ISL6252 Просмотр технического описания (PDF) - Intersil

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ISL6252

Highly Integrated Battery Charger Controller for Notebook Computers

Intersil 

ISL6252, ISL6252A

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.

PQ1, Switching =

(EQ. 23)

1--

2

VI

N

ILV

fsw

⎛

⎜

⎝

I--g---,---Qs---o-g--u--d--r--c---e-⎠⎟⎞

+

1--

2

VINILPfsw

⎛

⎜

⎝

I--g--Q-,---s-g--i-dn----k-⎠⎟⎞

+

QrrVINfsw

where the following are the peak gate-drive source/sink

current of Q1, respectively:

• Qgd: drain-to-gate charge

• Qrr: total reverse recovery charge of the body-diode in

low-side MOSFET

• ILV: inductor valley current

• ILP: Inductor peak current

• Ig,sink

• Ig,source

Low switching loss requires low drain-to-gate charge Qgd.

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):

PQ2

=

⎛

⎜

⎝

1

–

V----V-O---I-U-N---T--⎠⎟⎞

⋅

IB

2

AT

⋅

rD

S(

O

N

)

(EQ. 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

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 25:

QGAT

E

≤

1----G-----A---T----E--

fsw

(EQ. 25)

Where IGATE is the total gate drive current and should be

less than 24mA. Substituting IGATE = 24mA and fs = 300kHz

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:

CSNUB

=

------------------2------------------

(2πFring)2 ⋅ L

(EQ. 26)

RSNUB =

-----2-----⋅---L------

CSNUB

(EQ. 27)

Input Capacitor Selection

The input capacitor absorbs the ripple current from the

synchronous buck converter, which is given by Equation 28:

IRMS

=

IBAT

-----V----O----U-----T----⋅---(---V----I--N-----–-----V----O----U-----T----)

VIN

(EQ. 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.

17

FN6498.1

July 19, 2007

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