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參數(shù)資料
| 型號： | US3018 |
| 廠商： | Unisem Group |
| 英文描述： | 5 BIT PROGRAMMABLE SYNCHRONOUS BUCK PLUS LDO CONTROLLER AND 200mA LDO ON BOARD |
| 中文描述： | 5位可編程同步降壓普樂士LDO控制器和200mA LDO的船上 |
| 文件頁數(shù)： | 13/17頁 |
| 文件大?。?/td> | 108K |
| 代理商： | US3018 |
US3018
4-13
Rev. 1.4
12/8/00
Following the same procedure for the Schottcky diode
results in a heatsink with
θ
sa = 25
°
C/W. Although it is
possible to select a slightly smaller heatsink, for sim-
plicity the same heatsink as the one for the high side
MOSFET is also selected for the synchronous MOSFET.
Switcher Current Limit Protection
The US3018 uses the MOSFET Rds-on as the sensing
resistor to sense the MOSFET current and compares to
a programmed voltage which is set externally via a re-
sistor (Rcs) placed between the drain of the MOSFET
and the “CS+” terminal of the IC as shown in the appli-
cation circuit. For example, if the desired current limit
point is set to be 22A for the synchronous and 16A for
the non synchronous , and from our previous selection,
the maximum MOSFET Rds-on=19m
, then the cur-
rent sense resistor Rcs is calculated as :
Vcs=IcL*Rds=22*0.019=0.418V
Rcs=Vcs/Ib=(0.418V)/(200uA)=2.1k
Where: Ib=200uA is the internal current setting of the
US3018
Switcher Frequency Selection
The US3018 frequency is internally set at 200kHz with
no external timing resistor. However, it can be adjusted
up by using an external resistor from Rt pin to GND or
can be adjusted down if the resistor is connected to the
12V supply.
1.5V, GTL+ Supply LDO Power MOSFET Selection
The first step in selectiong the power MOSFET for the
1.5V linear regulator is to select its maximum Rds-on of
the pass transistor based on the input to output Dropout
voltage and the maximum load current.
Rds(max)=(Vin - Vo)/IL
For Vo=1.5V, and Vin=3.3V , IL=2A
Rds-max=(3.3 - 1.5)/2= 0.9
Note that since the MOSFETs Rds-on increases with
temperature, this number must be divided by
≈
1.5,
inorder to find the Rds-on max at room temperature. The
Motorola MTP3055VL has a maximum of 0.18
Rds-on
at room temperature, which meets our requirement.
To select the heatsink for the LDO Mosfet the first step
is to calculate the maximum power dissipation of the
device and then follow the same procedure as for the
switcher.
Pd = ( Vin - Vo ) * IL
Where :
Pd = Power Dissipation of the Linear Regulator
IL = Linear Regulator Load Current
For the 1.5V and 2A load:
Pd = (3.3 - 1.5)*2=3.6 W
Assuming Tj-max=125
°
C
Ts = Tj - Pd * (
θ
jc +
θ
cs)
Ts = 125 - 3.6 * (1.8 + 0.05) = 118
°
C
With the maximum heat sink temperature calculated in
the previous step, the Heat Sink to Air thermal resis-
tance (
θ
sa) is calculated as follows :
Assuming Ta=35
°
C
T = Ts - Ta = 118 - 35 = 83
°
C Temperature Rise
Above Ambient
θ
sa =
T/Pd
θ
sa = 83 / 3.6 = 23
°
C/W
The same heat sink as the one selected for the switcher
MOSFETs is also suitable for the 1.5V regulator.
2.5V, Clock Supply
The US3018 provides an internal
ultra low dropout
regulator with a minimum of 200mA
current capabil-
ity that converts 3.3V supply to a programmable regu-
lated 2.5V supply to power the clock chip. The internal
regulator has short circuit protection with internal ther-
mal shutdown.
1.5V and 2.5V Supply Resistor Divider Selection
Since the internal voltage reference for the linear regula-
tors is set at 1.26V for US3018, there is a need to use
external resistor dividers to step up the voltage. The re-
sistor dividers are selected using the following equations:
Vo=(1+Rt/Rb)*Vref
Where:
Rt=Top resistor divider
Rb=Bottom resistor divider
Vref=1.26V typical
For 1.5V supply :
Assuming Rb=100
Rt=Rb*
[
(Vo/Vref) - 1
]
Rt=100*
[
(1.5/1.26) - 1
]
=19.1
For 2.5V supply :
Assuming Rb=200
Rt=Rb*
[
(Vo/Vref) - 1
]
Rt=200*
[
(2.5/1.26) - 1
]
=197
Select Rt=200
Switcher Output Voltage Adjust
As it was discussed earlier,the trace resistance from
the output of the switching regulator to the Slot 1 can be
used to the circuit advantage and possibly reduce the
number of output capacitors, by level shifting the DC
相關(guān)PDF資料 |
PDF描述 |
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| US3018CW | 5 BIT PROGRAMMABLE SYNCHRONOUS BUCK PLUS LDO CONTROLLER AND 200mA LDO ON BOARD |
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| US3021BCW | 5 BIT PROGRAMMABLE SYNCHRONOUS BUCK PLUS TRIPPLE LDO CONTROLLER |
| US3021M | 5 BIT PROGRAMMABLE SYNCHRONOUS BUCK PLUS TRIPPLE LDO CONTROLLER |
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| US3018CW | 制造商:UNISEM 制造商全稱:UNISEM 功能描述:5 BIT PROGRAMMABLE SYNCHRONOUS BUCK PLUS LDO CONTROLLER AND 200mA LDO ON BOARD |
| US3021B | 制造商:UNISEM 制造商全稱:UNISEM 功能描述:5 BIT PROGRAMMABLE SYNCHRONOUS BUCK PLUS TRIPPLE LDO CONTROLLER |
| US3021BCW | 制造商:UNISEM 制造商全稱:UNISEM 功能描述:5 BIT PROGRAMMABLE SYNCHRONOUS BUCK PLUS TRIPPLE LDO CONTROLLER |
| US3021M | 制造商:UNISEM 制造商全稱:UNISEM 功能描述:5 BIT PROGRAMMABLE SYNCHRONOUS BUCK PLUS TRIPPLE LDO CONTROLLER |
| US3021MCW | 制造商:UNISEM 制造商全稱:UNISEM 功能描述:5 BIT PROGRAMMABLE SYNCHRONOUS BUCK PLUS TRIPPLE LDO CONTROLLER |
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