ISL6255HAZ-T View Datasheet(PDF) - Renesas Electronics

Part Name

Description

Manufacturer

ISL6255HAZ-T

Highly Integrated Battery Charger with Automatic Power Source Selector for Notebook Computers

Renesas Electronics 

ISL6255, ISL6255A

high sink current capability of the low-side MOSFET gate driver

help reduce the possibility of cross-conduction.

For the high-side MOSFET, the worst-case conduction losses

occur at the minimum input voltage:

PQ1,Conduction

 VOUT

VIN

I

2

BAT

R

DSON

The optimum efficiency occurs when the switching losses

equal the conduction losses. However, it is difficult to 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 provides a rough

estimate.

PQ1,Switching



1

2

VINILV

fs

Qgd

Ig ,source



1

2

VINILP

fs

Qgd

Ig ,sin k

 QrrVIN fs

Where 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 and Ig,source are the

peak gate-drive source/sink current of Q1, respectively.

To achieve low switching losses, it 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:

PQ2



1



VOUT

VIN



I

2

BAT

RDSON

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:

QGATE



IGATE

fs

Where IGATE is the total gate drive current and should be less

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

Irms  IBAT

VOUT VIN VOUT 

VIN

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.

TABLE 2. COMPONENT LIST

PARTS

PART NUMBERS AND MANUFACTURER

C1, C10 10F/25V ceramic capacitor, Taiyo Yuden

TMK325 MJ106MY X5R (3.2x2.5x1.9mm)

C2, C4, C8 0.1F/50V ceramic capacitor

C3, C7, C9 1F/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

10H/3.8A/26m, Sumida, CDRH104R-100

Q1, Q2 30V/35m, FDS6912A, Fairchild

Q3, Q4 -30V/30m, SI4835BDY, Siliconix

Q5

Signal P-channel MOSFET, NDS352AP

Q6

Signal N-channel MOSFET, 2N7002

FN9203 Rev 2.00

May 23, 2006

Page 17 of 22

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