IRU3037/IRU3037A & (PbF)
7
www.irf.com
For this design, IRF7301 is a good choice. The device
provides low on-resistance in a compact SOIC 8-Pin
package.
The IRF7301 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. With a
linear approximation, the total switching loss can be ex-
pressed as:
The switching time waveform is shown in figure 4.
Figure 4 - Switching time waveforms.
From IRF7301 data sheet we obtain:
These values are taken under a certain condition test.
For more detail please refer to the IRF7301 data sheet.
By using equation (6), we can calculate the switching
losses.
Feedback Compensation
The IRU3037 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 5). The Reso-
nant frequency of the LC filter expressed as follows:
Figure 5 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.
Figure 5 - Gain and phase of LC filter.
The IRU3037’s error amplifier is a differential-input
transconductance amplifier. The output is available for
DC gain control or AC phase compensation.
The E/A can be compensated with or without the use of
local feedback. When operated without local feedback
the transconductance properties of the E/A become evi-
dent and can be used to cancel one of the output filter
poles. This will be accomplished with a series RC circuit
from Comp pin to ground as shown in Figure 6.
VDSS = 20V
ID = 5.2A
RDS(ON) = 0.05Ω
Where:
V
DS(OFF) = Drain to Source Voltage at off time
tr = Rise Time
tf = Fall Time
T = Switching Period
ILOAD = Load Current
tr = 42ns
tf = 51ns
P
SW = 0.186W
PCON(TOTAL)=PCON(Upper Switch)+PCON(Lower Switch)
PCON(TOTAL) = ILOAD × RDS(ON) × ϑ
2
ϑ = 1.5 according to the IRF7301 data sheet for
150)C junction temperature
PCON(TOTAL) = 1.2W
FLC = ---(7)
1
2π× LO×CO
PSW = ILOAD ---(6)
tr + tf
T
VDS(OFF)
2
××
V
DS
V
GS
10%
90%
t
d
(ON)
t
d
(OFF)
t
r
t
f
Gain
F
LC
0dB
Phase
0
)
F
LC
-180
)
Frequency
Frequency
-40dB/decade
