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參數(shù)資料
| 型號： | IRU3018 |
| 廠商： | International Rectifier |
| 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的板載 |
| 文件頁數(shù)： | 14/16頁 |
| 文件大?。?/td> | 148K |
| 代理商： | IRU3018 |
14
Rev. 1.5
07/24/01
IRU3018
For 1.5V supply
Assuming R
B
=100
:
Rt = R
B
×
[
(Vo/Vref) - 1
]
Rt = 100
×
[
(1.5/1.26) - 1
]
= 19.1
For 2.5V supply
Assuming R
B
=200
:
Rt = R
B
×
[
(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
regulation point when transitioning from light load to full
load and vice versa. To account for the DC drop, the
output of the regulator is typically set about half the DC
drop that results from light load to full load. For example,
if the total resistance from the output capacitors to the
Slot 1 and back to the Gnd pin of the IRU3018 is 5m
and if the total
I, the change from light load to full load
is 14A, then the output voltage measured at the top of
the resistor divider which is also connected to the out-
put capacitors in this case, must be set at half of the
70mV or 35mV higher than the DAC voltage setting. To
do this, the top resistor of the resistor divider (R17 in the
application circuit) is set at 100
, and the R19 is calcu-
lated. For example, if DAC voltage setting is for 2.8V
and the desired output under light load is 2.835V, then
R19 is calculated using the following formula:
R19 = 100
×
[
V
DAC
/(Vo - 1.004
×
V
DAC
)
]
(
)
R19 = 100
×
[
2.8/(2.835 - 1.004
×
2.800)
]
= 11.76K
Select 11.8K
, 1%
Note:
The value of the top resistor must not exceed 100
.
The bottom resistor can then be adjusted to raise the
output voltage.
Soft-Start Capacitor Selection
The soft-start capacitor must be selected such that dur-
ing the start-up when the output capacitors are charging
up, the peak inductor current does not reach the current
limit threshold. A minimum of 1
μ
F capacitor insures this
for most applications. An internal resistor charges the
soft-start capacitor which slowly ramps up the inverting
input of the PWM comparator Vfb3. This insures the
output voltage to ramp at the same rate as the soft-start
cap thereby limiting the input current. For example, with
1
μ
F of soft-start capacitor, the ramp up rate is approxi-
mated to be 1V/20ms. For example if the output capaci-
tance is 9000
μ
F, the maximum start up current will be:
I = 9000
μ
F
×
(1V/20ms) = 0.45A
The other function of the soft-start cap is to provide an
off time between the current limit cycles(HICCUP) in or-
der for the synchronous MOSFET to cool off and survive
the short circuit condition. The off time between the cur-
rent limit cycles is approximated as:
Input Filter
It is recommended to place an inductor between the
system 5V supply and the input capacitors of the switch-
ing regulator to isolate the 5V supply from the switching
noise that occurs during the turn on and off of the switch-
ing components. Typically an inductor in the range of 1
to 3
μ
H will be sufficient in this type of application.
External Shutdown
The best way to shutdown the IRU3018 is to pull down
on the soft-start pin using an external small signal tran-
sistor such as 2N3904 or 2N7002 small signal MOSFET.
This allows slow ramp up of the output, the same as the
power up.
Layout Considerations
Switching regulators require careful attention to the lay-
out of the components, specifically power components
since they switch large currents. These switching com-
ponents can create large amount of voltage spikes and
high frequency harmonics if some of the critical compo-
nents are far away from each other and are connected
with inductive traces. The following is a guideline of how
to place the critical components and the connections
between them in order to minimize the above issues.
Start the layout by first placing the power components:
1) Place the input capacitor C14 and the high-side
MOSFET, Q3 as close to each other as possible.
2) Place the synchronous MOSFET, Q4 and the Q3 as
close to each other as possible with the intention
that the source of Q3 and drain of the Q4 has the
shortest length.
3) Place the snubber R15 & C13 between Q4 & Q3.
T
HICCUP
= 60
×
Css (ms)
For example if Css=1
μ
F, T
HICCUP
= 60
×
1 = 60ms
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