ISL6251 INTERSIL [Intersil Corporation], ISL6251 Datasheet - Page 16

• Manufacturer Part Number: ISL6251
• Description: Low Cost Multi-Chemistry Battery Charger Controller
• Manufacturer: INTERSIL [Intersil Corporation]

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:
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.
I
rms
C2, C4, C8 0.1 µ F/50V ceramic capacitor
C3, C7, C9 1 µ F/10V ceramic capacitor, Taiyo Yuden
C1, C10
R8, R11
PARTS
Q1, Q2
D2, D3
C11
R10
R12
R13
C5
C6
D1
D4
Q3
R1
R2
R3
R4
R5
R6
R7
R9
=
L
I
BAT
10 µ F/25V ceramic capacitor, Taiyo Yuden
TMK325 MJ106MY X5R (3.2x2.5x1.9mm)
LMK212BJ105MG
10nF ceramic capacitor
6.8nF ceramic capacitor
3300pF ceramic capacitor
30V/3A Schottky diode, EC31QS03L (optional)
100mA/30V Schottky Diode, Central Semiconductor
8A/30V Schottky rectifier, STPS8L30B (optional)
10 µ H/3.8A/26m Ω , Sumida, CDRH104R-100
30V/35m Ω , FDS6912A, Fairchild.
Signal N-channel MOSFET, 2N7002
40m Ω , ± 1%, LRC-LR2512-01-R040-F, IRC
20m Ω , ± 1%, LRC-LR2010-01-R020-F, IRC
18 Ω , ± 5%, (0805)
2.2 Ω , ± 5%, (0805)
100k Ω , ± 5%, (0805)
10k, ± 5%, (0805)
100 Ω , ± 5%, (0805)
130k, ± 1%, (0805)
10.2k Ω , ± 1%, (0805)
4.7 Ω , ± 5%, (0805)
20k Ω , ± 1%, (0805)
1.87k Ω , ± 1%, (0805)
V
OUT
TABLE 2. COMPONENT LIST
PART NUMBERS AND MANUFACTURER
(
V
V
IN
IN
−
V
OUT
16
)
ISL6251, ISL6251A
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
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
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 ISL6251, R
T
F
H
F
F
1
i
m
2
e
(
( )
( )
S
S
( )
S
S
=
)
=
=
v ˆ
=
=
comp
v ˆ
ω
R
i ˆ
d ˆ
d ˆ
T
n
d ˆ
L
PWM
ω
o
S
esr
T
and ω
is the trans-resistance in current loop. R
n
F
2
2
=
=
m
V
+
=
R
F
=
in
o
is the peak-peak voltage of the PWM ramp
ω
2
R
V
V
( ) ( )
n
+
n
c
S
S
ω
in
PWM
S
1
Q
C
R
o
1
are given by
2
2
H
o
n
1
2
1
L
i
(S) is expressed as the following
e
(S) from control to output voltage is:
+
(S) from control to inductor current is:
,
+
+
ω
S
S
ω
Q
1
ω
o
2
2
o
p
S
S
esr
Q
1
+
≈
p
+
ω
R
+
ω
o
o
S
S
m
1
Q
z
Q
p
:
C
L
n
o
+
1
=
,
−
, where
ω
π
2
o
T
,
=20R
=
ω
n
=
LC
1
π
e
1
ω
o
.
(S):
f
z
s
,
≈
T
June 17, 2005
R
is
o
1
FN9202.1
C
o
.