iru3039 International Rectifier Corp., iru3039 Datasheet - Page 10

• Manufacturer Part Number: iru3039
• Description: Ic Ctrlr Pwm Sync Switch 20-mlpq
• Manufacturer: International Rectifier Corp.

IRU3039(PbF)
Choose IRF7466 for control MOSFET and IRF7458 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 will be:
The switching loss is more difficult to calculate, even
though the switching transition is well understood. The
reason is the effect of the parasitic components and
switching times during the switching procedures such
as turn-on / turnoff delays and rise and fall times. The
control MOSFET contributes to the majority of the switch-
ing losses in synchronous Buck converter. The synchro-
nous MOSFET turns on under zero voltage conditions,
therefore, the turn on losses for synchronous MOSFET
can be neglected. With a linear approximation, the total
switching loss can be expressed as:
The switching time waveform is shown in Figure 10.
From IRF7466 data sheet we obtain:
10
P
P
P
IRF7466
t
t
IRF7466
V
I
R
Where:
V
t
t
T = Switching Period
I
D
LOAD
r
f
r
f
90%
10%
CON(TOTAL)
CON(TOTAL)
SW
DSS
DS(OFF)
DS(ON)
= Rise Time
= Fall Time
= 2.8ns
= 3.6ns
V
= 11A
V
GS
DS
Figure 10 - Switching time waveforms.
=
= Load Current
= 30V
t
d
V
= 12.5mΩ
(ON)
= Drain to Source Voltage at off time
DS(OFF)
2
= P
= 0.85W
CON(UPPER)
×
t
r
t
r
+
T
t
f
t
+ P
d
×
(OFF)
IRF7458
V
I
R
D
I
DSS
DS(ON)
CON(LOWER)
LOAD
= 14A
= 30V
= 8mΩ
t
f
---(12)
www.irf.com
These values are taken under a certain condition test.
For more details please refer to the IRF7466 and IRF7458
data sheets.
By using equation (12), we can calculate the total switch-
ing losses.
Programming the Over-Current Limit
The over-current threshold can be set by connecting a
resistor (R
OCSet pin. The resistor can be calculated by using equa-
tion (2).
The R
should be considered for the worse case operation.
Feedback Compensation
The IRU3039 is a voltage mode controller; the control
loop is a single voltage feedback path including error
amplifier and error comparator. To achieve fast transient
response and accurate output regulation, a compensa-
tion circuit is necessary. The goal of the compensation
network is to provide a closed loop transfer function with
the highest 0dB crossing frequency and adequate phase
margin (greater than 45 ).
The output LC filter introduces a double pole, –40dB/
decade gain slope above its corner resonant frequency,
and a total phase lag of 180 (see Figure 11). The Reso-
nant frequency of the LC filter is expressed as follows:
Figure 11 shows gain and phase of the LC filter. Since
we already have 180 phase shift just from the output
filter, the system risks being unstable.
0dB
Gain
R
I
(50% over nominal output current)
This results to:
R
F
P
SET
LC
SW(TOTAL)
DS(ON)
SET
DS(ON)
=
Figure 11 - Gain and phase of LC filter.
= 5.76KΩ
I
O(LIM)
2π× L
SET
= 8mΩ×1.5 = 12mΩ
has a positive temperature coefficient and it
) from drain of low side MOSFET to the
= 92mW
F
LC
= 8A×1.5 = 12A
1
Frequency
-40dB/decade
O
×C
O
-180
0
Phase
---(13)
F
LC
Frequency