iru3047 International Rectifier Corp., iru3047 Datasheet - Page 8

• Manufacturer Part Number: iru3047
• Description: Dual Synchronous Controller With Current Sharing Circuitry Controller
• Manufacturer: International Rectifier Corp.

IRU3047
The R
ered for the worst case operation. This is typically given
in the MOSFET data sheet. Ensure that the conduction
losses and switching losses do not exceed the package
ratings or violate the overall thermal budget.
Choose IRF7811A for control MOSFET and IRF7809A
for synchronous MOSFET. These devices provide low
on-resistance in a compact SOIC 8-Pin package.
The MOSFETs have the following data:
The total conduction losses for the master channel is:
The total conduction losses for the slave channel is:
The control MOSFET contributes to the majority of the
switching losses in synchronous Buck converter. The
synchronous MOSFET turns on under zero-voltage con-
dition, therefore the turn on losses for synchronous
MOSFET can be neglected. With a linear approxima-
tion, the total switching loss can be expressed as:
8
IRF7811A
V
I
R
V
P
P
P
Where:
V
t
t
T = Switching Period
I
D
LOAD
r
f
10%
DSS
DS(ON)
GS
CON(MASTER)
CON(SLAVE)
SW
DS(OFF)
90%
DS(ON)
= Rise Time
= Fall Time
= 11.2A @ 908C
V
V
GS
= 4.5V
D S
=
= Load Current
Figure 4 - Switching time waveforms.
= 28V
t
V
d
= 12mV @
temperature dependency should be consid-
(ON)
= Drain to Source Voltage at off time
DS(OFF)
2
For both: q = 1.5 for 1508C
= 0.5535W
(Junction Temperature)
= 0.498W
3
t
r
t
+ t
T
r
f
t
3
d
IRF7809A
V
I
R
V
(OFF)
D
DSS
DS(ON)
GS
I
= 14.2A @ 908C
LOAD
= 4.5V
= 20V
= 8.5mV @
---(7)
t
f
www.irf.com
From IRF7811A data sheet we obtain:
These values are taken under a certain condition test.
For more detail please refer to the IRF7811A and
IRF7809A data sheets.
By using equation (7), we can calculate the switching
losses.
Feedback Compensation
The control scheme for master and slave channels is
based on voltage mode control, but the compensation of
these two feedback loops is slightly different.
The Master channel sets the output voltage and its feed-
back loop should take care of double pole introduced by
the output filter as a regular voltage mode control loop.
The goal is to provide a close loop transfer function with
the highest 0dB crossing frequency and adequate phase
margin. The slave feedback loop acts slightly different
and its goal is using the current information for current
sharing.
The master feedback loop sees the output filter. The out-
put LC filter introduces a double pole, -40dB/decade gain
slope above its corner resonant frequency, and a total
phase lag of 1808 (see Figure 5). The Resonant frequency
of the LC filter expressed as follows:
Figure 5 shows gain and phase of the LC filter. Since we
already have 1808 phase shift just from the output filter,
the system risks being unstable.
0dB
Gain
F
IRF7811A
t
t
P
P
r
f
LC(MASTER)
SW(MASTER)
SW(SLAVE)
= 4ns
= 8ns
Figure 5 - Gain and phase of LC filter.
F
= 36mW
LC
=
= 86.4mW
2p
-40dB/decade
Frequency
Lo3Co
1
-180
0
Phase
8
8
---(8)
F
LC
Frequency
09/09/02
Rev. 1.0