IRU3007
For 2.5V supply
Assuming RB = 1.02KΩ:
The bottom resistor, R3 is calculated as follows:
R3 = R2×[2 / (Vo - 2)] (Ω)
Rt = RB×[(Vo / VREF) - 1]
R3 = 75×[2 / (3.5 - 2)] = 100Ω, 1%
Rt = 1.02×[(2.5 / 1.26) - 1] = 1KΩ
Note: The value of the top resistor, R2 must not exceed
100Ω.
Switcher Output Voltage Adjust
Vcore
As it was discussed earlier, the trace resistance from Soft-Start Capacitor Selection
the output of the switching regulator to the Slot 1 can be The soft-start capacitor must be selected such that dur-
used to the circuit advantage and possibly reduce the ing the start up when the output capacitors are charging
number of output capacitors, by level shifting the DC up, the peak inductor current does not reach the current
regulation point when transitioning from light load to full limit threshold. A minimum of 1µF capacitor insures this
load and vice versa. To account for the DC drop, the for most applications. An internal 10µA current source
output of the regulator is typically set about half the DC charges the soft-start capacitor which slowly ramps up
drop that results from light load to full load. For example, the inverting input of the PWM comparator Vfb3. This
if the total resistance from the output capacitors to the insures the output voltage to ramp at the same rate as
Slot 1 and back to the Gnd pin of the IRU3007 is 5mΩ the soft-start cap thereby limiting the input current. For
and if the total ∆I, the change from light load to full load example, with 1µF and the 10µA internal current source
is 14A, then the output voltage measured at the top of the ramp up rate is (∆V/∆t)=I/C=1V/100ms. Assuming
the resistor divider which is also connected to the out- that the output capacitance is 9000µF, the maximum
put capacitors in this case, must be set at half of the start up current will be:
70mV or 35mV higher than the DAC voltage setting. To
do this, the top resistor of the resistor divider (R12 in the
I = 9000µF × (1V / 100ms) = 0.09A
application circuit) is set at 100Ω, and the R19 is calcu- Input Filter
lated. For example, if DAC voltage setting is for 2.8V It is highly recommended to place an inductor between
and the desired output under light load is 2.835V, then the system 5V supply and the input capacitors of the
R19 is calculated using the following formula:
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 3µH will be sufficient in this type of appli-
cation.
R19 = 100×[VDAC / (Vo - 1.004×VDAC)] (Ω)
R19 = 100×[2.8 / (2.835 - 1.004×2.800)] = 11.76KΩ
Select 11.8KΩ, 1%
Note:The value of the top resistor must not exceed 100Ω. External Shutdown
The bottom resistor can then be adjusted to raise the The best way to shutdown the IRU3007 is to pull down
output voltage.
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
3.3V supply
The loop gain for the non-synchronous switching regula- power up.
tor is intentionally set low to take advantage of the level
shifting technique to reduce the number of output ca- Layout Considerations
pacitors. Typically there is a 1% drop in the output volt- Switching regulators require careful attention to the lay-
age from light load (discontinuous conduction mode) to out of the components, specifically power components
full load (continuous conduction mode) in the 3.3V sup- since they switch large currents. These switching com-
ply. To account for this, the output voltage is set at 3.5V ponents can create large amount of voltage spikes and
typically. The same procedure as for the synchronous is high frequency harmonics if some of the critical compo-
applied to the non-synch with the exception that the in- nents are far away from each other and are connected
ternal voltage reference of this regulator is internally set with inductive traces. The following is a guideline of how
at 2V. The following is the set of equations to use for the to place the critical components and the connections
output voltage setting for the non-synchronous assum- between them in order to minimize the above issues.
ing the Vo=3.5V and R2=75Ω (R2 is the top resistor in
the application circuit).
Rev. 2.1
08/20/02
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