iru3004 International Rectifier Corp., iru3004 Datasheet - Page 10

• Manufacturer Part Number: iru3004
• Description: 5-bit Programmable Synchronous Buck Controller Ic With Dual Ldo Controller
• Manufacturer: International Rectifier Corp.

IRU3004
APPLICATION INFORMATION
An example of how to calculate the components for the
application circuit is given below.
Assuming, two sets of output conditions that this regu-
lator must meet:
The regulator design will be done such that it meets the
worst case requirement of each condition.
Output Capacitor Selection
The first step is to select the output capacitor. This is
done primarily by selecting the maximum ESR value
that meets the transient voltage budget of the total DVo
specification. Assuming that the regulators DC initial
accuracy plus the output ripple is 2% of the output volt-
age, then the maximum ESR of the output capacitor is
calculated as:
The Sanyo MVGX series is a good choice to achieve
both the price and performance goals. The 6MV1500GX,
1500mF, 6.3V has an ESR of less than 36mV typical.
Selecting 6 of these capacitors in parallel has an ESR
of
Other type of Electrolytic capacitors from other manu-
facturers to consider are the Panasonic FA series or the
Nichicon PL series.
Reducing the Output Capacitors Using Voltage Level
Shifting Technique
The trace resistance or an external resistor 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 transition from light load to full load and vice versa.
To accomplish this, the output of the regulator is typi-
cally 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. This intentional voltage level shifting
during the load transient eases the requirement for the
10
a) Vo=2.8V, Io=14.2A, DVo=185mV, DIo=14.2A
b) Vo=2V, Io=14.2A, DVo=140mV, DIo=14.2A
ESR [
6mV which achieves our low ESR goal.
14.2
100
= 7mV
www.irf.com
output capacitor ESR at the cost of load regulation. One
can show that the new ESR requirement eases up by
half the total trace resistance. For example, if the ESR
requirement of the output capacitors without voltage level
shifting must be 7mV then after level shifting the new
ESR will only need to be 9.5mV if the trace resistance
is 5mV (7 + 5/2=9.5). However, one must be careful that
the combined “voltage level shifting” and the transient
response is still within the maximum tolerance of the
Intel specification. To insure this, the maximum trace
resistance must be less than:
For example, assuming:
Then the Rs is calculated to be:
However, if a resistor of this value is used, the maximum
power dissipated in the trace (or if an external resistor is
being used) must also be considered. For example if
Rs=12.6mV, the power dissipated is:
This is a lot of power to be dissipated in a system. So, if
the Rs=5mV, then the power dissipated is about 1W
which is much more acceptable. If level shifting is not
implemented, then the maximum output capacitor ESR
was shown previously to be 7mV which translated to 6
of the 1500mF, 6MV1500GX type Sanyo capacitors. With
Rs=5mV, the maximum ESR becomes 9.5mV which is
equivalent to
is that if a trace is being used to implement the resistor,
the power dissipated by the trace increases the case
temperature of the output capacitors which could seri-
ously effect the life time of the output capacitors.
Rs [ 2 3
Rs [ 2 3
Io
Where:
Rs = Total maximum trace resistance allowed
Vspec = Intel total voltage specification
Vo = Output voltage
DVo = Output ripple voltage
DI = load current step
Vspec = 140mV = 0.1V for 2V output
Vo = 2V
DVo = assume 10mV = 0.01V
DI = 14.2A
2
3Rs = 14.2
(Vspec - 0.02 3 Vo - DVo)
(0.140 - 0.02 3 2 - 0.01)
4 caps. Another important consideration
2
312.6 = 2.54W
14.2
DI
= 12.6mV
07/16/02
Rev. 1.7