ISL6232 View Datasheet(PDF) - Intersil
Part Name
Description
Manufacturer
ISL6232 Datasheet PDF : 25 Pages
| |||
ISL6232
Vo
R2
C3
V FB -
V REF GM
+
V COMP
R1
C2
C1
FIGURE 33. TYPE II COMPENSATOR
Figure 33 shows the type II compensator and its transfer
function is expressed as follows:
S)=
-vˆ---cvˆ---oF---m-B----p- =
C-----1--g--+--m---C-----2-
---1-----+------ω--------c-S-----z-------1---------1-----+------ω---------cS------z------2-----
S
1
+
ω----S-c---p-
(EQ. 28)
where
ωcz1
=
-------1------- ,
R1C1
ωcz2
=
-------1------- ,
R2C3
ωc
p
=
-C-----1-----+----C-----2--
R1C1C2
Compensator design goal:
High DC gain
Loop bandwidth fc:
1--
4
t
o
1--1--0--
fs
Gain margin: >10dB
Phase margin: 40°
The compensator design procedure is as follows:
Put compensator zero
ωcz1=
(
1
t
o
3
)
-------1-------
RoCo
Put one compensator pole at zero frequency to achieve high
DC gain, and put another compensator pole at either esr
zero frequency or half switching frequency, whichever is
lower. ωCZ2 is an internal zero due to 8pF and 600kΩ.
The loop gain Tv(S) at cross over frequency of fc has unity
gain. Therefore, the compensator resistance R1 is
determined by
R1
=
2----π----f--c---V----o----C----o----R----T--
gmVFB
(EQ. 29)
where gm is the trans-conductance of the voltage error
amplifier. Compensator capacitor C1 is then given by
C1
=
--------1---------
R1ωcz
,C2
=
------------1------------
2πR1fesr
(EQ. 30)
Example: Vin = 12V, Vo = 5V, Io = 5A, fs = 300kHz,
Co = 180µF/12mΩ, L = 6.8µH, gm = 100µS, RT = 0.128
(Rcs = 8mΩ, Ac = 16), VFB = 0.8V, Se = 1.5×105V/s,
Sn = 1.318×105V/s, fc = 45kHz, then compensator
resistance R1 = 400kΩ.
Put the compensator zero at 1.5kHz (~1.5x CoRo), and put
the compensator pole at esr zero which is 49kHz. The
compensator capacitors are:
C1 = 270pF, C2 = 10pF (There is approximately 8pF
parasitic capacitance from VCOMP to GND; Therefore, C2
optional).
Figure 34 shows the simulated voltage loop gain. It is shown
that it has 30kHz loop bandwidth with 85° phase margin and
20dB gain margin.
60
40
20
0
-20
-40
-60
100
VLOOP
1·103
1·104
1·105
FREQUENCY (Hz)
1·106
110
85
VLOOP
60
35
10
-15
-40
100
1·103
1·104
1·105
1·106
FIGURE 34. SIMULATED LOOP GAIN
12V Auxiliary Supply
A flyback transformer, or coupled inductor can be substituted
for the inductor in 5V or 3.3V supply to generate an 12V
auxiliary output as shown in Figure 35, which can be used to
drive N-channel MOSFETs. The ISL6232 is particularly well
suited for such applications because it can be configured in
ultrasonic or forced PWM mode to ensure good load
regulation when the main supplies are in light load
23
FN9116.0
April 18, 2005
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