IR3894 IRF [International Rectifier], IR3894 Datasheet - Page 31

• Manufacturer Part Number: IR3894
• Description: 12A HIGHLY INTERGRATED SUPLRBUCK
• Manufacturer: IRF [International Rectifier]

The small signal value may be obtained from the
manufacturer’s datasheets, design tools or spice models.
Alternatively, they may also be inferred from measuring the
power stage transfer function of the converter and measuring
the double pole frequency f
to compute the small signal c
These result to:
Select crossover frequency F
Since F
pole and zeros.
Detailed calculation of compensation Type III:
Desired Phase Boost Θ = 70°
Select:
Select C
Calculate R
Select R
C
C
2
F
F
F
3

LC
ESR
s
/2=300 kHz

=24.9 kHz
R
2 *
LC
=5.3 MHz
2 *

4
3
3
<F
= 2.2nF.
= 1.82 kΩ:


0
3
F
F
<Fs/2<F
, C
2 *
F
1
F
1
P
31
F
Z

Z
P
3
1
1
Z
3
2
*
and C
2
*


F
R

F
R
P
0.5*
F
3
o
3
F
3
o
C V
;
AUGUST 08, 2012 | DATA SHEET | Rev 3.1
ESR
*
o

;
4
C
2
L C V
C
, Type III is selected to place the
1 sin
1 sin
0.5*
:
*
1 sin
1 sin
o
2
F
3




*

Z

lc
in
2
and using equation (17)
o
354 pF, Select:
F
9.9 nF, Select:
.

o
s
0
*
=100 kHz





8.8 kHz and
300 kHz
osc


567.1 kHz
17.6 kHz
;
R
3
Single‐Input Voltage, Synchronous Buck Regulator


1.75 kΩ
C
C
2
3


220 pF
10 nF
- 31 -`
12A Highly Integrated SupIRBuck
Calculate R
Select R
Setting the Power Good Threshold
In this design IR3894 is used in normal (non‐tracking,
non‐sequencing) mode, therefore the PGood thresholds are
internally set at 90% and 120% of Vref. At startup as soon as
Vsns voltage reaches 0.9*0.5V=0.45V (Fig. 15), and after
1.28ms delay, PGood signal is asserted. As long as the Vsns
voltage is between the threshold range, Enable is high, and no
fault happens, the PGood remains high.
The following formula can be used to set the PGood
threshold. V
R8=2.87KΩ.
The PGood is an open drain output. Hence, it is necessary to
use a pull up resistor, R
of the pull‐up resistor must be chosen such as to limit the
current flowing into the PGood pin to be less than 5mA when
the output voltage is not in regulation. A typical value used
is 49.9kΩ.
OVP comparator also uses Vsns signal for over Voltage
dectection.With above values for R7 and R8, OVP trip point
(Vout_
Vref Bypass Capacitor
A minimum value of 100pF bypass capacitor is recommended
to be placed between Vref and Gnd pins.This capacitor should
be placed as close as possible to Vref pin.
Vout
R
6
R

4
_

V V
OVP)
OVP
5
o
2 *
= 4.02 kΩ:
V

-
is
ref
R

4
, R
out (PGood
5
ref
Vref
R
R
C

5
1
7 (
7 4.02
and R
4
* ;
2 *


*
R R

F
*1.2 * ( 7
5
V
P
_
out PGood TH
2
TH
0.9*
C
6
;
1
:
)
4
(
6
K
can be taken as 90% of Vout. Choose
PG
*

R

, from PGood pin to Vcc. The value
Vref
F
R
4
2.87 kΩ Select:
Z
_

2

127 Ω, Select:
-
)
R
R

4
8) / 8 1.44
;
1) * 8
R
R
5
R


4.1 kΩ,
IR3894
(34)
R
R
V
6
4


PD‐97745
2.87 kΩ
100 Ω
(35)