ISL6255 INTERSIL [Intersil Corporation], ISL6255 Datasheet - Page 17

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

include the MOSFET internal gate resistance, gate charge,
threshold voltage, stray inductance, pull-up and pull-down
resistance of the gate driver. The following switching loss
calculation provides a rough estimate.
Where Q
charge of the body-diode in low side MOSFET, I
valley current, I
I
respectively.
To achieve low switching losses, it requires low drain-to-gate
charge Q
the higher the on-resistance. Therefore, there is a trade-off
between the on-resistance and drain-to-gate charge. Good
MOSFET selection is based on the Figure of Merit (FOM),
which is a product of the total gate charge and on-
resistance. Usually, the smaller the value of FOM, the higher
the efficiency for the same application.
For the low-side MOSFET, the worst-case power dissipation
occurs at minimum battery voltage and maximum input
voltage:
Choose a low-side MOSFET that has the lowest possible
on-resistance with a moderate-sized package like the SO-8
and is reasonably priced. The switching losses are not an
issue for the low side MOSFET because it operates at zero-
voltage-switching.
Choose a Schottky diode in parallel with low-side MOSFET
Q2 with a forward voltage drop low enough to prevent the
low-side MOSFET Q2 body-diode from turning on during the
dead time. This also reduces the power loss in the high-side
MOSFET associated with the reverse recovery of the low-
side MOSFET Q2 body diode.
As a general rule, select a diode with DC current rating equal
to one-third of the load current. One option is to choose a
combined MOSFET with the Schottky diode in a single
package. The integrated packages may work better in
practice because there is less stray inductance due to a
short connection. This Schottky diode is optional and may be
removed if efficiency loss can be tolerated. In addition,
ensure that the required total gate drive current for the
selected MOSFETs should be less than 24mA. So, the total
gate charge for the high-side and low-side MOSFETs is
limited by the following equation:
Q
P
P
g
Q
,
Q
GATE
source
, 1
2
Switching
=




1
≤
gd
are the peak gate-drive source/sink current of Q1,
gd
−
I
: drain-to-gate charge, Q
V
GATE
. Generally, the lower the drain-to-gate charge,
V
f
=
OUT
s
IN
1
2
LP:
V




IN
I
Inductor peak current, I
BAT
2
I
LV
R
f
s
DSON
I
, g
Q
source
17
gd
+
rr
1
2
: total reverse recovery
V
IN
I
LP
g,sink
f
s
I
LV
, g
and
Q
sin
gd
: inductor
ISL6255, ISL6255A
k
+
Q
rr
V
IN
f
s
Where I
less than 24mA. Substituting I
into the previous equation yields that the total gate charge
should be less than 80nC. Therefore, the ISL6255,
ISL6255A easily drives the battery charge current up to 8A.
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 15.
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
Q3, Q4
C11
C5
C6
D1
D2
Q5
Q6
R1
R2
R3
R4
R5
R6
R7
=
L
I
GATE
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
10 µ H/3.8A/26m Ω , Sumida, CDRH104R-100
30V/35m Ω , FDS6912A, Fairchild
-30V/30m Ω , SI4835BDY, Siliconix
Signal P-channel MOSFET, NDS352AP
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)
is the total gate drive current and should be
V
OUT
TABLE 2. COMPONENT LIST
PART NUMBERS AND MANUFACTURER
(
V
V
IN
IN
−
V
OUT
GATE
)
=24mA and f
s
=300kHz
June 17, 2005
FN9203.1