ISL6255 INTERSIL [Intersil Corporation], ISL6255 Datasheet - Page 18

• Manufacturer Part Number: ISL6255
• Description: Highly Integrated Battery Charger with Automatic Power Source Selector for Notebook Computers
• Manufacturer: INTERSIL [Intersil Corporation]

Loop Compensation Design
ISL6255, ISL6255A uses a constant frequency current mode
control architecture to achieve fast loop transient response.
Accurate current sensing resistors in series with the output
inductor is used to regulate the charge current, and the
sensed current signal is injected into the voltage loop to
achieve current mode control to simplify the loop
compensation design. The inductor is not considered as a
state variable for current mode control and the system
becomes a single order system. It is much easier to design a
compensator to stabilize the voltage loop than voltage mode
control. Figure 17 shows the small signal model of the
synchronous buck regulator.
PWM Comparator Gain F
The PWM comparator gain Fm for peak current mode
control is given by:
Where V
signal.
Current Sampling Transfer Function H
In current loop, the current signal is sampled every switching
cycle. It has the following transfer function:
where Q
respectively.
Power Stage Transfer Functions
Transfer function F
Where
F
H
F
1
m
e
( )
PARTS
S
( )
S
=
R10
R12
R13
R9
=
v ˆ
=
comp
v ˆ
ω
d ˆ
d ˆ
n
PWM
S
ω
o
esr
TABLE 2. COMPONENT LIST (Continued)
and ω
2
2
n
=
V
+
=
10.2k Ω , ± 1%, (0805)
4.7 Ω , ± 5%, (0805)
20k Ω , ± 1%, (0805)
1.87k Ω , ± 1%, (0805)
=
in
is the peak-peak voltage of the PWM ramp
ω
R
V
n
c
n
ω
S
PWM
S
1
Q
C
PART NUMBERS AND MANUFACTURER
o
1
are given by
2
2
o
n
1
1
(S) from control to output voltage is:
+
,
+
+
ω
Q
1
ω
o
p
S
S
esr
Q
≈
p
R
+
o
18
m
1
Q
C
:
L
n
o
=
,
−
ω
π
2
o
,
=
ω
n
=
LC
1
π
e
o
(S):
f
s
,
ISL6255, ISL6255A
Transfer function F
Current loop gain T
equation:
where R
usually equal to the product of the current sensing resistance
of the current amplifier. For ISL6255, ISL6255A, R
The voltage gain with open current loop is:
Where
error amplifier. The Voltage loop gain with current loop
closed is given by:
If T
From the above equation, it is shown that the system is a
single order system, which has a single pole located at
before the half switching frequency. Therefore, simple type II
compensator can be easily used to stabilize the system.
Figure 17 shows the voltage loop compensator, and its
transfer function is expressed as follows
T
T
F
L
L
i
v
v
v
2
FIGURE 17. SMALL SIGNAL MODEL OF SYNCHRONOUS
(
+
+
(
(
(
( )
S
i
S
S
S
S
(S)>>1, then it can be simplified as follows:
i ˆ
i ˆ
v ˆ
v ˆ
in
in
in
in
)
)
)
)
=
=
=
=
=
R
K =
i ˆ
KF
1
d ˆ
T
V
L
V
T
T
+
FB
is the trans-resistance in current loop. R
o
F
m
v
=
T
m
V
( )
I
I
F
V
i
S
L
L
R
FB
R
1
( )
F
d ˆ
d ˆ
o
BUCK REGULATOR
S
Fm
Fm
o
2
( ) ( )
o
+
+
S
V
R
( ) ( )
+
S
in
, V
+
1:D
1:D
d ˆ
d ˆ
T
A
R
R
H
v
2
FB
L
i
L
(S) is expressed as the following
e
(S) from control to inductor current is:
S
1
ω
S
S
V
V
1
is the feedback voltage of the voltage
+
in
in
o
2
2
+
He(S)
He(S)
d ˆ
d ˆ
ω
1
+
ω
S
esr
S
+
ω
p
ω
o
i ˆ
i ˆ
L
L
S
S
T
T
Q
z
H
A
i
i
(S)
(S)
p
v
e
L
L
v ˆ
v ˆ
( )
( )
+
S
S
comp
comp
1
, where
ω
R
R
p
T
T
-Av(S)
-Av(S)
≈
R
o
1
C
ω
Rc
Rc
Co
Co
o
z
Ro
Ro
T
T
≈
v
v
T
T
(S)
(S)
June 17, 2005
R
=20R
v ˆ
v ˆ
is
o
1
o
o
FN9203.1
C
o
ω
1
K
K
.
p
.