ISL6251AHRZ-T View Datasheet(PDF) - Renesas Electronics

Part Name

Description

Manufacturer

ISL6251AHRZ-T

Low Cost Multi-Chemistry Battery Charger Controller

Renesas Electronics 

ISL6251, ISL6251A

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. Ensure that ISL6251, ISL6251A LGATE gate

driver can supply sufficient gate current to prevent it from

conduction, which is due to the injected current into the drain-to-

source parasitic capacitor (Miller capacitor Cgd), and caused by

the voltage rising rate at phase node at the time instant of the

high-side MOSFET turning on; otherwise, cross-conduction

problems may occur. Reasonably slowing turn-on speed of the

high-side MOSFET by connecting a resistor between the BOOT pin

and gate drive supply source, and the 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

RDSON

(EQ. 14)

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

(EQ. 15)

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

(EQ. 16)

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 17:

QGATE

 IGATE

fs

(EQ. 17)

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

than 24mA. Substituting IGATE = 24mA and fs = 300kHz into the

above equation yields that the total gate charge should be less

than 80nC. Therefore, the ISL6251, ISL6251A easily drives the

battery charge current up to 10A.

Input Capacitor Selection

The input capacitor absorbs the ripple current from the

synchronous buck converter, which is given by Equation 18:

Irms  IBAT

VOUT VIN VOUT 

VIN

(EQ. 18)

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 12.

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

FN9202 Rev 3.00

March 13, 2014

Page 15 of 20

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