |
| 文件大?。?/td> | 194K |
| 代理商： | LTC1700 |
LTC1700
8
Power MOSFET Selection
The LTC1700 requires two external power MOSFETs, one
for the main switch (N-channel) and one for the synchro-
nous rectifier (P-channel). Since the voltage operating
range of the LTC1700 is limited to less than 6V, the
breakdown voltage of the MOSFETs is not a concern.
Therefore the MOSFETs parameters that should be used
for selecting the power MOSFETs are threshold voltage
V
GS(TH)
, on-resistance R
DS(ON)
, reverse transfer capaci-
tance C
RSS
and maximum current I
D(MAX)
.
The gate drive voltage is set by the output voltage, V
OUT
.
Since the LTC1700 exits the start-up mode at 2.3V, sub-
logic level threshold MOSFETs should be used in LTC1700
applications. Newer MOSFETs with guaranteed R
DSON
at
gate voltage of 1.8V are now available and will work very
well with the LTC1700.
The MOSFETs on-resistance is chosen based on the
required load current. The maximum average output
current I
O(MAX)
is :
I
O(MAX)
= (I
PK
– 0.5
I)(1 – DC)
where:
I
PK
= Peak Inductor Current
I = Inductor Ripple Current
DC = Duty Cycle
The peak inductor current is inherently limited in a
current mode controller. The maximum V
DS
sense volt-
age of the main MOSFET is limited to 78mV. The LTC1700
will not allow peak inductor current to exceed 78mV/
R
DS(ON)(N-CHANNEL)
. The following equation is a good
guide for determining the required R
DS(ON)(MAX)
, allow-
ing some margin for ripple current, current limit and
variations in the LTC1700 and external component val-
ues:
R
V
I
DC
I
DS ON MAX
(
SENSE
1
2
O MAX
(
–
1
L
T
)(
)
)
+
( )
ρ
For 25
°
C operating condition, set
V
SENSE
= 65mV. For
conditions that vary over the full temperature range, set
V
SENSE
= 55mV.
The
ρ
T
is a normalized term accounting for the significant
variation in R
DS(ON)
with temperature, typically about
0.375%/
°
C as shown in Figure 2. Junction to case tem-
perature T
JC
is around 10
°
C in most applications. For a
maximum ambient temperature of 70
°
C, using
ρ
80
°
C
1.2
in the above equation is a reasonable choice. This equation
is plotted in Figure 3 to illustrate the dependence of
maximum output current on R
DS(ON)
, assuming
I = 0.4I
O(MAX)
.
APPLICATIO
S I
N
FOR
ATIO
U
W
U
Figure 2. R
DS(ON)
vs Temperature
Figure 3. Maximum Current vs R
DS(ON)
Power dissipated by the main and synchronous
MOSFETs depends upon their respective duty cycles and
R
DS(ON)
(m
)
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
M
1700 F03
0
10 20 30 40
50
60 70
80 90 100
DUTY CYCLE = 10%
DUTY CYCLE = 50%
DUTY CYCLE = 80%
TEMPERATURE (
°
C)
–55
ρ
T
65
1.5
1.4
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.6
1700 F02
–35
125
–15
5
25
45
85 105
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