# isl88731a Intersil Corporation, isl88731a Datasheet - Page 18

#### isl88731a

Manufacturer Part Number

isl88731a

Description

Smbus Level 2 Battery Charger

Manufacturer

Intersil Corporation

Datasheet

1.ISL88731A.pdf
(23 pages)

Low switching loss requires low drain-to-gate charge Q

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 10):

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.

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 11:

Where I

less than 24mA. Substituting I

into Equation 11 yields that the total gate charge should be

less than 80nC. Therefore, the ISL88731A easily drives the

battery charge current up to 8A.

Snubber Design

ISL88731A'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.

C

and 13:

C

P

Q

SNUB

SNUB

Q2

GATE

=

⎛

⎜

⎝

≤

=

GATE

and R

1

I

---------------- -

–

GATE

------------------------------------ -

(

f

2πF

sw

V

--------------- -

V

OUT

IN

is the total gate drive current and should be

SNUB

ring

2

⎞

⎟

⎠

)

⋅

2

(EQ. 12)

I

BAT

can be calculated from Equations 12

⋅

L

2

⋅

r

DS ON

18

GATE

(

R

SNUB

)

= 24mA and f

=

------------------- -

C

2 L

SNUB

⋅

s

= 400kHz

(EQ. 10)

(EQ. 13)

(EQ. 11)

gd

.

ISL88731A

I

Input Capacitor Selection

The input capacitor absorbs the ripple current from the

synchronous buck converter, which is given by Equation 14:

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.

Loop Compensation Design

ISL88731A has three closed loop control modes. One

controls the output voltage when the battery is fully charged

or absent. A second controls the current into the battery

when charging and the third limits current drawn from the

adapter. The charge current and input current control loops

are compensated by a single capacitor on the ICOMP pin.

The voltage control loop is compensated by a network on the

VCOMP pin. Descriptions of these control loops and

guidelines for selecting compensation components will be

given in the following sections. Which loop controls the

output is determined by the minimum current buffer and the

minimum voltage buffer shown in the “Functional Block

Diagram” (see Figure 1) on page 2. These three loops will be

described separately.

Transconductance Amplifiers GMV, GMI and GMS

ISL88731A uses several transconductance amplifiers (also

known as gm amps). Most commercially available op amps

are voltage controlled voltage sources with gain expressed

as A = V

sources with gain expressed as gm = I

appear in some of the equations for poles and zeros in the

compensation.

PWM Gain F

The Pulse Width Modulator in the ISL88731A converts

voltage at VCOMP to a duty cycle by comparing VCOMP to

a triangle wave (duty = VCOMP/V

filter formed by L and C

output voltage (Vo = V

wave amplitude is proportional to V

amplitude proportional to DCIN makes the gain from

VCOMP to the PHASE output a constant 11 and is

independent of DCIN. For small signal AC analysis, the

battery is modeled by its internal resistance. The total output

resistance is the sum of the sense resistor and the internal

resistance of the MOSFETs, inductor and capacitor.

Figure20 shows the small signal model of the pulse width

modulator (PWM), power stage, output filter and battery.

rms

=

I

BAT

OUT

/V

V

OUT

m

IN

. gm amps are voltage controlled current

(

V

V

IN

IN

DCIN

−

O

V

convert the duty cycle to a DC

OUT

*duty). In ISL88731A, the triangle

)

P-P RAMP

DCIN

OUT

. Making the ramp

/V

). The low-pass

IN

. gm will

January 7, 2009

(EQ. 14)

FN6738.1