ISL6721 datasheet(15/21 Pages) INTERSIL

15 / 21 page

FN9110.4

April 13, 2007

The final component of MOSFET loss is caused by the

charging of the gate capacitance through the device gate

resistance. Depending on the relative value of any external

resistance in the gate drive circuit, a portion of this power will

be dissipated externally.

Once the losses are known, the device package must be

selected and the heatsinking method designed. Since the

design requires a small surface mount part, a SOIC-8

package was selected. A Fairchild FDS2570 MOSFET was

selected based on these criteria. The overall losses are

estimated at 400mW.

Output Filter Design

In a flyback design, the primary concern for the design of the

output filter is the capacitor ripple current stress and the

ripple and noise specification of the output.

The current flowing in and out of the output capacitors is the

difference between the winding current and the output

current. The peak secondary current, ISPK, is 10.73A for the

3.3V output and 4.29A for the 1.8V output. The current

flowing into the output filter capacitor is the difference

between the winding current and the output current. Looking

at the 3.3V output, the peak winding current is

ISPK = 10.73A. The capacitor must store this amount minus

the output current of 2.5A, or 8.23A. The RMS ripple current

in the 3.3V output capacitor is about 3.5A RMS. The RMS

ripple current in the 1.8V output capacitor is about 1.4A RMS

Voltage deviation on the output during the switching cycle

(ripple and noise) is caused by the change in charge of the

output capacitance, the equivalent series resistance (ESR),

and equivalent series inductance (ESL). Each of these

components must be assigned a portion of the total ripple

and noise specification. How much to allow for each

contributor is dependent on the capacitor technology used.

For purposes of this discussion we will assume the following:

3.3V output: 100mV total output ripple and noise

ESR: 60mV

Capacitor

ΔQ: 10mV

ESL: 30mV

1.8V output: 50mV total output ripple and noise

ESR: 30mV

Capacitor

ΔQ: 5mV

ESL: 15mV

For the 3.3V output:

The change in voltage due to the change in charge of the

output capacitor,

ΔQ, determines how much capacitance is

required on the output.

ESL adds to the ripple and noise voltage in proportion to the

rate of change of current into the capacitor (V = L

• di/dt).

Capacitors having high capacitance usually do not have

sufficiently low ESL. High frequency capacitors such as

surface mount ceramic or film are connected in parallel with

the high capacitance capacitors to address the effects of

ESL. A combination of high frequency and high ripple

capability capacitors is used to achieve the desired overall

performance. The analysis of the 1.8V output is similar to

that of the 3.3V output and is omitted for brevity. Two

OSCON 4SEP560M (560

μF) electrolytic capacitors and a

μF X5R ceramic 1210 capacitor were selected for both the

3.3 and 1.8V outputs. The 4SEP560M electrolytic capacitors

are each rated at 4520mA ripple current and 13m

Ω of ESR.

The ripple current rating of just one of these capacitors is

adequate, but two are needed to meet the minimum ESR

and capacitance values.

The bias output is of such low power and current that it

places negligible stress on its filter capacitor. A single 0.1

μF

ceramic capacitor was selected.

Control Loop Design

The major components of the feedback control loop are a

programmable shunt regulator, an opto-coupler, and the

inverting amplifier of the ISL6721. The opto-coupler is used

to transfer the error signal across the isolation barrier. The

opto-coupler offers a convenient means to cross the

isolation barrier, but it adds complexity to the feedback

control loop. It adds a pole at about 10kHz and a significant

amount of gain variation due the current transfer ratio (CTR).

The CTR of the opto-coupler varies with initial tolerance,

temperature, forward current, and age.

Pgate

QgVgFsw

•

(EQ. 22)

ESR

ΔV

SPK

OUT

–

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

≤

0.060

10.73

2.5

–

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

7.3m

(EQ. 23)

Ispk

Iout

–

() Tr

•

ΔV

•

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

≥

10.73

2.5

–

() 2.33

–

×10

•

20.010

•

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

960

μF

(EQ. 24)

Vdt

•

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

≤

0.030

200

–

×10

•

10.73

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

0.56nH

(EQ. 25)

ISL6721

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