ISL6252HAZ Просмотр технического описания (PDF) - Renesas Electronics

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ISL6252HAZ

Highly Integrated Battery Charger Controller for Notebook Computers

Renesas Electronics 

ISL6252, ISL6252A

Inductor Selection

The inductor selection has trade-offs between cost, size,

crossover frequency and efficiency. For example, the lower the

inductance, the smaller the size, but ripple current is higher.

This also results in higher AC losses in the magnetic core and

the windings, which decrease the system efficiency. On the

other hand, the higher inductance results in lower ripple current

and smaller output filter capacitors, but it has higher DCR (DC

resistance of the inductor) loss, lower saturation current and

has slower transient response. So, the practical inductor

design is based on the inductor ripple current being ±15% to

±20% of the maximum operating DC current at maximum input

voltage. Maximum ripple is at 50% duty cycle or

VBAT = VIN,MAX/2. The required inductance can be calculated

from Equation 16:

L = 4--------f--S-V---W-I--N------M-I--R--A--I--PX---P----L----E--

(EQ. 16)

Where VIN,MAX and fSW are the maximum input voltage, and

switching frequency, respectively.

The inductor ripple current I is found from Equation 17:

IRIPPLE = 0.3  IL MAX

(EQ. 17)

where the maximum peak-to-peak ripple current is 30% of the

maximum charge current is used.

For VIN,MAX = 19V, VBAT = 16.8V, IBAT,MAX = 2.6A, and

fs = 300kHz, the calculated inductance is 8.3µH. Choosing the

closest standard value gives L = 10µH. Ferrite cores are often

the best choice since they are optimized at 300kHz to 600kHz

operation with low core loss. The core must be large enough

not to saturate at the peak inductor current IPeak in

Equation 18:

IPEAK = IL MAX + 12--  IRIPPLE

(EQ. 18)

Inductor saturation can lead to cascade failures due to very

high currents. Conservative design limits the peak and RMS

current in the inductor to less than 90% of the rated saturation

current.

Crossover frequency is heavily dependent on the inductor

value. fCO should be less than 20% of the switching frequency

and a conservative design has fCO less than 10% of the

switching frequency. The highest fCO is in voltage control

mode with the battery removed and may be calculated

(approximately) from Equation 19:

fCO = 5--------1----1---2----R-----S--L--E----N----S----E--

(EQ. 19)

Output Capacitor Selection

The output capacitor in parallel with the battery is used to

absorb the high frequency switching ripple current and smooth

the output voltage. The RMS value of the output ripple current

IRMS is given by Equation 20:

IRMS

=

------V----I--N-------M----A----X--------  D  1 – D

12  L  FSW

(EQ. 20)

where the duty cycle D is the ratio of the output voltage (battery

voltage) over the input voltage for continuous conduction mode

which is typical operation for the battery charger. During the

battery charge period, the output voltage varies from its initial

battery voltage to the rated battery voltage. So, the duty cycle

change can be in the range of between 0.53 and 0.88 for the

minimum battery voltage of 10V (2.5V/Cell) and the maximum

battery voltage of 16.8V. The maximum RMS value of the

output ripple current occurs at the duty cycle of 0.5 and is

expressed as Equation 21:

IRMS

=

---------V----I--N-------M----A----X-----------

4  12  L  fSW

(EQ. 21)

For VIN,MAX = 19V, VBAT = 16.8V, L = 10µH, and fs = 300kHz,

the maximum RMS current is 0.19A. A typical 10F ceramic

capacitor is a good choice to absorb this current and also has

very small size. Organic polymer capacitors have high

capacitance with small size and have a significant equivalent

series resistance (ESR). Although ESR adds to ripple voltage,

it also creates a high frequency zero that helps the closed loop

operation of the buck regulator.

EMI considerations usually make it desirable to minimize ripple

current in the battery leads. Beads may be added in series with

the battery pack to increase the battery impedance at 300kHz

switching frequency. Switching ripple current splits between

the battery and the output capacitor depending on the ESR of

the output capacitor and battery impedance. If the ESR of the

output capacitor is 10m and battery impedance is raised to

2 with a bead, then only 0.5% of the ripple current will flow in

the battery.

MOSFET Selection

The Notebook battery charger synchronous buck converter

has the input voltage from the AC adapter output. The

maximum AC adapter output voltage does not exceed 25V.

Therefore, 30V logic MOSFET should be used.

The high side MOSFET must be able to dissipate the

conduction losses plus the switching losses. For the battery

charger application, the input voltage of the synchronous buck

converter is equal to the AC adapter output voltage, which is

relatively constant. The maximum efficiency is achieved by

selecting a high side MOSFET that has the conduction losses

equal to the switching losses. Switching losses in the low-side

FET are very small. The choice of low-side FET is a trade-off

between conduction losses (rDS(ON)) and cost. A good rule of

thumb for the rDS(ON) of the low-side FET is 2x the rDS(ON) of

the high-side FET.

FN6498 Rev 3.00

Aug 25, 2010

Page 16 of 25

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