14

Rev. 2.1

08/20/02

IRU3007

www.irf.com

For 2.5V supply

V:

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

V

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 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

V, 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

V.

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

V (R2 is the top resistor in

the application circuit).

3

Rt = 1.02

3

[(2.5 / 1.26) - 1] = 1KV

R19 = 100

3

(

V)

R19 = 100

3

[2.8 / (2.835 - 1.00432.800)] = 11.76KV

Select 11.8K

V, 1%

I = 9000

mF 3 (1V / 100ms) = 0.09A

R3 = R2

3

[2 / (Vo - 2)]

(

V)

R3 = 75

3

[2 / (3.5 - 2)] = 100V, 1%

The bottom resistor, R3 is calculated as follows:

Note: The value of the top resistor, R2 must not exceed

100

V.

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

mF capacitor insures this

for most applications. An internal 10

mA 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

mF and the 10mA internal current source

the ramp up rate is (

DV/Dt)=I/C=1V/100ms. Assuming

that the output capacitance is 9000

mF, 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

mH 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.