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參數(shù)資料
| 型號： | IRU3007 |
| 廠商： | International Rectifier |
| 英文描述： | 5-BIT PROGRAMMABLE SYNCHRONOUS BUCK, NON-SYNCHRONOUS,ADJUSTABLE LDO AND 200mA ON-BOARD LDO |
| 中文描述： | 5位可編程同步降壓，非同步，可調(diào)LDO和200mA的板上穩(wěn)壓器 |
| 文件頁數(shù)： | 14/17頁 |
| 文件大?。?/td> | 100K |
| 代理商： | IRU3007 |
14
Rev. 2.1
08/20/02
IRU3007
www.irf.com
For 2.5V supply
Assuming R
B
= 1.02K
:
Rt = R
B
×
[
(Vo / V
REF
) - 1
]
Rt = 1.02
×
[
(2.5 / 1.26) - 1
]
= 1K
Switcher Output Voltage Adjust
Vcore
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 IRU3007 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 (R12 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:
Note:
The value of the top resistor must not exceed 100
.
The bottom resistor can then be adjusted to raise the
output voltage.
3.3V supply
The loop gain for the non-synchronous switching regula-
tor is intentionally set low to take advantage of the level
shifting technique to reduce the number of output ca-
pacitors. Typically there is a 1% drop in the output volt-
age from light load (discontinuous conduction mode) to
full load (continuous conduction mode) in the 3.3V sup-
ply. To account for this, the output voltage is set at 3.5V
typically. The same procedure as for the synchronous is
applied to the non-synch with the exception that the in-
ternal voltage reference of this regulator is internally set
at 2V. The following is the set of equations to use for the
output voltage setting for the non-synchronous assum-
ing the Vo=3.5V and R2=75
(R2 is the top resistor in
the application circuit).
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%
I = 9000
μ
F
×
(1V / 100ms) = 0.09A
R3 = R2
×
[
2 / (Vo - 2)
]
(
)
R3 = 75
×
[
2 / (3.5 - 2)
]
= 100
, 1%
The bottom resistor, R3 is calculated as follows:
Note:
The value of the top resistor, R2 must not exceed
100
.
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 10
μ
A current source
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 and the 10
μ
A internal current source
the ramp up rate is (
V/
t)=I/C=1V/100ms. Assuming
that the output capacitance is 9000
μ
F, the maximum
start up current will be:
Input Filter
It is highly recommended to place an inductor between
the system 5V supply and the input capacitors of the
switching regulator to isolate the 5V supply from the
switching noise that occurs during the turn on and off of
the switching components. Typically an inductor in the
range of 1 to 3
μ
H will be sufficient in this type of appli-
cation.
External Shutdown
The best way to shutdown the IRU3007 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.
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