10
Rev. 1.6
08/20/02
IRU3011
www.irf.com
RB = 1003[VDAC /(Vo - 1.0043VDAC)] [V]
RB = 1003[2.8 /(2.835 - 1.00432.800)] = 11.76KV
I = 9000mF3(1V / 100ms) = 0.09A
Next, a heat sink with lower uSA than the one calculated
in the previous step must be selected. One way to do
this is to simply look at the graphs of the “Heat Sink
Temp Rise Above the Ambient” vs. the “Power Dissipa-
tion” given in the heat sink manufacturers’ catalog and
select a heat sink that results in lower temperature rise
than the one calculated in previous step. The following
AAVID and Thermalloy heat sinks, meet this criteria.
Co. Part #
Thermalloy............................6078B
AAVID...................................577002
Following the same procedure for the Schottky diode
results in a heatsink with uSA=258C/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 PWM controller uses the MOSFET RDS(ON) as the
sensing resistor to sense the MOSFET current and com-
pares to a programmed voltage which is set externally
via a resistor (Rcs) placed between the drain of the
MOSFET and the “CS+” terminal of the IC as shown in
the application circuit. For example, if the desired cur-
rent limit point is set to be 22A and from our previous
selection, the maximum MOSFET RDS(ON)=19mV, then
the current sense resistor, Rcs is calculated as:
Where:
IB = 200mA is the internal current setting of the
IRU3011
Switcher Timing Capacitor Selection
The switching frequency can be programmed using an
external timing capacitor. The value of Ct can be ap-
proximated using the equation below:
Where:
Ct = Timing Capacitor
FSW = Switching Frequency
If, FSW = 200KHz:
Vcs = ICL3RDS
= 2230.019 = 0.418V
Rcs = Vcs / IB = (0.418V) / (200mA) = 2.1KV
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 device is 5mV and
if the total DI, 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 output
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, RTOP is set at 100V,
and the bottom resistor, RB is calculated. For example,
if DAC voltage setting is for 2.8V and the desired output
under light load is 2.835V, then RB is calculated using
the following formula:
Select 11.8KV, 1%
Note: The value of the top resistor must not exceed 100V.
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 threshhold. A minimum of 1mF capacitor insures
this for most applications. An internal 10mA 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 cur-
rent. For example, with 1mF and the 10mA internal cur-
rent source the ramp up rate is (DV/Dt)=I/C=1V/100ms.
Assuming that the output capacitance is 9000µF, the
maximum start up current will be:
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 3mH will be sufficient in this type of application.
Fsw ≅≅
3.5 3 10
-5
Ct
Ct ≅≅ = 175pF
3.5 3 10
-5
200 3 10
3
