
16
FN9202.1
June 17, 2005
should be less than 80nC. Therefore, the ISL6251 easily
drives the battery charge current up to 10A.
Input Capacitor Selection
The input capacitor absorbs the ripple current from the
synchronous buck converter, which is given by:
(
OUT
IN
OUT
BAT
rms
V
This RMS ripple current must be smaller than the rated RMS
current in the capacitor datasheet. Non-tantalum chemistries
(ceramic, aluminum, or OSCON) are preferred due to their
resistance to power-up surge currents when the AC adapter
is plugged into the battery charger. For Notebook battery
charger applications, it is recommend that ceramic
capacitors or polymer capacitors from Sanyo be used due to
their small size and reasonable cost.
Table 2 shows the component lists for the typical application
circuit in Figure 12.
Loop Compensation Design
ISL6251 uses 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 single order system. It is much easier to design a
compensator to stabilize the voltage loop than voltage mode
control. Figure 14 shows the small signal model of the
synchronous buck regulator.
PWM Comparator Gain F
m
:
The PWM comparator gain Fm for peak current mode
control is given by:
Where V
PWM
is the peak-peak voltage of the PWM ramp
signal.
Current Sampling Transfer Function H
e
(S):
In current loop, the current signal is sampled every switching
cycle. It has the following transfer function:
where Q
n
and
ω
n
are given by
respectively.
ω
n
=
π
f
s
,
Power Stage Transfer Functions
Transfer function F
1
(S) from control to output voltage is:
Where
,
Transfer function F
2
(S) from control to inductor current is:
, where
.
Current loop gain T
i
(S) is expressed as the following
equation:
where R
T
is the trans-resistance in current loop. R
T
is
usually equal to the product of the current sensing resistance
of the current amplifier. For ISL6251, R
T
=20R
1
.
TABLE 2. COMPON
E
NT LIST
PARTS
PART NUMBERS AND MANUFACTURER
C1, C10
10
μ
F/25V ceramic capacitor, Taiyo Yuden
TMK325 MJ106MY X5R (3.2x2.5x1.9mm)
C2, C4, C8
0.1
μ
F/50V ceramic capacitor
C3, C7, C9
1
μ
F/10V ceramic capacitor, Taiyo Yuden
LMK212BJ105MG
C5
10nF ceramic capacitor
C6
6.8nF ceramic capacitor
C11
3300pF ceramic capacitor
D1
30V/3A Schottky diode, EC31QS03L (optional)
D2, D3
100mA/30V Schottky Diode, Central Semiconductor
D4
8A/30V Schottky rectifier, STPS8L30B (optional)
L
10
μ
H/3.8A/26m
, Sumida, CDRH104R-100
Q1, Q2
30V/35m
, FDS6912A, Fairchild.
Q3
Signal N-channel MOSFET, 2N7002
R1
40m
,
±
1%, LRC-LR2512-01-R040-F, IRC
R2
20m
,
±
1%, LRC-LR2010-01-R020-F, IRC
R3
18
,
±
5%, (0805)
R4
2.2
,
±
5%, (0805)
R5
100k
,
±
5%, (0805)
R6
10k,
±
5%, (0805)
R7
100
,
±
5%, (0805)
R8, R11
130k,
±
1%, (0805)
R9
10.2k
,
±
1%, (0805)
R10
4.7
,
±
5%, (0805)
R12
20k
,
±
1%, (0805)
R13
1.87k
,
±
1%, (0805)
)
IN
V
V
V
I
I
=
PWM
V
comp
m
1
v
d
F
=
=
( )
S
1
Q
S
S
ω
H
n
n
2
n
2
e
+
+
=
ω
,
2
π
Q
n
=
( )
S
1
Q
S
S
ω
S
1
V
d
v
F
p
o
2
o
2
esr
in
o
1
+
+
+
=
=
ω
ω
,
C
R
1
o
c
esr
=
ω
L
C
R
Q
o
o
p
≈
o
o
LC
1
=
ω
( )
S
1
Q
S
S
ω
S
1
R
R
V
d
i
F
p
o
2
o
2
z
L
o
in
+
L
2
+
+
+
=
=
ω
ω
o
o
z
C
R
1
≈
ω
( )
S
( )
S
H
F
F
R
)
S
(
T
e
2
m
T
i
=
ISL6251, ISL6251A