ISL6256 Datasheet, PDF (19/26 Pages) Intersil Corporation – Highly Integrated Battery Charger with Automatic Power Source Selector for Notebook Computers

ISL6256, ISL6256A
• I_g,_source
Low switching loss requires low drain-to-gate charge Q_gd.
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 26):
P_Q2
=
⎛
⎜
⎝
1
–
V_{----V-O---I-U-N---T--⎠⎟⎞}
⋅
I_B
2
AT
⋅
r_D
S(
O
N
)
(EQ. 26)
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 Equation 27:
Q_GAT
E
≤
1_{----G-----A---T----E--}
f_sw
(EQ. 27)
Where I_GATE is the total gate drive current and should be
less than 24mA. Substituting I_GATE = 24mA and f_s = 300kHz
into Equation 27 yields that the total gate charge should be
less than 80nC. Therefore, the ISL6256 easily drives the
battery charge current up to 10A.
Snubber Design
ISL6256'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 28
and 29:
C_SNUB
=
-----------------²-------------------
(2πF_ring)² ⋅ L
(EQ. 28)
R_SNUB =
-----²-----^â‹---^L------
C_SNUB
(EQ. 29)
Input Capacitor Selection
The input capacitor absorbs the ripple current from the
synchronous buck converter, which is given by Equation 30:
I_RMS
=
I_B
A
T
-----^V----^O----^U-----^T----^â‹---⁽---^V----^I--^N-----–-----^V----^O----^U-----^T----⁾
V_IN
(EQ. 30)
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.
Table 2 shows the component lists for the typical application
circuit in Figure 2.
TABLE 2. COMPONENT LIST
PARTS
PART NUMBERS AND MANUFACTURER
C₁, C₁₀ 10µF/25V ceramic capacitor, Taiyo Yuden
TMK325 MJ106MY X5R (3.2mmx2.5mmx1.9mm)
C₂, C₄, C₈ 0.1µF/50V ceramic capacitor
C₃, C₇, C₉ 1µF/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
100mA/30V Schottky Diode, Central Semiconductor
L
10µH/3.8A/26mΩ, Sumida, CDRH104R-100
Q₁, Q₂
Q₃, Q₄
Q₅
Q₆
30V/35mΩ, FDS6912A, Fairchild
-30V/30mΩ, SI4835BDY, Siliconix
Signal P-Channel MOSFET, NDS352AP
Signal N-Channel MOSFET, 2N7002
19
FN6499.3
September 14, 2010