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參數(shù)資料
| 型號： | MIC5191BMM TR |
| 廠商： | Micrel Inc |
| 文件頁數(shù)： | 10/15頁 |
| 文件大?。?/td> | 845K |
| 描述： | IC REG CTRLR SGL POS ADJ 10-MSOP |
| 標準包裝： | 2,500 |
| 類型： | 正，可調(diào)式 |
| 輸出數(shù)： | 1 |
| 輸出電壓： | 可調(diào) |
| 電流 - 電源： | 15mA |
| 輸入電壓： | 1 V ~ 5.5 V |
| 工作溫度： | -40°C ~ 125°C |
| 封裝/外殼： | 10-TFSOP，10-MSOP（0.118"，3.00mm 寬） |
| 供應商設備封裝： | 10-MSOP |
| 包裝： | 帶卷 (TR) |
| 其它名稱： | MIC5191BMMTR MIC5191BMMTR-ND |
Micrel, Inc.
MIC5191
December 2006
10
M9999-122206
compensation resistor, creating a higher mid-band gain.
-20
0
20
40
60
80
100
0.01
0.1
1
10
100
1000 10000 100000
Frequency (KHz)
-45
0
45
90
135
180
225
Increasing Cout reduces
the load resistance and
output capacitor pole
allowing for an increase
in mid-band gain
Figure 7. Increasing Output Capacitance
This will have the effect of both decreasing the voltage
drop as well as returning closer and faster to the
regulated voltage during the recovery time.
MOSFET Selection
The typical pass element for the MIC5191 is an N-
Channel MOSFET. There are multiple considerations
when choosing a MOSFET. These include:
" V
IN
to V
OUT
differential
" Output Current
" Case Size/Thermal Characteristics
" Gate Capacitance (C
ISS
<10nF)
" Gate to Source threshold
The V
IN(min)
to V
OUT
ratio and current will determine the
maximum R
DSON
required. For example, for a 1.8V (?%)
to 1.5V conversion at 5A of load current, dropout voltage
can be calculated as follows (using V
IN(min)
:
(
)
OUT
OUT
IN
DSON
I
V
V
R
=
(
)
5A
V
5
.
1
1.71V
R
DSON
=
R
DSON
= 42m&
For performance reasons, we do not want to run the N-
Channel in dropout. This will seriously affect transient
response and PSRR (power supply ripple rejection). For
this reason, we want to select a MOSFET that has lower
than 42m& for our example application.
Size is another important consideration. Most import-
antly, the design must be able to handle the amount of
power being dissipated.
The amount of power dissipated can be calculated as
follows (using V
IN(max)
):
P
D
= (V
IN
V
OUT
) ?I
OUT
P
D
= (1.89V 1.5V) ?5A
P
D
= 1.95W
Now that we know the amount of power we will be
dissipating, we will need to know the maximum ambient
air temperature. For our case were going to assume a
maximum of 65癈 ambient temperature, though different
MOSFETs have different maximum operating junction
temperatures. Most MOSFETs are rated to 150癈, while
others are rated as high as 175癈. In this case, were
going to limit our maximum junction temperature to
125癈. The MIC5191 has no internal thermal protection
for the MOSFET so it is important that the design
provides margin for the maximum junction temperature.
Our design will maintain better than 125癈 junction
temperature with 1.95W of power dissipation at an
ambient temperature of 65癈. Our thermal resistance
calculates as follows:
D
J
J
JA
P
(ambient)
T
(max)
T
?/DIV>
=
1.95W
C
65
C
125
?/DIV>
JA
?/DIV>
?/DIV>
=
?/DIV>
JA
= 31癈 /W
So our package must have a thermal resistance less
than 31癈 /W. Table 1 shows a good approximation of
power dissipation and package recommendation.
Package
Power Dissipation
TSOP-6
<850mW
TSSOP-8
<950mW
TSSOP-8
<1W
PowerPAK" 1212-8
<1.1W
SO-8
<1.125W
PowerPAK" SO-8 D-Pack
<1.4W
TO-220/TO-263 (D
2
pack)
>1.4W
Table 1. Power Dissipation and
Package Recommendation
In our example, our power dissipation is greater than
1.4W, so well choose a TO-263 (D
2
Pack) N-Channel
MOSFET. ?/DIV>
JA
is calculated as follows:
?/DIV>
JA
= ?/DIV>
JC
+ ?/DIV>
CS
+ ?/DIV>
SA
Where ?/DIV>
JC
is the junction to case resistance, ?/DIV>
CS
is the
case-to-sink resistance and the ?/DIV>
SA
is the sink-to-ambi-
ent air resistance.
In the D
2
package weve selected, the ?/DIV>
JC
is 2癈/W. The
?/DIV>
CS
, assuming we are using the PCB as the heat sink,
can be approximated to 0.2癈/W. This allows us to
calculate the minimum ?/DIV>
SA
:
?/DIV>
SA
= ?/DIV>
JA
?/DIV>
CS
?/DIV>
JC
?/DIV>
SA
= 31癈/W 0.2癈/W 2癈/W
?/DIV>
SA
= 28.8癈/W
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