IRU3137CS IRF [International Rectifier], IRU3137CS Datasheet - Page 9

• Manufacturer Part Number: IRU3137CS
• Description: 8-PIN SYNCHRONOUS PWM CONTROLLER
• Manufacturer: IRF [International Rectifier]

Choose IRF7832 for both control MOSFET and synchro-
nous MOSFET. This device provides low on-resistance
in a compact SOIC 8-Pin package.
The MOSFET has 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 7.
From IRF7832 data sheet we obtain:
Rev. 1.0
06/22/04
P
P
P
IRF7832
t
t
IRF7832
V
I
R
Where:
V
t
t
T = Switching Period
I
D
LOAD
r
f
r
f
10%
90%
CON(TOTAL)
CON(TOTAL)
SW
DSS
DS(ON)
DS(OFF)
= Fall Time
= 12.3ns
= 21ns
= Rise Time
V
= 20A @ 25 C
V
GS
DS
=
= Load Current
Figure 7 - Switching time waveforms.
= 30V
t
d
V
= 4m @ V
(ON)
= Drain to Source Voltage at off time
DS(OFF)
2
= 1.166W
= P
CON(UPPER)
t
r
t
GS
r
+
T
=10V
t
f
t
+ P
d
(OFF)
I
CON(LOWER)
LOAD
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.
Feedback Compensation
The IRU3137 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 8). The Reso-
nant frequency of the LC filter is expressed as follows:
Figure 9 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
P
F
LC
SW(TOTAL)
=
Figure 8 - Gain and phase of LC filter.
2
F
= 250mW
LC
1
Frequency
-40dB/decade
L
O
C
O
-180
Phase
0
---(13)
IRU3137
F
LC
Frequency
9