IR3621F International Rectifier, IR3621F Datasheet - Page 15

• Manufacturer Part Number: IR3621F
• Description: IC PWM DUAL SYNC PREBIAS 28TSSOP
• Manufacturer: International Rectifier

Specifications of IR3621F

• Pwm Type: Voltage Mode
• Number Of Outputs: 2
• Frequency - Max: 345kHz
• Duty Cycle: 86.5%
• Voltage - Supply: 5.5 V ~ 14.5 V
• Buck: Yes
• Boost: No
• Flyback: No
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: No
• Operating Temperature: -40°C ~ 125°C
• Package / Case: 28-TSSOP
• Frequency-max: 345kHz
• Package: 28-Pin TSSOP
• Circuit: Dual Sync PWM Controller or 2Phase Single Output
• Vcc (min): 4.7
• Vcc (max): 16
• Vout (min): 0.8
• Vout (max): Vcc * 0.90
• Switch Freq (khz): programmable to 500kHz
• Pbf: PbF Option Available
• For Use With: IRDC3621 - BOARD EVAL DUAL SYNC BUCK CTRLR
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

To cancel one of the LC filter poles, place the zero be-
fore the LC filter resonant frequency pole:
Using equations (13) and (15) to calculate C
Same calcuation For V
C
One more capacitor is sometimes added in parallel with
C
used to suppress the switching noise. The additional
pole is given by:
The pole sets to one half of switching frequency which
results in the capacitor C
For V
V
V
F
F
C
C
8
9
for F
O1
ESR
IN
OSC
F
F
For:
Lo = 1.1µH
Co = 990µF
= 10nF
and R
POLE
R
9
Where:
V
V
F
F
F
R
g
This results to R
Choose R
Z
Z
4
= 12V
= 40KHz
m
O1
ESR
LC
IN
OSC
5
= 13.3kHz
=
= 1.25V
8.3nF; Choose C
2.5V
P
and R
= Error Amplifier Transconductance
=
75%F
0.75×
= Maximum Input Voltage
= Crossover Frequency
= Resonant Frequency of the Output Filter
V
<<
4
V
= Zero Frequency of the Output Capacitor
= Oscillator Ramp Voltage
:
. This introduces one more pole which is mainly
π×R
OSC
IN
f
2
S
F
×
9
LC
P
4
= Resistor Dividers for Output Voltage
2π
=5K
4
F
=
×f
Programming
1
O1
2π×R
F
S
×F
LC
-
L
4
1
=4.8K
O
2
C
ESR
1
× C
1.8V
9
9
4
=8.2nF
POLE:
×
×
will result to: R
1
F
R
R
g
O
C
R
C
m
LC
5
9
F
R
π×R
9
9
5
= 1400µmho
= 1K
= 2.14K
Z
4
×C
R
+ R
= 5.06kHz
+ C
= 3.61kHz
= 5K
5
1
4
POLE
9
POLE
×f
×
---(15)
S
g
1
m
3
= 4.2K and
9
, we get:
---(14)
www.irf.com
For a general solution for unconditional stability for ce-
ramic output capacitor with very low ESR or any type of
output capacitors, in a wide range of ESR values we
should implement local feedback with a compensation
network. The typically used compensation network for a
voltage-mode controller is shown in Figure 16.
In such configuration, the transfer function is given by:
The error amplifier gain is independent of the transcon-
ductance under the following condition:
By replacing Z
former function can be expressed as:
H(s) =
As known, transconductance amplifier has high imped-
ance (current source) output, therefore, consider should
be taken when loading the E/A output. It may exceed its
source/sink output current capability, so that the ampli-
fier will not be able to swing its output voltage over the
necessary range.
The compensation network has three poles and two ze-
ros and they are expressed as follows:
H(s) dB
g
m
V
Z
Figure 16- Compensation network with local
V
Z
OUT
f
sR
IN
e
>> 1
Gain(dB)
feedback and its asymptotic gain plot.
6
R
=
(C
C
8
10
1
12
1 + g
F
1 - g
+C
IN
V
Z
and
1
OUT
and Z
11
m
R
m
R
)
Z
Z
×
6
5
IN
f
[
f
V
g
according to Figure 16, the trans-
1+sR
(1+sR
P2
Fb
m
F
=V
Z
Z
IN
2
REF
IR3621 & (PbF)
7
>>1
(
7
R
C
C
C
7
F
11
E/A
12
P
12
)×[1+sC
+C
2
C
C
11
11
12
)]
F
×(1+sR
---(16)
C
P
10
Comp
3
11
(R
Frequency
6
+R
Z
8
8
Ve
f
C
)]
15
10
)