isl6252a Intersil Corporation, isl6252a Datasheet - Page 17

• Manufacturer Part Number: isl6252a
• Description: Highly Integrated Battery Charger Controller For Notebook Computers
• Manufacturer: Intersil Corporation

calculate the switching losses in the high-side MOSFET
since it must allow for difficult-to-quantify factors that
influence the turn-on and turn-off times. These factors
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 (Equation 23)
provides a rough estimate.
where the following are the peak gate-drive source/sink
current of Q
• Q
• Q
• I
• I
• I
• I
Low switching loss requires low drain-to-gate charge Q
Generally, the lower the drain-to-gate charge, 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 (Equation 24):
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
Q
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 Q
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
P
1
-- - V
2
P
Q1 Switching
Q2
2
low-side MOSFET
LV
LP
g,sink
g
IN
with a forward voltage drop low enough to prevent the
gd
rr
,
,
source
I
: inductor valley current
=
: total reverse recovery charge of the body-diode in
: Inductor peak current
LV
: drain-to-gate charge
⎛
⎜
⎝
f
1
sw
–
⎛
⎜
⎝
V
--------------- -
1
------------------------ -
I
V
g source
OUT
, respectively:
,
IN
Q
=
gd
⎞
⎟
⎠
⋅
I
BAT
⎞
⎟
⎠
2
+
body diode.
2
1
-- - V
2
⋅
r
IN
DS ON
I
17
LP
(
f
sw
)
⎛
⎜
⎝
---------------- -
I
g
Q
,
sin
gd
k
⎞
⎟
⎠
+
Q
rr
V
IN
f
ISL6252, ISL6252A
sw
(EQ. 24)
(EQ. 23)
gd
.
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 Equation 25:
Where I
less than 24mA. Substituting I
into Equation 25 yields that the total gate charge should be
less than 80nC. Therefore, the ISL6252 easily drives the
battery charge current up to 10A.
Snubber Design
ISL6252's buck regulator operates in discontinuous current
mode (DCM) when the load current is less than half the
peak-to-peak current in the inductor. After the low-side FET
turns off, the phase voltage rings due to the high impedance
with both FETs off. This can be seen in Figure 9. Adding a
snubber (resistor in series with a capacitor) from the phase
node to ground can greatly reduce the ringing. In some
situations a snubber can improve output ripple and
regulation.
The snubber capacitor should be approximately twice the
parasitic capacitance on the phase node. This can be
estimated by operating at very low load current (100mA) and
measuring the ringing frequency.
CSNUB and RSNUB can be calculated from Equations 26
and 27:
Input Capacitor Selection
The input capacitor absorbs the ripple current from the
synchronous buck converter, which is given by Equation 28:
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 recommended that ceramic
capacitors or polymer capacitors from Sanyo be used due to
their small size and reasonable cost.
Q
C
R
I
RMS
GATE
SNUB
SNUB
=
≤
GATE
=
=
I
1
------------------ -
BAT
GATE
------------------------------------ -
(
f
2πF
sw
------------------- -
C
------------------------------------------------------------ -
2 L
SNUB
is the total gate drive current and should be
V
⋅
ring
OUT
2
)
2
⋅
⋅
(
L
V
V
IN
IN
–
V
OUT
GATE
)
= 24mA and f
s
= 300kHz
July 19, 2007
(EQ. 26)
(EQ. 27)
(EQ. 25)
(EQ. 28)
FN6498.1